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Standard ECMA-262

5.1 Edition / June 2011

ECMAScript® Language Specification

This is the HTML rendering of Ecma-262 Edition 5.1, The ECMAScript Language Specification.

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The PDF version is the definitive specification. Any discrepancies between this HTML version and the PDF version are unintentional.

Copyright notice

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Contents

  1. Copyright notice
  2. Introduction
  3. 1 Scope
  4. 2 Conformance
  5. 3 Normative references
  6. 4 Overview
    1. 4.1 Web Scripting
    2. 4.2 Language Overview
      1. 4.2.1 Objects
      2. 4.2.2 The Strict Variant of ECMAScript
    3. 4.3 Terms and definitions
      1. 4.3.1 type
      2. 4.3.2 primitive value
      3. 4.3.3 object
      4. 4.3.4 constructor
      5. 4.3.5 prototype
      6. 4.3.6 native object
      7. 4.3.7 built-in object
      8. 4.3.8 host object
      9. 4.3.9 undefined value
      10. 4.3.10 Undefined type
      11. 4.3.11 null value
      12. 4.3.12 Null type
      13. 4.3.13 Boolean value
      14. 4.3.14 Boolean type
      15. 4.3.15 Boolean object
      16. 4.3.16 String value
      17. 4.3.17 String type
      18. 4.3.18 String object
      19. 4.3.19 Number value
      20. 4.3.20 Number type
      21. 4.3.21 Number object
      22. 4.3.22 Infinity
      23. 4.3.23 NaN
      24. 4.3.24 function
      25. 4.3.25 built-in function
      26. 4.3.26 property
      27. 4.3.27 method
      28. 4.3.28 built-in method
      29. 4.3.29 attribute
      30. 4.3.30 own property
      31. 4.3.31 inherited property
  7. 5 Notational Conventions
    1. 5.1 Syntactic and Lexical Grammars
      1. 5.1.1 Context-Free Grammars
      2. 5.1.2 The Lexical and RegExp Grammars
      3. 5.1.3 The Numeric String Grammar
      4. 5.1.4 The Syntactic Grammar
      5. 5.1.5 The JSON Grammar
      6. 5.1.6 Grammar Notation
    2. 5.2 Algorithm Conventions
  8. 6 Source Text
  9. 7 Lexical Conventions
    1. 7.1 Unicode Format-Control Characters
    2. 7.2 White Space
    3. 7.3 Line Terminators
    4. 7.4 Comments
    5. 7.5 Tokens
    6. 7.6 Identifier Names and Identifiers
      1. 7.6.1 Reserved Words
    7. 7.7 Punctuators
    8. 7.8 Literals
      1. 7.8.1 Null Literals
      2. 7.8.2 Boolean Literals
      3. 7.8.3 Numeric Literals
      4. 7.8.4 String Literals
      5. 7.8.5 Regular Expression Literals
    9. 7.9 Automatic Semicolon Insertion
      1. 7.9.1 Rules of Automatic Semicolon Insertion
      2. 7.9.2 Examples of Automatic Semicolon Insertion
  10. 8 Types
    1. 8.1 The Undefined Type
    2. 8.2 The Null Type
    3. 8.3 The Boolean Type
    4. 8.4 The String Type
    5. 8.5 The Number Type
    6. 8.6 The Object Type
      1. 8.6.1 Property Attributes
      2. 8.6.2 Object Internal Properties and Methods
    7. 8.7 The Reference Specification Type
      1. 8.7.1 GetValue (V)
      2. 8.7.2 PutValue (V, W)
    8. 8.8 The List Specification Type
    9. 8.9 The Completion Specification Type
    10. 8.10 The Property Descriptor and Property Identifier Specification Types
      1. 8.10.1 IsAccessorDescriptor ( Desc )
      2. 8.10.2 IsDataDescriptor ( Desc )
      3. 8.10.3 IsGenericDescriptor ( Desc )
      4. 8.10.4 FromPropertyDescriptor ( Desc )
      5. 8.10.5 ToPropertyDescriptor ( Obj )
    11. 8.11 The Lexical Environment and Environment Record Specification Types
    12. 8.12 Algorithms for Object Internal Methods
      1. 8.12.1 [[GetOwnProperty]] (P)
      2. 8.12.2 [[GetProperty]] (P)
      3. 8.12.3 [[Get]] (P)
      4. 8.12.4 [[CanPut]] (P)
      5. 8.12.5 [[Put]] ( P, V, Throw )
      6. 8.12.6 [[HasProperty]] (P)
      7. 8.12.7 [[Delete]] (P, Throw)
      8. 8.12.8 [[DefaultValue]] (hint)
      9. 8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)
  11. 9 Type Conversion and Testing
    1. 9.1 ToPrimitive
    2. 9.2 ToBoolean
    3. 9.3 ToNumber
      1. 9.3.1 ToNumber Applied to the String Type
    4. 9.4 ToInteger
    5. 9.5 ToInt32: (Signed 32 Bit Integer)
    6. 9.6 ToUint32: (Unsigned 32 Bit Integer)
    7. 9.7 ToUint16: (Unsigned 16 Bit Integer)
    8. 9.8 ToString
      1. 9.8.1 ToString Applied to the Number Type
    9. 9.9 ToObject
    10. 9.10 CheckObjectCoercible
    11. 9.11 IsCallable
    12. 9.12 The SameValue Algorithm
  12. 10 Executable Code and Execution Contexts
    1. 10.1 Types of Executable Code
      1. 10.1.1 Strict Mode Code
    2. 10.2 Lexical Environments
      1. 10.2.1 Environment Records
      2. 10.2.2 Lexical Environment Operations
      3. 10.2.3 The Global Environment
    3. 10.3 Execution Contexts
      1. 10.3.1 Identifier Resolution
    4. 10.4 Establishing an Execution Context
      1. 10.4.1 Entering Global Code
      2. 10.4.2 Entering Eval Code
      3. 10.4.3 Entering Function Code
    5. 10.5 Declaration Binding Instantiation
    6. 10.6 Arguments Object
  13. 11 Expressions
    1. 11.1 Primary Expressions
      1. 11.1.1 Thethis Keyword
      2. 11.1.2 Identifier Reference
      3. 11.1.3 Literal Reference
      4. 11.1.4 Array Initialiser
      5. 11.1.5 Object Initialiser
      6. 11.1.6 The Grouping Operator
    2. 11.2 Left-Hand-Side Expressions
      1. 11.2.1 Property Accessors
      2. 11.2.2 Thenew Operator
      3. 11.2.3 Function Calls
      4. 11.2.4 Argument Lists
      5. 11.2.5 Function Expressions
    3. 11.3 Postfix Expressions
      1. 11.3.1 Postfix Increment Operator
      2. 11.3.2 Postfix Decrement Operator
    4. 11.4 Unary Operators
      1. 11.4.1 Thedelete Operator
      2. 11.4.2 Thevoid Operator
      3. 11.4.3 Thetypeof Operator
      4. 11.4.4 Prefix Increment Operator
      5. 11.4.5 Prefix Decrement Operator
      6. 11.4.6 Unary+ Operator
      7. 11.4.7 Unary- Operator
      8. 11.4.8 Bitwise NOT Operator (~ )
      9. 11.4.9 Logical NOT Operator (! )
    5. 11.5 Multiplicative Operators
      1. 11.5.1 Applying the* Operator
      2. 11.5.2 Applying the/ Operator
      3. 11.5.3 Applying the% Operator
    6. 11.6 Additive Operators
      1. 11.6.1 The Addition operator (+ )
      2. 11.6.2 The Subtraction Operator (- )
      3. 11.6.3 Applying the Additive Operators to Numbers
    7. 11.7 Bitwise Shift Operators
      1. 11.7.1 The Left Shift Operator (<< )
      2. 11.7.2 The Signed Right Shift Operator (>> )
      3. 11.7.3 The Unsigned Right Shift Operator (>>> )
    8. 11.8 Relational Operators
      1. 11.8.1 The Less-than Operator (< )
      2. 11.8.2 The Greater-than Operator (> )
      3. 11.8.3 The Less-than-or-equal Operator (<= )
      4. 11.8.4 The Greater-than-or-equal Operator (>= )
      5. 11.8.5 The Abstract Relational Comparison Algorithm
      6. 11.8.6 The instanceof operator
      7. 11.8.7 The in operator
    9. 11.9 Equality Operators
      1. 11.9.1 The Equals Operator (== )
      2. 11.9.2 The Does-not-equals Operator (!= )
      3. 11.9.3 The Abstract Equality Comparison Algorithm
      4. 11.9.4 The Strict Equals Operator (=== )
      5. 11.9.5 The Strict Does-not-equal Operator (!== )
      6. 11.9.6 The Strict Equality Comparison Algorithm
    10. 11.10 Binary Bitwise Operators
    11. 11.11 Binary Logical Operators
    12. 11.12 Conditional Operator (? : )
    13. 11.13 Assignment Operators
      1. 11.13.1 Simple Assignment (= )
      2. 11.13.2 Compound Assignment (op= )
    14. 11.14 Comma Operator (, )
  14. 12 Statements
    1. 12.1 Block
    2. 12.2 Variable Statement
      1. 12.2.1 Strict Mode Restrictions
    3. 12.3 Empty Statement
    4. 12.4 Expression Statement
    5. 12.5 Theif Statement
    6. 12.6 Iteration Statements
      1. 12.6.1 Thedo-while Statement
      2. 12.6.2 Thewhile Statement
      3. 12.6.3 Thefor Statement
      4. 12.6.4 Thefor-in Statement
    7. 12.7 Thecontinue Statement
    8. 12.8 Thebreak Statement
    9. 12.9 Thereturn Statement
    10. 12.10 Thewith Statement
      1. 12.10.1 Strict Mode Restrictions
    11. 12.11 Theswitch Statement
    12. 12.12 Labelled Statements
    13. 12.13 Thethrow Statement
    14. 12.14 Thetry Statement
      1. 12.14.1 Strict Mode Restrictions
    15. 12.15 Thedebugger statement
  15. 13 Function Definition
    1. 13.1 Strict Mode Restrictions
    2. 13.2 Creating Function Objects
      1. 13.2.1 [[Call]]
      2. 13.2.2 [[Construct]]
      3. 13.2.3 The [[ThrowTypeError]] Function Object
  16. 14 Program
    1. 14.1 Directive Prologues and the Use Strict Directive
  17. 15 Standard Built-in ECMAScript Objects
    1. 15.1 The Global Object
      1. 15.1.1 Value Properties of the Global Object
      2. 15.1.2 Function Properties of the Global Object
      3. 15.1.3 URI Handling Function Properties
      4. 15.1.4 Constructor Properties of the Global Object
      5. 15.1.5 Other Properties of the Global Object
    2. 15.2 Object Objects
      1. 15.2.1 The Object Constructor Called as a Function
      2. 15.2.2 The Object Constructor
      3. 15.2.3 Properties of the Object Constructor
      4. 15.2.4 Properties of the Object Prototype Object
      5. 15.2.5 Properties of Object Instances
    3. 15.3 Function Objects
      1. 15.3.1 The Function Constructor Called as a Function
      2. 15.3.2 The Function Constructor
      3. 15.3.3 Properties of the Function Constructor
      4. 15.3.4 Properties of the Function Prototype Object
      5. 15.3.5 Properties of Function Instances
    4. 15.4 Array Objects
      1. 15.4.1 The Array Constructor Called as a Function
      2. 15.4.2 The Array Constructor
      3. 15.4.3 Properties of the Array Constructor
      4. 15.4.4 Properties of the Array Prototype Object
      5. 15.4.5 Properties of Array Instances
    5. 15.5 String Objects
      1. 15.5.1 The String Constructor Called as a Function
      2. 15.5.2 The String Constructor
      3. 15.5.3 Properties of the String Constructor
      4. 15.5.4 Properties of the String Prototype Object
      5. 15.5.5 Properties of String Instances
    6. 15.6 Boolean Objects
      1. 15.6.1 The Boolean Constructor Called as a Function
      2. 15.6.2 The Boolean Constructor
      3. 15.6.3 Properties of the Boolean Constructor
      4. 15.6.4 Properties of the Boolean Prototype Object
      5. 15.6.5 Properties of Boolean Instances
    7. 15.7 Number Objects
      1. 15.7.1 The Number Constructor Called as a Function
      2. 15.7.2 The Number Constructor
      3. 15.7.3 Properties of the Number Constructor
      4. 15.7.4 Properties of the Number Prototype Object
      5. 15.7.5 Properties of Number Instances
    8. 15.8 The Math Object
      1. 15.8.1 Value Properties of the Math Object
      2. 15.8.2 Function Properties of the Math Object
    9. 15.9 Date Objects
      1. 15.9.1 Overview of Date Objects and Definitions of Abstract Operators
      2. 15.9.2 The Date Constructor Called as a Function
      3. 15.9.3 The Date Constructor
      4. 15.9.4 Properties of the Date Constructor
      5. 15.9.5 Properties of the Date Prototype Object
      6. 15.9.6 Properties of Date Instances
    10. 15.10 RegExp (Regular Expression) Objects
      1. 15.10.1 Patterns
      2. 15.10.2 Pattern Semantics
      3. 15.10.3 The RegExp Constructor Called as a Function
      4. 15.10.4 The RegExp Constructor
      5. 15.10.5 Properties of the RegExp Constructor
      6. 15.10.6 Properties of the RegExp Prototype Object
      7. 15.10.7 Properties of RegExp Instances
    11. 15.11 Error Objects
      1. 15.11.1 The Error Constructor Called as a Function
      2. 15.11.2 The Error Constructor
      3. 15.11.3 Properties of the Error Constructor
      4. 15.11.4 Properties of the Error Prototype Object
      5. 15.11.5 Properties of Error Instances
      6. 15.11.6 Native Error Types Used in This Standard
      7. 15.11.7NativeError Object Structure
    12. 15.12 The JSON Object
      1. 15.12.1 The JSON Grammar
      2. 15.12.2 parse ( text [ , reviver ] )
      3. 15.12.3 stringify ( value [ , replacer [ , space ] ] )
  18. 16 Errors
  19. Annex A(informative) Grammar Summary
    1. A.1 Lexical Grammar
    2. A.2 Number Conversions
    3. A.3 Expressions
    4. A.4 Statements
    5. A.5 Functions and Programs
    6. A.6 Universal Resource Identifier Character Classes
    7. A.7 Regular Expressions
    8. A.8 JSON
      1. A.8.1 JSON Lexical Grammar
      2. A.8.2 JSON Syntactic Grammar
  20. Annex B(informative) Compatibility
    1. B.1 Additional Syntax
      1. B.1.1 Numeric Literals
      2. B.1.2 String Literals
    2. B.2 Additional Properties
      1. B.2.1 escape (string)
      2. B.2.2 unescape (string)
      3. B.2.3 String.prototype.substr (start, length)
      4. B.2.4 Date.prototype.getYear ( )
      5. B.2.5 Date.prototype.setYear (year)
      6. B.2.6 Date.prototype.toGMTString ( )
  21. Annex C(informative) The Strict Mode of ECMAScript
  22. Annex D(informative) Corrections and Clarifications in the 5th Edition with Possible 3rd Edition Compatibility Impact
  23. Annex E(informative) Additions and Changes in the 5th Edition that Introduce Incompatibilities with the 3rd Edition
  24. Annex F(informative) Technically Significant Corrections and Clarifications in the 5.1 Edition
  25. Bibliography

Introduction

This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of forthcoming internationalisation facilities and future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

Since publication of the third edition, ECMAScript has achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. Although that work was not completed and not published as the fourth edition of ECMAScript, it informs continuing evolution of the language. The fifth edition of ECMAScript (published as ECMA-262 5th edition) codifies de facto interpretations of the language specification that have become common among browser implementations and adds support for new features that have emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security.

This present edition 5.1 of the ECMAScript Standard is fully aligned with third edition of the international standard ISO/IEC 16262:2011.

ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technical enhancement will continue with future editions of this specification.

This Ecma Standard has been adopted by the General Assembly of June 2011.

ECMAScript Language Specification

1 Scope

This Standard defines the ECMAScript scripting language.

2 Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of this Standard shall interpret characters in conformance with the Unicode Standard, Version 3.0 or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding form is not otherwise specified, it presumed to be the UTF-16 encoding form.

A conforming implementation of ECMAScript is permitted to provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript is permitted to provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript is permitted to support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript is permitted to support program syntax that makes use of the “future reserved words” listed in7.6.1.2 of this specification.

3 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC 9899:1996, Programming Languages – C, including amendment 1 and technical corrigenda 1 and 2

ISO/IEC 10646-1:1993, Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus its amendments and corrigenda

4 Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specifichost objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

Ascripting language is a programming language that is used to manipulate, customise, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.

ECMAScript was originally designed to be aWeb scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript can provide core scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this document apart from any particular host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular Java™, Self, and Scheme as described in:

Gosling, James, Bill Joy and Guy Steele.The Java™ Language Specification. Addison Wesley Publishing Co., 1996.

Ungar, David, and Smith, Randall B.Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp. 227–241, Orlando, FL, October 1987.

IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.

4.1 Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customised user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.

4.2 Language Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. An ECMAScriptobject is a collection ofproperties each with zero or moreattributes that determine how each property can be used—for example, when the Writable attribute for a property is set tofalse, any attempt by executed ECMAScript code to change the value of the property fails. Properties are containers that hold other objects,primitive values, orfunctions. A primitive value is a member of one of the following built-in types:Undefined,Null,Boolean,Number, andString; an object is a member of the remaining built-in typeObject; and a function is a callable object. A function that is associated with an object via a property is amethod.

ECMAScript defines a collection ofbuilt-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object, theObject object, theFunction object, theArray object, theString object, theBoolean object, theNumber object, theMath object, theDate object, theRegExp object, theJSON object, and the Error objectsError, EvalError,RangeError, ReferenceError, SyntaxError, TypeError andURIError.

ECMAScript also defines a set of built-inoperators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

4.2.1 Objects

ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or viaconstructors which create objects and then execute code that initialises all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named “prototype” that is used to implementprototype-based inheritance andshared properties. Objects are created by using constructors innew expressions; for example,new Date(2009,11) creates a new Date object. Invoking a constructor without usingnew has consequences that depend on the constructor. For example,Date() produces a string representation of the current date and time rather than an object.

Every object created by a constructor has an implicit reference (called the object’sprototype) to the value of its constructor’s “prototype” property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called theprototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

An image of lots of boxes and arrows.
Figure 1 — Object/Prototype Relationships

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, and structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by usingnew expressions:cf1,cf2,cf3,cf4, andcf5. Each of these objects contains properties namedq1 andq2. The dashed lines represent the implicit prototype relationship; so, for example,cf3’s prototype isCFp. The constructor,CF, has two properties itself, namedP1 andP2, which are not visible toCFp,cf1,cf2,cf3,cf4, orcf5. The property namedCFP1 inCFp is shared bycf1,cf2,cf3,cf4, andcf5 (but not byCF), as are any properties found inCFp’s implicit prototype chain that are not namedq1,q2, orCFP1. Notice that there is no implicit prototype link betweenCF andCFp.

Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object’s properties. In the above diagram, one could add a new shared property forcf1,cf2,cf3,cf4, andcf5by assigning a new value to the property inCFp.

4.2.2 The Strict Variant of ECMAScript

The ECMAScript Language recognises the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as thestrict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript code units. Because strict mode is selected at the level of a syntactic code unit, strict mode only imposes restrictions that have local effect within such a code unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple code units. A complete ECMAScript program may be composed for both strict mode and non-strict mode ECMAScript code units. In this case, strict mode only applies when actually executing code that is defined within astrict mode code unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict mode variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted andstrict mode code units into a single composite program.

4.3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

4.3.1 type

set of data values as defined inClause 8 of this specification

4.3.2 primitive value

member of one of the types Undefined, Null, Boolean, Number, or String as defined inClause 8

NOTE A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.3.3 object

member of the type Object

NOTE An object is a collection of properties and has a single prototype object. The prototype may be the null value.

4.3.4 constructor

function object that creates and initialises objects

NOTE The value of a constructor’s “prototype” property is a prototype object that is used to implement inheritance and shared properties.

4.3.5 prototype

object that provides shared properties for other objects

NOTE When a constructor creates an object, that object implicitly references the constructor’s “prototype” property for the purpose of resolving property references. The constructor’s “prototype” property can be referenced by the program expressionconstructor.prototype, and properties added to an object’s prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using theObject.create built-in function.

4.3.6 native object

object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host environment

NOTE Standard native objects are defined in this specification. Some native objects are built-in; others may be constructed during the course of execution of an ECMAScript program.

4.3.7 built-in object

object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the execution of an ECMAScript program

NOTE Standard built-in objects are defined in this specification, and an ECMAScript implementation may specify and define others. Every built-in object is a native object. Abuilt-in constructor is a built-in object that is also a constructor.

4.3.8 host object

object supplied by the host environment to complete the execution environment of ECMAScript

NOTE Any object that is not native is a host object.

4.3.9 undefined value

primitive value used when a variable has not been assigned a value

4.3.10 Undefined type

type whose sole value is the undefined value

4.3.11 null value

primitive value that represents the intentional absence of any object value

4.3.12 Null type

type whose sole value is the null value

4.3.13 Boolean value

member of the Boolean type

NOTE There are only two Boolean values,true andfalse.

4.3.14 Boolean type

type consisting of the primitive valuestrue andfalse

4.3.15 Boolean object

member of the Object type that is an instance of the standard built-inBoolean constructor

NOTE A Boolean object is created by using theBoolean constructor in anew expression, supplying a Boolean value as an argument. The resulting object has an internal property whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.3.16 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer

NOTE A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

4.3.17 String type

set of all possible String values

4.3.18 String object

member of the Object type that is an instance of the standard built-inString constructor

NOTE A String object is created by using theString constructor in anew expression, supplying a String value as an argument. The resulting object has an internal property whose value is the String value. A String object can be coerced to a String value by calling theString constructor as a function (15.5.1).

4.3.19 Number value

primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value

NOTE A Number value is a member of the Number type and is a direct representation of a number.

4.3.20 Number type

set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity

4.3.21 Number object

member of the Object type that is an instance of the standard built-inNumber constructor

NOTE A Number object is created by using theNumber constructor in anew expression, supplying a Number value as an argument. The resulting object has an internal property whose value is the Number value. A Number object can be coerced to a Number value by calling theNumber constructor as a function (15.7.1).

4.3.22 Infinity

number value that is the positive infinite Number value

4.3.23 NaN

number value that is a IEEE 754 “Not-a-Number” value

4.3.24 function

member of the Object type that is an instance of the standard built-inFunction constructor and that may be invoked as a subroutine

NOTE In addition to its named properties, a function contains executable code and state that determine how it behaves when invoked. A function’s code may or may not be written in ECMAScript.

4.3.25 built-in function

built-in object that is a function

NOTE Examples of built-in functions includeparseInt andMath.exp. An implementation may provide implementation-dependent built-in functions that are not described in this specification.

4.3.26 property

association between a name and a value that is a part of an object

NOTE Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.

4.3.27 method

function that is the value of a property

NOTE When a function is called as a method of an object, the object is passed to the function as itsthis value.

4.3.28 built-in method

method that is a built-in function

NOTE Standard built-in methods are defined in this specification, and an ECMAScript implementation may specify and provide other additional built-in methods.

4.3.29 attribute

internal value that defines some characteristic of a property

4.3.30 own property

property that is directly contained by its object

4.3.31 inherited property

property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

Acontext-free grammar consists of a number ofproductions. Each production has an abstract symbol called anonterminal as itsleft-hand side, and a sequence of zero or more nonterminal andterminal symbols as itsright-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

Starting from a sentence consisting of a single distinguished nonterminal, called thegoal symbol, a given context-free grammar specifies alanguage, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars

Alexical grammar for ECMAScript is given inclause 7. This grammar has as its terminal symbols characters (Unicode code units) that conform to the rules forSourceCharacter defined inClause 6. It defines a set of productions, starting from the goal symbolInputElementDiv orInputElementRegExp, that describe how sequences of such characters are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScripttokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process ofautomatic semicolon insertion (7.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. AMultiLineComment (that is, a comment of the form “/*…*/” regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if aMultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

ARegExp grammar for ECMAScript is given in15.10. This grammar also has as its terminal symbols the characters as defined bySourceCharacter. It defines a set of productions, starting from the goal symbolPattern, that describe how sequences of characters are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbolsSourceCharacter. This grammar appears in9.3.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.

5.1.4 The Syntactic Grammar

Thesyntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from the goal symbolProgram, that describe how sequences of tokens can form syntactically correct ECMAScript programs.

When a stream of characters is to be parsed as an ECMAScript program, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The program is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminalProgram, with no tokens left over.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account of which token sequences are accepted as correct ECMAScript programs. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a terminator character appears in certain “awkward” places.

5.1.5 The JSON Grammar

The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual objects. The JSON grammar is given in15.12.1.

The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON lexical grammar is used to translate character sequences into tokens and is similar to parts of the ECMAScript lexical grammar. The JSON syntactic grammar describes how sequences of tokens from the JSON lexical grammar can form syntactically correct JSON object descriptions.

Productions of the JSON lexical grammar are distinguished by having two colons “::” as separating punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The JSON syntactic grammar is similar to parts of the ECMAScript syntactic grammar. Productions of the JSON syntactic grammar are distinguished by using one colon “:” as separating punctuation.

5.1.6 Grammar Notation

Terminal symbols of the lexical, RegExp, and numeric string grammars, and some of the terminal symbols of the other grammars, are shown infixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a program exactly as written. All terminal symbol characters specified in this way are to be understood as the appropriate Unicode character from the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown initalic type. The definition of a nonterminal is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement:
while(Expression)Statement

states that the nonterminalWhileStatement represents the tokenwhile, followed by a left parenthesis token, followed by anExpression, followed by a right parenthesis token, followed by aStatement. The occurrences ofExpression andStatement are themselves nonterminals. As another example, the syntactic definition:

ArgumentList:
AssignmentExpression
ArgumentList,AssignmentExpression

states that anArgumentList may represent either a singleAssignmentExpression or anArgumentList, followed by a comma, followed by anAssignmentExpression. This definition ofArgumentList is recursive, that is, it is defined in terms of itself. The result is that anArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is anAssignmentExpression. Such recursive definitions of nonterminals are common.

The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration:
IdentifierInitialiseropt

is a convenient abbreviation for:

VariableDeclaration:
Identifier
IdentifierInitialiser

and that:

IterationStatement:
for(ExpressionNoInopt;Expressionopt;Expressionopt)Statement

is a convenient abbreviation for:

IterationStatement:
for(;Expressionopt;Expressionopt)Statement
for(ExpressionNoIn;Expressionopt;Expressionopt)Statement

which in turn is an abbreviation for:

IterationStatement:
for(;;Expressionopt)Statement
for(;Expression;Expressionopt)Statement
for(ExpressionNoIn;;Expressionopt)Statement
for(ExpressionNoIn;Expression;Expressionopt)Statement

which in turn is an abbreviation for:

IterationStatement:
for(;;)Statement
for(;;Expression)Statement
for(;Expression;)Statement
for(;Expression;Expression)Statement
for(ExpressionNoIn;;)Statement
for(ExpressionNoIn;;Expression)Statement
for(ExpressionNoIn;Expression;)Statement
for(ExpressionNoIn;Expression;Expression)Statement

so the nonterminalIterationStatement actually has eight alternative right-hand sides.

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

NonZeroDigit::one of
123456789

which is merely a convenient abbreviation for:

NonZeroDigit::
1
2
3
4
5
6
7
8
9

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead ∉set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token is a member of the givenset. Theset can be written as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. For example, given the definitions

DecimalDigit::one of
0123456789
DecimalDigits::
DecimalDigit
DecimalDigitsDecimalDigit

the definition

LookaheadExample::
n[lookahead ∉ {1,3,5,7,9}]DecimalDigits
DecimalDigit[lookahead ∉DecimalDigit]

matches either the lettern followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

If the phrase “[noLineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production isa restricted production: it may not be used if aLineTerminator occurs in the input stream at the indicated position. For example, the production:

ThrowStatement:
throw[noLineTerminator here]Expression;

indicates that the production may not be used if aLineTerminator occurs in the program between thethrow token and theExpression.

Unless the presence of aLineTerminator is forbidden by a restricted production, any number of occurrences ofLineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the program.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-character token, it represents the sequence of characters that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

Identifier::
IdentifierNamebut notReservedWord

means that the nonterminalIdentifier may be replaced by any sequence of characters that could replaceIdentifierName provided that the same sequence of characters could not replaceReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:

SourceCharacter::
any Unicode code unit

5.2 Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.

In order to facilitate their use in multiple parts of this specification, some algorithms, calledabstractoperations, are named and written in parameterised functional form so that they may be referenced by name from within other algorithms.

When an algorithm is to produce a value as a result, the directive “returnx” is used to indicate that the result of the algorithm is the value ofx and that the algorithm should terminate. The notationResult(n) is used as shorthand for “theresult of stepn”.

For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

  1. Top-level step
    1. Substep.
    2. Substep
      1. Subsubstep.
      2. Subsubstep.
        1. Subsubsubstep
          1. Subsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step may assert an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point number must be finite, and if it is+0 or−0 then the corresponding mathematical value is simply0.

The mathematical functionabs(x) yields the absolute value ofx, which is−x ifx is negative (less than zero) and otherwise isx itself.

The mathematical functionsign(x) yields1 ifx is positive and−1 ifx is negative. The sign function is not used in this standard for cases whenx is zero.

The notation “x moduloy” (y must be finite and nonzero) computes a valuek of the same sign asy (or zero) such thatabs(k) < abs(y) andx−k =q×y for some integerq.

The mathematical functionfloor(x) yields the largest integer (closest to positive infinity) that is not larger thanx.

NOTEfloor(x) =x−(x modulo 1).

If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no result is returned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals with the exception, using terminology such as “If an exception was thrown…”. Once such an algorithm step has been encountered the exception is no longer considered to have occurred.

6 Source Text

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later. The text is expected to have been normalised to Unicode Normalization Form C (canonical composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be processed as if it was first converted to UTF-16.

Syntax

SourceCharacter::
any Unicode code unit

Throughout the rest of this document, the phrase “code unit” and the word “character” will be used to refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase “Unicode character” will be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be longer than 16 bits and thus may be represented by more than one code unit). The phrase “code point” refers to such a Unicode scalar value. “Unicode character” only refers to entities represented by single Unicode scalar values: the components of a combining character sequence are still individual “Unicode characters,” even though a user might think of the whole sequence as a single character.

In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed as a Unicode escape sequence consisting of six characters, namely\u plus four hexadecimal digits. Within a comment, such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal, the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence contributes one character to the identifier.

NOTE Although this document sometimes refers to a “transformation” between a “character” within a “string” and the 16-bit unsigned integer that is the code unit of that character, there is actually no transformation because a “character” within a “string” is actually represented using that 16-bit unsigned value.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence\u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode character000A is line feed) and therefore the next character is not part of the comment. Similarly, if the Unicode escape sequence\u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write\n instead of\u000A to cause a line feed to be part of the string value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes a character to the String value of the literal and is never interpreted as a line terminator or as a quote mark that might terminate the string literal.

7 Lexical Conventions

The source text of an ECMAScript program is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of characters as the next input element.

There are two goal symbols for the lexical grammar. TheInputElementDiv symbol is used in those syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. TheInputElementRegExp symbol is used in other syntactic grammar contexts.

NOTE There are no syntactic grammar contexts where both a leading division or division-assignment, and a leadingRegularExpressionLiteral are permitted. This is not affected bysemicolon insertion (see 7.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment character after aLineTerminator is slash (/) and the syntactic context allows division or division-assignment, no semicolon is inserted at theLineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementDiv::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
InputElementRegExp::
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral

7.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as left-to-right mark or right-to-left mark) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals and regular expression literals.

<ZWNJ> and<ZWJ> are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text,<ZWNJ> and<ZWJ> may also be used in an identifier after the first character.

<BOM> is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order.<BOM> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. <BOM> characters are treated as white space characters (see 7.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarised in Table 1.

Table 1 — Format-Control Character Usage
Code UnitValueNameFormal NameUsage
\u200CZero width non-joiner<ZWNJ>IdentifierPart
\u200DZero width joiner<ZWJ>IdentifierPart
\uFEFFByte Order Mark<BOM>Whitespace

7.2 White Space

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space characters may occur between any two tokens and at the start or end of input. White space characters may also occur within aStringLiteral or aRegularExpressionLiteral (where they are considered significant characters forming part of the literal value) or within aComment, but cannot appear within any other kind of token.

The ECMAScript white space characters are listed in Table 2.

Table 2 — Whitespace Characters
Code Unit ValueNameFormal Name
\u0009Tab<TAB>
\u000BVertical Tab<VT>
\u000CForm Feed<FF>
\u0020Space<SP>
\u00A0No-break space<NBSP>
\uFEFF

Other category “Zs”

Byte Order Mark

Any other Unicode “space separator”

<BOM>

<USP>

ECMAScript implementations must recognise all of the white space characters defined in Unicode 3.0. Later editions of the Unicode Standard may define other white space characters. ECMAScript implementations may recognise white space characters from later editions of the Unicode Standard.

Syntax

WhiteSpace::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<BOM>
<USP>

7.3 Line Terminators

Like white space characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space characters, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process ofautomatic semicolon insertion (7.9). A line terminator cannot occur within any token except aStringLiteral. Line terminators may only occur within aStringLiteral token as part of aLineContinuation.

A line terminator can occur within aMultiLineComment (7.4) but cannot occur within aSingleLineComment.

Line terminators are included in the set of white space characters that are matched by the\s class in regular expressions.

The ECMAScript line terminator characters are listed in Table 3.

Table 3 — Line Terminator Characters
Code Unit ValueNameFormal Name
\u000ALine Feed<LF>
\u000DCarriage Return<CR>
\u2028Line separator<LS>
\u2029Paragraph separator<PS>

Only the characters in Table 3 are treated as line terminators. Other new line or line breaking characters are treated as white space but not as line terminators. The character sequence <CR><LF> is commonly used as a line terminator. It should be considered a single character for the purpose of reporting line numbers.

Syntax

LineTerminator::
<LF>
<CR>
<LS>
<PS>
LineTerminatorSequence::
<LF>
<CR>[lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>

7.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any character except aLineTerminator character, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all characters from the// marker to the end of the line. However, theLineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognised separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process ofautomatic semicolon insertion (see 7.9).

Comments behave like white space and are discarded except that, if aMultiLineComment contains a line terminator character, then the entire comment is considered to be aLineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment::
MultiLineComment
SingleLineComment
MultiLineComment::
/*MultiLineCommentCharsopt*/
MultiLineCommentChars::
MultiLineNotAsteriskCharMultiLineCommentCharsopt
*PostAsteriskCommentCharsopt
PostAsteriskCommentChars::
MultiLineNotForwardSlashOrAsteriskCharMultiLineCommentCharsopt
*PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar::
SourceCharacterbut not*
MultiLineNotForwardSlashOrAsteriskChar::
SourceCharacterbut not one of/or*
SingleLineComment::
//SingleLineCommentCharsopt
SingleLineCommentChars::
SingleLineCommentCharSingleLineCommentCharsopt
SingleLineCommentChar::
SourceCharacterbut notLineTerminator

7.5 Tokens

Syntax

Token::
IdentifierName
Punctuator
NumericLiteral
StringLiteral

NOTE TheDivPunctuator andRegularExpressionLiteral productions define tokens, but are not included in theToken production.

7.6 Identifier Names and Identifiers

Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section of chapter 5 of the Unicode standard, with some small modifications. AnIdentifier is anIdentifierName that is not aReservedWord (see 7.6.1). The Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard. The characters in the specified categories in version 3.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations.

This standard specifies specific character additions: The dollar sign ($) and the underscore (_) are permitted anywhere in anIdentifierName.

Unicode escape sequences are also permitted in anIdentifierName, where they contribute a single character to theIdentifierName, as computed by the CV of theUnicodeEscapeSequence (see 7.8.4). The\ preceding theUnicodeEscapeSequence does not contribute a character to theIdentifierName. AUnicodeEscapeSequence cannot be used to put a character into anIdentifierName that would otherwise be illegal. In other words, if a\UnicodeEscapeSequence sequence were replaced by itsUnicodeEscapeSequence's CV, the result must still be a validIdentifierName that has the exact same sequence of characters as the originalIdentifierName. All interpretations of identifiers within this specification are based upon their actual characters regardless of whether or not an escape sequence was used to contribute any particular characters.

TwoIdentifierName that are canonically equivalent according to the Unicode standard arenot equal unless they are represented by the exact same sequence of code units (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on IdentifierName values). The intent is that the incoming source text has been converted to normalised form C before it reaches the compiler.

ECMAScript implementations may recognise identifier characters defined in later editions of the Unicode Standard. If portability is a concern, programmers should only employ identifier characters defined in Unicode 3.0.

Syntax

Identifier::
IdentifierNamebut notReservedWord
IdentifierName::
IdentifierStart
IdentifierNameIdentifierPart
IdentifierStart::
UnicodeLetter
$
_
\UnicodeEscapeSequence
IdentifierPart::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
<ZWNJ>
<ZWJ>
UnicodeLetter::
any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.
UnicodeCombiningMark::
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”
UnicodeDigit::
any character in the Unicode category “Decimal number (Nd)”
UnicodeConnectorPunctuation::
any character in the Unicode category “Connector punctuation (Pc)”

The definitions of the nonterminalUnicodeEscapeSequence is given in7.8.4

7.6.1 Reserved Words

A reserved word is anIdentifierName that cannot be used as anIdentifier.

Syntax

ReservedWord::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

7.6.1.1 Keywords

The following tokens are ECMAScript keywords and may not be used asIdentifiers in ECMAScript programs.

Syntax

Keyword::one of
breakdoinstanceoftypeof
caseelsenewvar
catchfinallyreturnvoid
continueforswitchwhile
debuggerfunctionthiswith
defaultifthrow
deleteintry

7.6.1.2 Future Reserved Words

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.

Syntax

FutureReservedWord::one of
classenumextendssuper
constexportimport

The following tokens are also considered to beFutureReservedWords when they occur withinstrict mode code (see 10.1.1). The occurrence of any of these tokens withinstrict mode code in any context where the occurrence of aFutureReservedWord would produce an error must also produce an equivalent error:

implementsletprivatepublicyield
interfacepackageprotectedstatic

7.7 Punctuators

Syntax

Punctuator::one of
{}()[]
.;,<><=
>===!====!==
+-*%++--
<<>>>>>&|^
!~&&||?:
=+=-=*=%=<<=
>>=>>>=&=|=^=
DivPunctuator::one of
//=

7.8 Literals

Syntax

Literal::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral

7.8.1 Null Literals

Syntax

NullLiteral::
null

Semantics

The value of the null literalnull is the sole value of the Null type, namelynull.

7.8.2 Boolean Literals

Syntax

BooleanLiteral::
true
false

Semantics

The value of the Boolean literaltrue is a value of the Boolean type, namelytrue.

The value of the Boolean literalfalse is a value of the Boolean type, namelyfalse.

7.8.3 Numeric Literals

Syntax

NumericLiteral::
DecimalLiteral
HexIntegerLiteral
DecimalLiteral::
DecimalIntegerLiteral.DecimalDigitsoptExponentPartopt
.DecimalDigitsExponentPartopt
DecimalIntegerLiteralExponentPartopt
DecimalIntegerLiteral::
0
NonZeroDigitDecimalDigitsopt
DecimalDigits::
DecimalDigit
DecimalDigitsDecimalDigit
DecimalDigit::one of
0123456789
NonZeroDigit::one of
123456789
ExponentPart::
ExponentIndicatorSignedInteger
ExponentIndicator::one of
eE
SignedInteger::
DecimalDigits
+DecimalDigits
-DecimalDigits
HexIntegerLiteral::
0xHexDigit
0XHexDigit
HexIntegerLiteralHexDigit
HexDigit::one of
0123456789abcdefABCDEF

The source character immediately following aNumericLiteral must not be anIdentifierStart orDecimalDigit.

NOTE For example:

3in

is an error and not the two input elements3 andin.

Semantics

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

  • The MV ofNumericLiteral::DecimalLiteral is the MV ofDecimalLiteral.

  • The MV ofNumericLiteral::HexIntegerLiteral is the MV ofHexIntegerLiteral.

  • The MV ofDecimalLiteral::DecimalIntegerLiteral. is the MV ofDecimalIntegerLiteral.

  • The MV ofDecimalLiteral::DecimalIntegerLiteral.DecimalDigits is the MV ofDecimalIntegerLiteral plus (the MV ofDecimalDigits times 10–n), wheren is the number of characters inDecimalDigits.

  • The MV ofDecimalLiteral::DecimalIntegerLiteral.ExponentPart is the MV ofDecimalIntegerLiteral times 10e, wheree is the MV ofExponentPart.

  • The MV ofDecimalLiteral::DecimalIntegerLiteral.DecimalDigitsExponentPart is (the MV ofDecimalIntegerLiteral plus (the MV ofDecimalDigits times 10–n)) times 10e, wheren is the number of characters inDecimalDigitsand e is the MV ofExponentPart.

  • The MV ofDecimalLiteral::.DecimalDigits is the MV ofDecimalDigits times 10–n, wheren is the number of characters inDecimalDigits.

  • The MV ofDecimalLiteral::.DecimalDigitsExponentPart is the MV ofDecimalDigits times 10e–n, wheren is the number of characters inDecimalDigits ande is the MV ofExponentPart.

  • The MV ofDecimalLiteral::DecimalIntegerLiteral is the MV ofDecimalIntegerLiteral.

  • The MV ofDecimalLiteral::DecimalIntegerLiteralExponentPart is the MV ofDecimalIntegerLiteral times 10e, wheree is the MV ofExponentPart.

  • The MV ofDecimalIntegerLiteral::0 is 0.

  • The MV ofDecimalIntegerLiteral::NonZeroDigit is the MV ofNonZeroDigit.

  • The MV ofDecimalIntegerLiteral::NonZeroDigitDecimalDigits is (the MV ofNonZeroDigit times 10n) plus the MV ofDecimalDigits, wheren is the number of characters inDecimalDigits.

  • The MV ofDecimalDigits::DecimalDigit is the MV ofDecimalDigit.

  • The MV ofDecimalDigits::DecimalDigitsDecimalDigit is (the MV ofDecimalDigits times 10) plus the MV ofDecimalDigit.

  • The MV ofExponentPart::ExponentIndicatorSignedInteger is the MV ofSignedInteger.

  • The MV ofSignedInteger::DecimalDigits is the MV ofDecimalDigits.

  • The MV ofSignedInteger::+DecimalDigits is the MV ofDecimalDigits.

  • The MV ofSignedInteger::-DecimalDigits is the negative of the MV ofDecimalDigits.

  • The MV ofDecimalDigit::0 or ofHexDigit::0 is 0.

  • The MV ofDecimalDigit::1 or ofNonZeroDigit::1 or ofHexDigit::1 is 1.

  • The MV ofDecimalDigit::2 or ofNonZeroDigit::2 or ofHexDigit::2 is 2.

  • The MV ofDecimalDigit::3 or ofNonZeroDigit::3 or ofHexDigit::3 is 3.

  • The MV ofDecimalDigit::4 or ofNonZeroDigit::4 or ofHexDigit::4 is 4.

  • The MV ofDecimalDigit::5 or ofNonZeroDigit::5 or ofHexDigit::5 is 5.

  • The MV ofDecimalDigit::6 or ofNonZeroDigit::6 or ofHexDigit::6 is 6.

  • The MV ofDecimalDigit::7 or ofNonZeroDigit::7 or ofHexDigit::7 is 7.

  • The MV ofDecimalDigit::8 or ofNonZeroDigit::8 or ofHexDigit::8 is 8.

  • The MV ofDecimalDigit::9 or ofNonZeroDigit::9 or ofHexDigit::9 is 9.

  • The MV ofHexDigit::a or ofHexDigit::A is 10.

  • The MV ofHexDigit::b or ofHexDigit::B is 11.

  • The MV ofHexDigit::c or ofHexDigit::C is 12.

  • The MV ofHexDigit::d or ofHexDigit::D is 13.

  • The MV ofHexDigit::e or ofHexDigit::E is 14.

  • The MV ofHexDigit::f or ofHexDigit::F is 15.

  • The MV ofHexIntegerLiteral::0xHexDigit is the MV ofHexDigit.

  • The MV ofHexIntegerLiteral::0XHexDigit is the MV ofHexDigit.

  • The MV ofHexIntegerLiteral::HexIntegerLiteralHexDigit is (the MV ofHexIntegerLiteral times 16) plus the MV ofHexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is+0; otherwise, the rounded value must be the Number value for the MV (as specified in8.5), unless the literal is aDecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a0 digit and then incrementing the literal at the 20th significant digit position. A digit issignificant if it is not part of anExponentPart and

  • it is not0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in theExponentPart, to its right.

A conforming implementation, when processingstrict mode code (see 10.1.1), must not extend the syntax ofNumericLiteral to includeOctalIntegerLiteral as described inB.1.1.

7.8.4 String Literals

A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash, carriage return, line separator, paragraph separator, and line feed. Any character may appear in the form of an escape sequence.

Syntax

StringLiteral::
"DoubleStringCharactersopt"
'SingleStringCharactersopt'
DoubleStringCharacters::
DoubleStringCharacterDoubleStringCharactersopt
SingleStringCharacters::
SingleStringCharacterSingleStringCharactersopt
DoubleStringCharacter::
SourceCharacterbut not one of"or\orLineTerminator
\EscapeSequence
LineContinuation
SingleStringCharacter::
SourceCharacterbut not one of'or\orLineTerminator
\EscapeSequence
LineContinuation
LineContinuation::
\LineTerminatorSequence
EscapeSequence::
CharacterEscapeSequence
0[lookahead ∉DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter::one of
'"\bfnrtv
NonEscapeCharacter::
SourceCharacterbut not one ofEscapeCharacterorLineTerminator
EscapeCharacter::
SingleEscapeCharacter
DecimalDigit
x
u
HexEscapeSequence::
xHexDigitHexDigit
UnicodeEscapeSequence::
uHexDigitHexDigitHexDigitHexDigit

The definition of the nonterminalHexDigit is given in7.8.3.SourceCharacter is defined inclause 6.

Semantics

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of character values (CV) contributed by the various parts of the string literal. As part of this process, some characters within the string literal are interpreted as having a mathematical value (MV), as described below or in7.8.3.

  • The SV ofStringLiteral::"" is the empty character sequence.

  • The SV ofStringLiteral::'' is the empty character sequence.

  • The SV ofStringLiteral::"DoubleStringCharacters" is the SV ofDoubleStringCharacters.

  • The SV ofStringLiteral::'SingleStringCharacters' is the SV ofSingleStringCharacters.

  • The SV ofDoubleStringCharacters::DoubleStringCharacter is a sequence of one character, the CV ofDoubleStringCharacter.

  • The SV ofDoubleStringCharacters::DoubleStringCharacterDoubleStringCharacters is a sequence of the CV ofDoubleStringCharacter followed by all the characters in the SV ofDoubleStringCharacters in order.

  • The SV ofSingleStringCharacters::SingleStringCharacter is a sequence of one character, the CV ofSingleStringCharacter.

  • The SV ofSingleStringCharacters::SingleStringCharacter SingleStringCharacters is a sequence of the CV ofSingleStringCharacter followed by all the characters in the SV ofSingleStringCharacters in order.

  • The SV ofLineContinuation::\LineTerminatorSequence is the empty character sequence.

  • The CV ofDoubleStringCharacter::SourceCharacterbut not one of"or\orLineTerminator is theSourceCharacter character itself.

  • The CV ofDoubleStringCharacter::\EscapeSequence is the CV of theEscapeSequence.

  • The CV ofDoubleStringCharacter::LineContinuation is the empty character sequence.

  • The CV ofSingleStringCharacter::SourceCharacterbut not one of'or\orLineTerminator is theSourceCharacter character itself.

  • The CV ofSingleStringCharacter::\EscapeSequence is the CV of theEscapeSequence.

  • The CV ofSingleStringCharacter::LineContinuation is the empty character sequence.

  • The CV ofEscapeSequence::CharacterEscapeSequence is the CV of theCharacterEscapeSequence.

  • The CV ofEscapeSequence::0[lookahead ∉DecimalDigit] is a <NUL> character (Unicode value 0000).

  • The CV ofEscapeSequence::HexEscapeSequence is the CV of theHexEscapeSequence.

  • The CV ofEscapeSequence::UnicodeEscapeSequence is the CV of theUnicodeEscapeSequence.

  • The CV ofCharacterEscapeSequence::SingleEscapeCharacter is the character whose code unit value is determined by theSingleEscapeCharacter according to Table 4:

Table 4 — String Single Character Escape Sequences
Escape SequenceCode Unit ValueNameSymbol
\b\u0008backspace<BS>
\t\u0009horizontal tab<HT>
\n\u000Aline feed (new line)<LF>
\v\u000Bvertical tab<VT>
\f\u000Cform feed<FF>
\r\u000Dcarriage return<CR>
\"\u0022double quote"
\'\u0027single quote'
\\\u005Cbackslash\

  • The CV ofCharacterEscapeSequence::NonEscapeCharacter is the CV of theNonEscapeCharacter.

  • The CV ofNonEscapeCharacter::SourceCharacterbut not one ofEscapeCharacterorLineTerminator is theSourceCharacter character itself.

  • The CV ofHexEscapeSequence::xHexDigitHexDigit is the character whose code unit value is (16 times the MV of the firstHexDigit) plus the MV of the secondHexDigit.

  • The CV ofUnicodeEscapeSequence::uHexDigitHexDigitHexDigitHexDigit is the character whose code unit value is (4096 times the MV of the firstHexDigit) plus (256 times the MV of the secondHexDigit) plus (16 times the MV of the thirdHexDigit) plus the MV of the fourthHexDigit.

A conforming implementation, when processingstrict mode code (see 10.1.1), may not extend the syntax ofEscapeSequence to includeOctalEscapeSequence as described inB.1.2.

NOTE A line terminator character cannot appear in a string literal, except as part of aLineContinuation to produce the empty character sequence. The correct way to cause a line terminator character to be part of the String value of a string literal is to use an escape sequence such as\n or\u000A.

7.8.5 Regular Expression Literals

A regular expression literal is an input element that is converted to a RegExp object (see 15.10) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as=== to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime bynew RegExp (see 15.10.4) or calling theRegExp constructor as a function (15.10.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The Strings of characters comprising theRegularExpressionBody and theRegularExpressionFlags are passed uninterpreted to the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation may extend the regular expression constructor's grammar, but it must not extend theRegularExpressionBody andRegularExpressionFlags productions or the productions used by these productions.

Syntax

RegularExpressionLiteral::
/RegularExpressionBody/RegularExpressionFlags
RegularExpressionBody::
RegularExpressionFirstCharRegularExpressionChars
RegularExpressionChars::
[empty]
RegularExpressionCharsRegularExpressionChar
RegularExpressionFirstChar::
RegularExpressionNonTerminatorbut not one of*or\or/or[
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar::
RegularExpressionNonTerminatorbut not one of\or/or[
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence::
\RegularExpressionNonTerminator
RegularExpressionNonTerminator::
SourceCharacterbut notLineTerminator
RegularExpressionClass::
[RegularExpressionClassChars]
RegularExpressionClassChars::
[empty]
RegularExpressionClassCharsRegularExpressionClassChar
RegularExpressionClassChar::
RegularExpressionNonTerminatorbut not one of]or\
RegularExpressionBackslashSequence
RegularExpressionFlags::
[empty]
RegularExpressionFlagsIdentifierPart

NOTE Regular expression literals may not be empty; instead of representing an empty regular expression literal, the characters// start a single-line comment. To specify an empty regular expression, use:/(?:)/.

Semantics

A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in constructor RegExp. This value is determined in two steps: first, the characters comprising the regular expression'sRegularExpressionBody andRegularExpressionFlags production expansions are collected uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new object is created as if by the expressionnew RegExp(Pattern, Flags) where RegExp is the standard built-in constructor with that name. The newly constructed object becomes the value of theRegularExpressionLiteral. If the call tonew RegExp would generate an error as specified in15.10.4.1, the error must be treated as an early error (Clause 16).

7.9 Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, variable statement, expression statement,do-while statement,continue statement,break statement,return statement, andthrow statement) must be terminated with semicolons. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

7.9.1 Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

  1. When, as the program is parsed from left to right, a token (called theoffending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
    • The offending token is separated from the previous token by at least oneLineTerminator.
    • The offending token is}.
  2. When, as the program is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScriptProgram, then a semicolon is automatically inserted at the end of the input stream.
  3. When, as the program is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is arestricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation“[noLineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least oneLineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of afor statement (see 12.6.3).

NOTE The following are the only restricted productions in the grammar:

PostfixExpression:
LeftHandSideExpression[noLineTerminator here]++
LeftHandSideExpression[noLineTerminator here]--
ContinueStatement:
continue[noLineTerminator here]Identifier;
BreakStatement:
break[noLineTerminator here]Identifier;
ReturnStatement:
return[noLineTerminator here]Expression;
ThrowStatement:
throw[noLineTerminator here]Expression;

The practical effect of these restricted productions is as follows:

When a++ or-- token is encountered where the parser would treat it as a postfix operator, and at least oneLineTerminator occurred between the preceding token and the++ or-- token, then a semicolon is automatically inserted before the++ or-- token.

When acontinue,break,return, orthrow token is encountered and aLineTerminator is encountered before the next token, a semicolon is automatically inserted after thecontinue,break,return, orthrow token.

The resulting practical advice to ECMAScript programmers is:

A postfix++ or-- operator should appear on the same line as its operand.

AnExpression in areturn orthrow statement should start on the same line as thereturn orthrow token.

AnIdentifier in abreak orcontinue statement should be on the same line as thebreak orcontinue token.

7.9.2 Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of afor statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of afor statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

NOTE The expressiona + b is not treated as a value to be returned by thereturn statement, because aLineTerminator separates it from the tokenreturn.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

NOTE The token++ is not treated as a postfix operator applying to the variableb, because aLineTerminator occurs betweenb and++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before theelse token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

isnot transformed by automatic semicolon insertion, because the parenthesised expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

8 Types

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Number, and Object.

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types areReference,List,Completion,Property Descriptor,Property Identifier,Lexical Environment, andEnvironment Record. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

Within this specification, the notation “Type(x)” is used as shorthand for “the type ofx” where “type” refers to the ECMAScript language and specification types defined in this clause.

8.1 The Undefined Type

The Undefined type has exactly one value, calledundefined. Any variable that has not been assigned a value has the valueundefined.

8.2 The Null Type

The Null type has exactly one value, callednull.

8.3 The Boolean Type

The Boolean type represents a logical entity having two values, calledtrue andfalse.

8.4 The String Type

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values (“elements”). The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a code unit value (see Clause 6). Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at position 0, the next element (if any) at position 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.

NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.

8.5 The Number Type

The Number type has exactly18437736874454810627 (that is,264−253+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the9007199254740990 (that is,253−2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single specialNaN value. (Note that theNaN value is produced by the program expressionNaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

There are two other special values, calledpositive Infinity andnegative Infinity. For brevity, these values are also referred to for expository purposes by the symbols+∞ and−∞, respectively. (Note that these two infinite Number values are produced by the program expressions+Infinity (or simplyInfinity) and-Infinity.)

The other18437736874454810624 (that is,264−253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.

Note that there is both apositive zero and anegative zero. For brevity, these values are also referred to for expository purposes by the symbols+0 and−0, respectively. (Note that these two different zero Number values are produced by the program expressions+0 (or simply0) and-0.)

The18437736874454810622 (that is,264−253−2) finite nonzero values are of two kinds:

18428729675200069632 (that is,264−254) of them are normalised, having the form

s ×m × 2e

wheres is+1 or−1,m is a positive integer less than253 but not less than252, ande is an integer ranging from−1074 to971, inclusive.

The remaining9007199254740990 (that is,253−2) values are denormalised, having the form

s ×m × 2e

wheres is+1 or−1,m is a positive integer less than252, ande is−1074.

Note that all the positive and negative integers whose magnitude is no greater than253 are representable in the Number type (indeed, the integer0 has two representations,+0 and-0).

A finite number has anodd significand if it is nonzero and the integerm used to express it (in one of the two forms shown above) is odd. Otherwise, it has aneven significand.

In this specification, the phrase “the Number value forx” wherex represents an exact nonzero real mathematical quantity (which might even be an irrational number such asπ) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with−0 removed and with two additional values added to it that are not representable in the Number type, namely21024 (which is+1 × 253× 2971) and−21024 (which is−1 × 253× 2971). Choose the member of this set that is closest in value tox. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values21024 and−21024 are considered to have even significands. Finally, if21024 was chosen, replace it with+∞; if−21024 was chosen, replace it with−∞; if+0 was chosen, replace it with−0 if and only ifx is less than zero; any other chosen value is used unchanged. The result is the Number value forx. (This procedure corresponds exactly to the behaviour of the IEEE 754 “round to nearest” mode.)

Some ECMAScript operators deal only with integers in the range−231 through231−1, inclusive, or in the range0 through232−1, inclusive. These operators accept any value of the Number type but first convert each such value to one of232 integer values. See the descriptions of theToInt32 andToUint32 operators in9.5 and9.6, respectively.

8.6 The Object Type

An Object is a collection of properties. Each property is either a named data property, a named accessor property, or an internal property:

There are two kinds of access for named (non-internal) properties:get andput, corresponding to retrieval and assignment, respectively.

8.6.1 Property Attributes

Attributes are used in this specification to define and explain the state of named properties. A named data property associates a name with the attributes listed in Table 5

Table 5 — Attributes of a Named Data Property
Attribute NameValue DomainDescription
[[Value]]Any ECMAScript language typeThe value retrieved by reading the property.
[[Writable]]BooleanIffalse, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] will not succeed.
[[Enumerable]]BooleanIftrue, the property will be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]]BooleanIffalse, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]]) will fail.

A named accessor property associates a name with the attributes listed in Table 6.

Table 6 — Attributes of a Named Accessor Property
Attribute NameValue DomainDescription
[[Get]]Objector UndefinedIf the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an empty arguments list to return the property value each time a get access of the property is performed.
[[Set]]Objector UndefinedIf the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]BooleanIftrue, the property is to be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]]BooleanIffalse, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the value of an attribute is not explicitly specified by this specification for a named property, the default value defined in Table 7 is used.

Table 7 — Default Attribute Values
Attribute NameDefault Value
[[Value]]undefined
[[Get]]undefined
[[Set]]undefined
[[Writable]]false
[[Enumerable]]false
[[Configurable]]false

8.6.2 Object Internal Properties and Methods

This specification uses various internal properties to define the semantics of object values. These internal properties are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not implement the indicated internal property, aTypeError exception is thrown.

The Table 8 summarises the internal properties used by this specification that are applicable to all ECMAScript objects. The Table 9 summarises the internal properties used by this specification that are only applicable to some ECMAScript objects. The descriptions in these tables indicate their behaviour for native ECMAScript objects, unless stated otherwise in this document for particular kinds of native ECMAScript objects. Host objects may support these internal properties with any implementation-dependent behaviour as long as it is consistent with the specific host object restrictions stated in this document.

The “Value Type Domain” columns of the following tables define the types of values associated with internal properties. The type names refer to the types defined inClause 8 augmented by the following additional names. “any” means the value may be any ECMAScript language type. “primitive” means Undefined, Null, Boolean, String, or Number. “SpecOp” means the internal property is an internal method, an implementation provided procedure defined by an abstract operation specification. “SpecOp” is followed by a list of descriptive parameter names. If a parameter name is the same as a type name then the name describes the type of the parameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and the type of the returned value.

Table 8 — Internal Properties Common to All Objects
Internal PropertyValue Type DomainDescription
[[Prototype]]Objector NullThe prototype of this object.
[[Class]]StringA String value indicating a specification defined classification of objects.
[[Extensible]]BooleanIftrue, own properties may be added to the object.
[[Get]]SpecOp(propertyName)→ anyReturns the value of the named property.
[[GetOwnProperty]]

SpecOp (propertyName)→

UndefinedorProperty Descriptor

Returns theProperty Descriptor of the named own property of this object, orundefined if absent.
[[GetProperty]]

SpecOp (propertyName)→

UndefinedorProperty Descriptor

Returns the fully populatedProperty Descriptor of the named property of this object, orundefined if absent.
[[Put]]SpecOp (propertyName,any, Boolean)Sets the specified named property to the value of the second parameter. The flag controls failure handling.
[[CanPut]]SpecOp (propertyName)→ BooleanReturns a Boolean value indicating whether a [[Put]] operation withPropertyName can be performed.
[[HasProperty]]SpecOp (propertyName)→BooleanReturns a Boolean value indicating whether the object already has a property with the given name.
[[Delete]]SpecOp (propertyName, Boolean)→ BooleanRemoves the specified named own property from the object. The flag controls failure handling.
[[DefaultValue]]SpecOp (Hint)→primitiveHint is a String. Returns a default value for the object.
[[DefineOwnProperty]]SpecOp (propertyName, PropertyDescriptor, Boolean)→ BooleanCreates or alters the named own property to have the state described by aProperty Descriptor. The flag controls failure handling.

Every object (including host objects) must implement all of the internal properties listed in Table 8. However, the [[DefaultValue]] internal method may, for some objects, simply throw aTypeError exception.

All objects have an internal property called [[Prototype]]. The value of this property is eithernull or an object and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] internal property must eventually lead to anull value). Named data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for put access. Named accessor properties are inherited for both get access and put access.

Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether or not named properties may be added to the object. If the value of the [[Extensible]] internal property isfalse then additional named properties may not be added to the object. In addition, if [[Extensible]] isfalse the value of the [[Class]] and [[Prototype]] internal properties of the object may not be modified. Once the value of an [[Extensible]] internal property has been set tofalse it may not be subsequently changed totrue.

NOTE This specification defines no ECMAScript language operators or built-in functions that permit a program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of [[Extensible]] fromfalse totrue. Implementation specific extensions that modify [[Class]], [[Prototype]] or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The value of the [[Class]] internal property of a host object may be any String value except one of"Arguments","Array","Boolean","Date","Error","Function","JSON","Math","Number","Object","RegExp", and"String". The value of a [[Class]] internal property is used internally to distinguish different kinds of objects. Note that this specification does not provide any means for a program to access that value except throughObject.prototype.toString (see 15.2.4.2).

Unless otherwise specified, the common internal methods of native ECMAScript objects behave as described in8.12. Array objects have a slightly different implementation of the [[DefineOwnProperty]] internal method (see 15.4.5.1) and String objects have a slightly different implementation of the [[GetOwnProperty]] internal method (see 15.5.5.2). Arguments objects (10.6) have different implementations of [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]]. Function objects (15.3) have a different implementation of [[Get]].

Host objects may implement these internal methods in any manner unless specified otherwise; for example, one possibility is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but [[HasProperty]] always generatesfalse. However, if any specified manipulation of a host object's internal properties is not supported by an implementation, that manipulation must throw aTypeError exception when attempted.

The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for each property of the host object:

  • If a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must betrue even if no mechanism to change the value is exposed via the other internal methods.

  • If a property is described as a data property and its [[Writable]] and [[Configurable]] are bothfalse, then theSameValue (according to 9.12) must be returned for the [[Value]] attribute of the property on all calls to [[GetOwnProperty]].

  • If the attributes other than [[Writable]] may change over time or if the property might disappear, then the [[Configurable]] attribute must betrue.

  • If the [[Writable]] attribute may change fromfalse totrue, then the [[Configurable]] attribute must betrue.

  • If the value of the host object’s [[Extensible]] internal property has been observed by ECMAScript code to befalse, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls must also describe that property as non-existent.

The [[DefineOwnProperty]] internal method of a host object must not permit the addition of a new property to a host object if the [[Extensible]] internal property of that host object has been observed by ECMAScript code to befalse.

If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to befalse then it must not subsequently becometrue.

Table 9 — Internal Properties Only Defined for Some Objects
Internal PropertyValue Type DomainDescription
[[PrimitiveValue]]primitiveInternal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]].
[[Construct]]SpecOp(aList ofany)→ ObjectCreates an object. Invoked via thenew operator. The arguments to the SpecOp are the arguments passed to thenew operator. Objects that implement this internal method are calledconstructors.
[[Call]]SpecOp(any, aList ofany)→any orReferenceExecutes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method arecallable. Only callable objects that are host objects may returnReference values.
[[HasInstance]]SpecOp(any)→ BooleanReturns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]].
[[Scope]]Lexical EnvironmentAlexical environment that defines the environment in which a Function object is executed. Of the standard built-in ECMAScript objects, only Function objects implement [[Scope]].
[[FormalParameters]]List of StringsA possibly emptyList containing the identifier Strings of a Function’sFormalParameterList. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameterList]].
[[Code]]ECMAScript codeThe ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]].
[[TargetFunction]]ObjectThe target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property.
[[BoundThis]]anyThe pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property.
[[BoundArguments]]List ofanyThe pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property.
[[Match]]SpecOp(String,index)→MatchResultTests for a regular expression match and returns a MatchResult value (see 15.10.2.1). Of the standard built-in ECMAScript objects, only RegExp objects implement [[Match]].
[[ParameterMap]]ObjectProvides a mapping between the properties of an arguments object (see 10.6) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property.

8.7 The Reference Specification Type

The Reference type is used to explain the behaviour of such operators asdelete,typeof, and the assignment operators. For example, the left-hand operand of an assignment is expected to produce a reference. The behaviour of assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a user-defined function to return a reference. (Another reason not to use a syntactic case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)

AReference is a resolved name binding. A Reference consists of three components, thebase value, thereferenced name and the Boolean valuedstrict reference flag. The base value is eitherundefined, an Object, a Boolean, a String, a Number, or anenvironment record (10.2.1). A base value ofundefined indicates that the reference could not be resolved to a binding. The referenced name is a String.

The following abstract operations are used in this specification to access the components of references:

The following abstract operations are used in this specification to operate on references:

8.7.1 GetValue (V)

  1. IfType(V) is notReference, returnV.
  2. Letbase be the result of callingGetBase(V).
  3. IfIsUnresolvableReference(V), throw aReferenceError exception.
  4. IfIsPropertyReference(V), then
    1. IfHasPrimitiveBase(V) isfalse, then letget be the [[Get]] internal method ofbase, otherwise letget be the special [[Get]] internal method defined below.
    2. Return the result of calling theget internal method usingbase as itsthis value, and passingGetReferencedName(V) for the argument.
  5. Else,base must be an environment record.
    1. Return the result of calling the GetBindingValue (see 10.2.1) concrete method ofbase passingGetReferencedName(V) andIsStrictReference(V) as arguments.

The following [[Get]] internal method is used by GetValue whenV is a property reference with a primitive base value. It is called usingbase as itsthis value and with propertyP as its argument. The following steps are taken:

  1. LetO beToObject(base).
  2. Letdesc be the result of calling the [[GetProperty]] internal method ofO with property nameP.
  3. Ifdesc isundefined, returnundefined.
  4. IfIsDataDescriptor(desc) istrue, returndesc.[[Value]].
  5. Otherwise,IsAccessorDescriptor(desc) must betrue so, letgetter bedesc.[[Get]] (see 8.10).
  6. Ifgetter isundefined, returnundefined.
  7. Return the result calling the [[Call]] internal method ofgetter providingbase as thethis value and providing no arguments.

NOTE The object that may be created in step 1 is not accessible outside of the above method. An implementation might choose to avoid the actual creation of the object. The only situation where such an actual property access that uses this internal method can have visible effect is when it invokes an accessor function.

8.7.2 PutValue (V, W)

  1. IfType(V) is notReference, throw aReferenceError exception.
  2. Letbase be the result of callingGetBase(V).
  3. IfIsUnresolvableReference(V), then
    1. IfIsStrictReference(V) istrue, then
      1. ThrowReferenceError exception.
    2. Call the [[Put]] internal method of the global object, passingGetReferencedName(V) for the property name,W for the value, andfalse for theThrow flag.
  4. Else ifIsPropertyReference(V), then
    1. IfHasPrimitiveBase(V) isfalse, then letput be the [[Put]] internal method ofbase, otherwise letput be the special [[Put]] internal method defined below.
    2. Call theput internal method usingbase as itsthis value, and passingGetReferencedName(V) for the property name,W for the value, andIsStrictReference(V) for theThrow flag.
  5. Elsebase must be a reference whose base is an environment record. So,
    1. Call the SetMutableBinding (10.2.1) concrete method ofbase, passingGetReferencedName(V),W, andIsStrictReference(V) as arguments.
  6. Return.

The following [[Put]] internal method is used by PutValue whenV is a property reference with a primitive base value. It is called usingbase as itsthis value and with propertyP, valueW, and Boolean flagThrow as arguments. The following steps are taken:

  1. LetO beToObject(base).
  2. If the result of calling the [[CanPut]] internal method ofO with argumentP isfalse, then
    1. IfThrow istrue, then throw aTypeError exception.
    2. Else return.
  3. LetownDesc be the result of calling the [[GetOwnProperty]] internal method ofO with argumentP.
  4. IfIsDataDescriptor(ownDesc) istrue, then
    1. IfThrow istrue, then throw aTypeError exception.
    2. Else return.
  5. Letdesc be the result of calling the [[GetProperty]] internal method ofO with argumentP. This may be either an own or inherited accessor property descriptor or an inherited data property descriptor.
  6. IfIsAccessorDescriptor(desc) istrue, then
    1. Letsetter bedesc.[[Set]] (see 8.10) which cannot beundefined.
    2. Call the [[Call]] internal method ofsetter providingbase as thethis value and an argument list containing onlyW.
  7. Else, this is a request to create an own property on the transient objectO
    1. IfThrow istrue, then throw aTypeError exception.
  8. Return.

NOTE The object that may be created in step 1 is not accessible outside of the above method. An implementation might choose to avoid the actual creation of that transient object. The only situations where such an actual property assignment that uses this internal method can have visible effect are when it either invokes an accessor function or is in violation of aThrow predicated error check. WhenThrow istrue any property assignment that would create a new property on the transient object throws an error.

8.8 The List Specification Type

The List type is used to explain the evaluation of argument lists (see 11.2.4) innew expressions, in function calls, and in other algorithms where a simple list of values is needed. Values of the List type are simply ordered sequences of values. These sequences may be of any length.

8.9 The Completion Specification Type

The Completion type is used to explain the behaviour of statements (break,continue,return andthrow) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type,value,target), wheretype is one ofnormal,break,continue,return, orthrow,value is any ECMAScript language value orempty, andtarget is any ECMAScript identifier orempty. Ifcv is a completion value thencv.type,cv.value, andcv.target may be used to directly refer to its constituent values.

The term “abrupt completion” refers to any completion with atype other thannormal.

8.10 The Property Descriptor and Property Identifier Specification Types

The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in8.6.1. In addition, any field may be present or absent.

Property Descriptor values may be further classified as data property descriptors and accessor property descriptors based upon the existence or use of certain fields. A data property descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor property descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any property descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data property descriptor and an accessor property descriptor; however, it may be neither. A generic property descriptor is a Property Descriptor value that is neither a data property descriptor nor an accessor property descriptor. A fully populated property descriptor is one that is either an accessor property descriptor or a data property descriptor and that has all of the fields that correspond to the property attributes defined in either 8.6.1 Table 5 or Table 6.

For notational convenience within this specification, an object literal-like syntax can be used to define a property descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]:false, [[Configurable]]:true} defines a data property descriptor. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Property Descriptor. For example, if D is a property descriptor then D.[[Value]] is shorthand for “the field of D named [[Value]]”.

The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor value.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

8.10.1 IsAccessorDescriptor ( Desc )

When the abstract operation IsAccessorDescriptor is called with property descriptorDesc, the following steps are taken:

  1. IfDesc isundefined, then returnfalse.
  2. If bothDesc.[[Get]] andDesc.[[Set]] are absent, then returnfalse.
  3. Returntrue.

8.10.2 IsDataDescriptor ( Desc )

When the abstract operation IsDataDescriptor is called with property descriptorDesc, the following steps are taken:

  1. IfDesc isundefined, then returnfalse.
  2. If bothDesc.[[Value]] andDesc.[[Writable]] are absent, then returnfalse.
  3. Returntrue.

8.10.3 IsGenericDescriptor ( Desc )

When the abstract operation IsGenericDescriptor is called with property descriptorDesc, the following steps are taken:

  1. IfDesc isundefined, then returnfalse.
  2. IfIsAccessorDescriptor(Desc) andIsDataDescriptor(Desc) are bothfalse, then returntrue.
  3. Returnfalse.

8.10.4 FromPropertyDescriptor ( Desc )

When the abstract operation FromPropertyDescriptor is called with property descriptorDesc, the following steps are taken:

The following algorithm assumes thatDesc is a fully populatedProperty Descriptor, such as that returned from [[GetOwnProperty]] (see 8.12.1).

  1. IfDesc isundefined, then returnundefined.
  2. Letobj be the result of creating a new object as if by the expressionnew Object() whereObject is the standard built-in constructor with that name.
  3. IfIsDataDescriptor(Desc) istrue, then
    1. Call the [[DefineOwnProperty]] internal method ofobj with arguments "value",Property Descriptor {[[Value]]:Desc.[[Value]], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    2. Call the [[DefineOwnProperty]] internal method ofobj with arguments "writable",Property Descriptor {[[Value]]:Desc.[[Writable]], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  4. Else,IsAccessorDescriptor(Desc) must betrue, so
    1. Call the [[DefineOwnProperty]] internal method ofobj with arguments "get",Property Descriptor {[[Value]]:Desc.[[Get]], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    2. Call the [[DefineOwnProperty]] internal method ofobj with arguments "set",Property Descriptor {[[Value]]:Desc.[[Set]], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  5. Call the [[DefineOwnProperty]] internal method ofobj with arguments "enumerable",Property Descriptor {[[Value]]:Desc.[[Enumerable]], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  6. Call the [[DefineOwnProperty]] internal method ofobj with arguments "configurable",Property Descriptor {[[Value]]:Desc.[[Configurable]], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  7. Returnobj.

8.10.5 ToPropertyDescriptor ( Obj )

When the abstract operation ToPropertyDescriptor is called with objectObj, the following steps are taken:

  1. IfType(Obj) is not Object throw aTypeError exception.
  2. Letdesc be the result of creating a newProperty Descriptor that initially has no fields.
  3. If the result of calling the [[HasProperty]] internal method ofObj with argument "enumerable" istrue, then
    1. Letenum be the result of calling the [[Get]] internal method ofObj with "enumerable".
    2. Set the [[Enumerable]] field ofdesc toToBoolean(enum).
  4. If the result of calling the [[HasProperty]] internal method ofObj with argument "configurable" istrue, then
    1. Letconf be the result of calling the [[Get]] internal method ofObj with argument "configurable".
    2. Set the [[Configurable]] field ofdesc toToBoolean(conf).
  5. If the result of calling the [[HasProperty]] internal method ofObj with argument "value" istrue, then
    1. Letvalue be the result of calling the [[Get]] internal method ofObj with argument “value”.
    2. Set the [[Value]] field ofdesc tovalue.
  6. If the result of calling the [[HasProperty]] internal method ofObj with argument "writable" istrue, then
    1. Letwritable be the result of calling the [[Get]] internal method ofObj with argument "writable".
    2. Set the [[Writable]] field ofdesc toToBoolean(writable).
  7. If the result of calling the [[HasProperty]] internal method ofObj with argument "get" istrue, then
    1. Letgetter be the result of calling the [[Get]] internal method ofObj with argument "get".
    2. IfIsCallable(getter) isfalse andgetter is notundefined, then throw aTypeError exception.
    3. Set the [[Get]] field ofdesc togetter.
  8. If the result of calling the [[HasProperty]] internal method ofObj with argument "set" istrue, then
    1. Letsetter be the result of calling the [[Get]] internal method ofObj with argument "set".
    2. IfIsCallable(setter) isfalse andsetter is notundefined, then throw aTypeError exception.
    3. Set the [[Set]] field ofdesc tosetter.
  9. If eitherdesc.[[Get]] ordesc.[[Set]] are present, then
    1. If eitherdesc.[[Value]] ordesc.[[Writable]] are present, then throw aTypeError exception.
  10. Returndesc.

8.11 The Lexical Environment and Environment Record Specification Types

TheLexical Environment andEnvironment Record types are used to explain the behaviour of name resolution in nested functions and blocks. These types and the operations upon them are defined inClause 10.

8.12 Algorithms for Object Internal Methods

In the following algorithm descriptions, assumeO is a native ECMAScript object,P is a String,Desc is a Property Description record, andThrow is a Boolean flag.

8.12.1 [[GetOwnProperty]] (P)

When the [[GetOwnProperty]] internal method ofO is called with property nameP, the following steps are taken:

  1. IfO doesn’t have an own property with nameP, returnundefined.
  2. LetD be a newly createdProperty Descriptor with no fields.
  3. LetX beO’s own property namedP.
  4. IfX is a data property, then
    1. SetD.[[Value]] to the value ofX’s [[Value]] attribute.
    2. SetD.[[Writable]] to the value ofX’s [[Writable]] attribute
  5. ElseX is an accessor property, so
    1. SetD.[[Get]] to the value ofX’s [[Get]] attribute.
    2. SetD.[[Set]] to the value ofX’s [[Set]] attribute.
  6. SetD.[[Enumerable]] to the value ofX’s [[Enumerable]] attribute.
  7. SetD.[[Configurable]] to the value ofX’s [[Configurable]] attribute.
  8. ReturnD.

However, ifO is a String object it has a more elaborate [[GetOwnProperty]] internal method defined in15.5.5.2.

8.12.2 [[GetProperty]] (P)

When the [[GetProperty]] internal method ofO is called with property nameP, the following steps are taken:

  1. Letprop be the result of calling the [[GetOwnProperty]] internal method ofO with property nameP.
  2. Ifprop is notundefined, returnprop.
  3. Letproto be the value of the [[Prototype]] internal property ofO.
  4. Ifproto isnull, returnundefined.
  5. Return the result of calling the [[GetProperty]] internal method ofproto with argumentP.

8.12.3 [[Get]] (P)

When the [[Get]] internal method ofO is called with property nameP, the following steps are taken:

  1. Letdesc be the result of calling the [[GetProperty]] internal method ofO with property nameP.
  2. Ifdesc isundefined, returnundefined.
  3. IfIsDataDescriptor(desc) istrue, returndesc.[[Value]].
  4. Otherwise,IsAccessorDescriptor(desc) must be true so, letgetter bedesc.[[Get]].
  5. Ifgetter isundefined, returnundefined.
  6. Return the result calling the [[Call]] internal method ofgetter providingO as thethis value and providing no arguments.

8.12.4 [[CanPut]] (P)

When the [[CanPut]] internal method ofO is called with property nameP, the following steps are taken:

  1. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO with argumentP.
  2. Ifdesc is notundefined, then
    1. IfIsAccessorDescriptor(desc) istrue, then
      1. Ifdesc.[[Set]] isundefined, then returnfalse.
      2. Else returntrue.
    2. Else,desc must be a DataDescriptor so return the value ofdesc.[[Writable]].
  3. Letproto be the [[Prototype]] internal property ofO.
  4. Ifproto isnull, then return the value of the [[Extensible]] internal property ofO.
  5. Letinherited be the result of calling the [[GetProperty]] internal method ofproto with property nameP.
  6. Ifinherited isundefined, return the value of the [[Extensible]] internal property ofO.
  7. IfIsAccessorDescriptor(inherited) istrue, then
    1. Ifinherited.[[Set]] isundefined, then returnfalse.
    2. Else returntrue.
  8. Else,inherited must be a DataDescriptor
    1. If the [[Extensible]] internal property ofO isfalse, returnfalse.
    2. Else return the value ofinherited.[[Writable]].

Host objects may define additional constraints upon [[Put]] operations. If possible, host objects should not allow [[Put]] operations in situations where this definition of [[CanPut]] returns false.

8.12.5 [[Put]] ( P, V, Throw )

When the [[Put]] internal method ofO is called with propertyP, valueV, and Boolean flagThrow, the following steps are taken:

  1. If the result of calling the [[CanPut]] internal method ofO with argumentP isfalse, then
    1. IfThrow istrue, then throw aTypeError exception.
    2. Else return.
  2. LetownDesc be the result of calling the [[GetOwnProperty]] internal method ofO with argumentP.
  3. IfIsDataDescriptor(ownDesc) istrue, then
    1. LetvalueDesc be theProperty Descriptor {[[Value]]:V}.
    2. Call the [[DefineOwnProperty]] internal method ofO passingP,valueDesc, andThrow as arguments.
    3. Return.
  4. Letdesc be the result of calling the [[GetProperty]] internal method ofO with argumentP. This may be either an own or inherited accessor property descriptor or an inherited data property descriptor.
  5. IfIsAccessorDescriptor(desc) istrue, then
    1. Letsetter bedesc.[[Set]] which cannot beundefined.
    2. Call the [[Call]] internal method ofsetter providingO as thethis value and providingV as the sole argument.
  6. Else, create a named data property namedP on objectO as follows
    1. LetnewDesc be theProperty Descriptor
      {[[Value]]:V, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.
    2. Call the [[DefineOwnProperty]] internal method ofO passingP,newDesc, andThrow as arguments.
  7. Return.

8.12.6 [[HasProperty]] (P)

When the [[HasProperty]] internal method ofO is called with property nameP, the following steps are taken:

  1. Letdesc be the result of calling the [[GetProperty]] internal method ofO with property nameP.
  2. Ifdesc isundefined, then returnfalse.
  3. Else returntrue.

8.12.7 [[Delete]] (P, Throw)

When the [[Delete]] internal method ofO is called with property nameP and the Boolean flagThrow, the following steps are taken:

  1. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO with property nameP.
  2. Ifdesc isundefined, then returntrue.
  3. Ifdesc.[[Configurable]] istrue, then
    1. Remove the own property with nameP fromO.
    2. Returntrue.
  4. Else ifThrow, then throw aTypeError exception.
  5. Returnfalse.

8.12.8 [[DefaultValue]] (hint)

When the [[DefaultValue]] internal method ofO is called with hint String, the following steps are taken:

  1. LettoString be the result of calling the [[Get]] internal method of objectO with argument "toString".
  2. IfIsCallable(toString) istrue then,
    1. Letstr be the result of calling the [[Call]] internal method oftoString, withO as thethis value and an empty argument list.
    2. Ifstr is a primitive value, returnstr.
  3. LetvalueOf be the result of calling the [[Get]] internal method of objectO with argument "valueOf".
  4. IfIsCallable(valueOf) istrue then,
    1. Letval be the result of calling the [[Call]] internal method ofvalueOf, withO as the this value and an empty argument list.
    2. Ifval is a primitive value, returnval.
  5. Throw aTypeError exception.

When the [[DefaultValue]] internal method ofO is called with hint Number, the following steps are taken:

  1. LetvalueOf be the result of calling the [[Get]] internal method of objectO with argument "valueOf".
  2. IfIsCallable(valueOf) istrue then,
    1. Letval be the result of calling the [[Call]] internal method ofvalueOf, withO as thethis value and an empty argument list.
    2. Ifval is a primitive value, returnval.
  3. LettoString be the result of calling the [[Get]] internal method of objectO with argument "toString".
  4. IfIsCallable(toString) istrue then,
    1. Letstr be the result of calling the [[Call]] internal method oftoString, withO as the this value and an empty argument list.
    2. Ifstr is a primitive value, returnstr.
  5. Throw aTypeError exception.

When the [[DefaultValue]] internal method ofO is called with no hint, then it behaves as if the hint were Number, unlessO is a Date object (see 15.9.6), in which case it behaves as if the hint were String.

The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object implements its own [[DefaultValue]] internal method, it must ensure that its [[DefaultValue]] internal method can return only primitive values.

8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)

In the following algorithm, the term “Reject” means “IfThrow istrue, then throw aTypeError exception, otherwise returnfalse”. The algorithm contains steps that test various fields of theProperty DescriptorDesc for specific values. The fields that are tested in this manner need not actually exist inDesc. If a field is absent then its value is considered to befalse.

When the [[DefineOwnProperty]] internal method ofO is called with property nameP, property descriptorDesc, and Boolean flagThrow, the following steps are taken:

  1. Letcurrent be the result of calling the [[GetOwnProperty]] internal method ofO with property nameP.
  2. Letextensible be the value of the [[Extensible]] internal property ofO.
  3. Ifcurrent isundefined andextensible isfalse, then Reject.
  4. Ifcurrent isundefined andextensible istrue, then
    1. IfIsGenericDescriptor(Desc) orIsDataDescriptor(Desc) istrue, then
      1. Create an own data property namedP of objectO whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described byDesc. If the value of an attribute field ofDesc is absent, the attribute of the newly created property is set to its default value.
    2. Else,Desc must be an accessorProperty Descriptor so,
      1. Create an own accessor property namedP of objectO whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described byDesc. If the value of an attribute field ofDesc is absent, the attribute of the newly created property is set to its default value.
    3. Returntrue.
  5. Returntrue, if every field inDesc is absent.
  6. Returntrue, if every field inDesc also occurs incurrent and the value of every field inDesc is the same value as the corresponding field incurrent when compared usingthe SameValue algorithm (9.12).
  7. If the [[Configurable]] field ofcurrent isfalse then
    1. Reject, if the [[Configurable]] field ofDesc istrue.
    2. Reject, if the [[Enumerable]] field ofDesc is present and the [[Enumerable]] fields ofcurrent andDesc are the Boolean negation of each other.
  8. IfIsGenericDescriptor(Desc) istrue, then no further validation is required.
  9. Else, ifIsDataDescriptor(current) andIsDataDescriptor(Desc) have different results, then
    1. Reject, if the [[Configurable]] field ofcurrent isfalse.
    2. IfIsDataDescriptor(current) istrue, then
      1. Convert the property namedP of objectO from a data property to an accessor property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
    3. Else,
      1. Convert the property namedP of objectO from an accessor property to a data property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
  10. Else, ifIsDataDescriptor(current) andIsDataDescriptor(Desc) are bothtrue, then
    1. If the [[Configurable]] field ofcurrent isfalse, then
      1. Reject, if the [[Writable]] field ofcurrent isfalse and the [[Writable]] field ofDesc istrue.
      2. If the [[Writable]] field ofcurrent isfalse, then
        1. Reject, if the [[Value]] field ofDesc is present andSameValue(Desc.[[Value]],current.[[Value]]) isfalse.
    2. else, the [[Configurable]] field ofcurrent istrue, so any change is acceptable.
  11. Else,IsAccessorDescriptor(current) andIsAccessorDescriptor(Desc) are bothtrue so,
    1. If the [[Configurable]] field ofcurrent isfalse, then
      1. Reject, if the [[Set]] field ofDesc is present andSameValue(Desc.[[Set]],current.[[Set]]) isfalse.
      2. Reject, if the [[Get]] field ofDesc is present andSameValue(Desc.[[Get]],current.[[Get]]) isfalse.
  12. For each attribute field ofDesc that is present, set the correspondingly named attribute of the property namedP of objectO to the value of the field.
  13. Returntrue.

However, ifO is an Array object, it has a more elaborate [[DefineOwnProperty]] internal method defined in15.4.5.1.

NOTE Step 10.b allows any field of Desc to be different from the corresponding field of current if current’s [[Configurable]] field istrue. This even permits changing the [[Value]] of a property whose [[Writable]] attribute isfalse. This is allowed because atrue [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] is first set totrue, a new [[Value]] is set, and then [[Writable]] is set tofalse.

9 Type Conversion and Testing

The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. These abstract operations are not a part of the language; they are defined here to aid the specification of the semantics of the language. The conversion abstract operations are polymorphic; that is, they can accept a value of any ECMAScript language type, but not of specification types.

9.1 ToPrimitive

The abstract operation ToPrimitive takes aninput argument and an optional argumentPreferredType. The abstract operation ToPrimitive converts itsinput argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hintPreferredType to favour that type. Conversion occurs according to Table 10:

Table 10 — ToPrimitive Conversions
Input TypeResult
UndefinedThe result equals theinput argument (no conversion).
NullThe result equals theinput argument (no conversion).
BooleanThe result equals theinput argument (no conversion).
NumberThe result equals theinput argument (no conversion).
StringThe result equals theinput argument (no conversion).
ObjectReturn a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hintPreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in8.12.8.

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

Table 11 — ToBoolean Conversions
Argument TypeResult
Undefinedfalse
Nullfalse
BooleanThe result equals the input argument (no conversion).
NumberThe result isfalse if the argument is+0,−0, orNaN; otherwise the result istrue.
StringThe result isfalse if the argument is the empty String (its length is zero); otherwise the result istrue.
Objecttrue

9.3 ToNumber

The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:

Table 12 — To Number Conversions
Argument TypeResult
UndefinedNaN
Null+0
BooleanThe result is1 if the argument istrue. The result is+0 if the argument isfalse.
NumberThe result equals the input argument (no conversion).
StringSee grammar and note below.
Object

Apply the following steps:

  1. LetprimValue beToPrimitive(input argument, hint Number).
  2. Return ToNumber(primValue).

9.3.1 ToNumber Applied to the String Type

ToNumber applied to Strings applies the following grammar to the input String. If the grammar cannot interpret the String as an expansion ofStringNumericLiteral, then the result ofToNumber isNaN.

Syntax

StringNumericLiteral:::
StrWhiteSpaceopt
StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt
StrWhiteSpace:::
StrWhiteSpaceCharStrWhiteSpaceopt
StrWhiteSpaceChar:::
WhiteSpace
LineTerminator
StrNumericLiteral:::
StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral:::
StrUnsignedDecimalLiteral
+StrUnsignedDecimalLiteral
-StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral:::
Infinity
DecimalDigits.DecimalDigitsoptExponentPartopt
.DecimalDigitsExponentPartopt
DecimalDigitsExponentPartopt
DecimalDigits:::
DecimalDigit
DecimalDigitsDecimalDigit
DecimalDigit:::one of
0123456789
ExponentPart:::
ExponentIndicatorSignedInteger
ExponentIndicator:::one of
eE
SignedInteger:::
DecimalDigits
+DecimalDigits
-DecimalDigits
HexIntegerLiteral:::
0xHexDigit
0XHexDigit
HexIntegerLiteralHexDigit
HexDigit:::one of
0123456789abcdefABCDEF

Some differences should be noted between the syntax of aStringNumericLiteral and aNumericLiteral (see 7.8.3):

  • AStringNumericLiteral may be preceded and/or followed by white space and/or line terminators.

  • AStringNumericLiteral that is decimal may have any number of leading0 digits.

  • AStringNumericLiteral that is decimal may be preceded by+ or- to indicate its sign.

  • AStringNumericLiteral that is empty or contains only white space is converted to+0.

The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 7.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below.

  • The MV ofStringNumericLiteral:::[empty] is 0.

  • The MV ofStringNumericLiteral:::StrWhiteSpace is 0.

  • The MV ofStringNumericLiteral:::StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt is the MV ofStrNumericLiteral, no matter whether white space is present or not.

  • The MV ofStrNumericLiteral:::StrDecimalLiteral is the MV ofStrDecimalLiteral.

  • The MV ofStrNumericLiteral:::HexIntegerLiteral is the MV ofHexIntegerLiteral.

  • The MV ofStrDecimalLiteral:::StrUnsignedDecimalLiteral is the MV ofStrUnsignedDecimalLiteral.

  • The MV ofStrDecimalLiteral:::+StrUnsignedDecimalLiteral is the MV ofStrUnsignedDecimalLiteral.

  • The MV ofStrDecimalLiteral:::-StrUnsignedDecimalLiteral is the negative of the MV ofStrUnsignedDecimalLiteral. (Note that if the MV ofStrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless mathematical zero to a floating-point+0 or−0 as appropriate.)

  • The MV ofStrUnsignedDecimalLiteral:::Infinity is1010000 (a value so large that it will round to+∞).

  • The MV ofStrUnsignedDecimalLiteral:::DecimalDigits. is the MV ofDecimalDigits.

  • The MV ofStrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigits is the MV of the firstDecimalDigits plus (the MV of the secondDecimalDigits times10−n), wheren is the number of characters in the secondDecimalDigits.

  • The MV ofStrUnsignedDecimalLiteral:::DecimalDigits.ExponentPart is the MV ofDecimalDigits times 10e, wheree is the MV ofExponentPart.

  • The MV ofStrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigitsExponentPart is (the MV of the firstDecimalDigits plus (the MV of the secondDecimalDigits times 10−n)) times 10e, wheren is the number of characters in the secondDecimalDigits ande is the MV ofExponentPart.

  • The MV ofStrUnsignedDecimalLiteral:::.DecimalDigits is the MV ofDecimalDigits times 10−n, wheren is the number of characters inDecimalDigits.

  • The MV ofStrUnsignedDecimalLiteral:::.DecimalDigitsExponentPart is the MV ofDecimalDigits times 10e−n, wheren is the number of characters inDecimalDigits ande is the MV ofExponentPart.

  • The MV ofStrUnsignedDecimalLiteral:::DecimalDigits is the MV ofDecimalDigits.

  • The MV ofStrUnsignedDecimalLiteral:::DecimalDigitsExponentPart is the MV ofDecimalDigits times 10e, wheree is the MV ofExponentPart.

  • The MV ofDecimalDigits:::DecimalDigit is the MV ofDecimalDigit.

  • The MV ofDecimalDigits:::DecimalDigitsDecimalDigit is (the MV ofDecimalDigits times 10) plus the MV ofDecimalDigit.

  • The MV ofExponentPart:::ExponentIndicatorSignedInteger is the MV ofSignedInteger.

  • The MV ofSignedInteger:::DecimalDigits is the MV ofDecimalDigits.

  • The MV ofSignedInteger:::+DecimalDigits is the MV ofDecimalDigits.

  • The MV ofSignedInteger:::-DecimalDigits is the negative of the MV ofDecimalDigits.

  • The MV ofDecimalDigit:::0 or ofHexDigit:::0 is 0.

  • The MV ofDecimalDigit:::1 or ofHexDigit:::1 is 1.

  • The MV ofDecimalDigit:::2 or ofHexDigit:::2 is 2.

  • The MV ofDecimalDigit:::3 or ofHexDigit:::3 is 3.

  • The MV ofDecimalDigit:::4 or ofHexDigit:::4 is 4.

  • The MV ofDecimalDigit:::5 or ofHexDigit:::5 is 5.

  • The MV ofDecimalDigit:::6 or ofHexDigit:::6 is 6.

  • The MV ofDecimalDigit:::7 or ofHexDigit:::7 is 7.

  • The MV ofDecimalDigit:::8 or ofHexDigit:::8 is 8.

  • The MV ofDecimalDigit:::9 or ofHexDigit:::9 is 9.

  • The MV ofHexDigit:::a or ofHexDigit:::A is 10.

  • The MV ofHexDigit:::b or ofHexDigit:::B is 11.

  • The MV ofHexDigit:::c or ofHexDigit:::C is 12.

  • The MV ofHexDigit:::d or ofHexDigit:::D is 13.

  • The MV ofHexDigit:::e or ofHexDigit:::E is 14.

  • The MV ofHexDigit:::f or ofHexDigit:::F is 15.

  • The MV ofHexIntegerLiteral:::0xHexDigit is the MV ofHexDigit.

  • The MV ofHexIntegerLiteral:::0XHexDigit is the MV ofHexDigit.

  • The MV ofHexIntegerLiteral:::HexIntegerLiteralHexDigit is (the MV ofHexIntegerLiteral times 16) plus the MV ofHexDigit.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space character in the String numeric literal is ‘-’, in which case the rounded value is −0. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in8.5), unless the literal includes aStrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th digit position. A digit issignificant if it is not part of anExponentPart and

  • it is not0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in theExponentPart, to its right.

9.4 ToInteger

The abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as follows:

  1. Letnumber be the result of callingToNumber on the input argument.
  2. Ifnumber isNaN, return+0.
  3. Ifnumber is+0,−0,+∞, or−∞, returnnumber.
  4. Return the result of computingsign(number) ×floor(abs(number)).

9.5 ToInt32: (Signed 32 Bit Integer)

The abstract operation ToInt32 converts its argument to one of232 integer values in the range−231 through231−1, inclusive. This abstract operation functions as follows:

  1. Letnumber be the result of callingToNumber on the input argument.
  2. Ifnumber isNaN,+0,−0,+∞, or−∞, return+0.
  3. LetposInt besign(number) *floor(abs(number)).
  4. Letint32bit beposIntmodulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such that the mathematical difference ofposInt and k is mathematically an integer multiple of 232.
  5. Ifint32bit is greater than or equal to 231, returnint32bit − 232, otherwise returnint32bit.

NOTE Given the above definition of ToInt32:

  • The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

  • ToInt32(ToUint32(x)) is equal toToInt32(x) for all values ofx. (It is to preserve this latter property that +∞ and −∞ are mapped to+0.)

  • ToInt32 maps−0 to+0.

9.6 ToUint32: (Unsigned 32 Bit Integer)

The abstract operation ToUint32 converts its argument to one of232 integer values in the range0 through232−1, inclusive. This abstraction operation functions as follows:

  1. Letnumber be the result of callingToNumber on the input argument.
  2. Ifnumber isNaN, +0, −0, +∞, or −∞, return+0.
  3. LetposInt besign(number) ×floor(abs(number)).
  4. Letint32bit beposIntmodulo 232; that is, a finite integer value k of Number type with positive sign and less than 232 in magnitude such that the mathematical difference ofposInt and k is mathematically an integer multiple of 232.
  5. Returnint32bit.

NOTE Given the above definition of ToUInt32:

  • Step 5 is the only difference between ToUint32 andToInt32.

  • The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

  • ToUint32(ToInt32(x)) is equal toToUint32(x) for all values ofx. (It is to preserve this latter property that+∞ and−∞ are mapped to+0.)

  • ToUint32 maps−0 to+0.

9.7 ToUint16: (Unsigned 16 Bit Integer)

The abstract operation ToUint16 converts its argument to one of216 integer values in the range0 through216−1, inclusive. This abstract operation functions as follows:

  1. Letnumber be the result of callingToNumber on the input argument.
  2. Ifnumber isNaN, +0, −0, +∞, or −∞, return+0.
  3. LetposInt besign(number) ×floor(abs(number)).
  4. Letint16bit beposIntmodulo 216; that is, a finite integer valuek of Number type with positive sign and less than 216 in magnitude such that the mathematical difference ofposInt andk is mathematically an integer multiple of 216.
  5. Returnint16bit.

NOTE Given the above definition of ToUint16:

  • The substitution of216 for232 in step 4 is the only difference betweenToUint32 and ToUint16.
  • ToUint16 maps−0 to+0.

9.8 ToString

The abstract operation ToString converts its argument to a value of type String according to Table 13:

Table 13 — ToString Conversions
Argument TypeResult
Undefined"undefined"
Null"null"
Boolean

If the argument istrue, then the result is"true".

If the argument isfalse, then the result is"false".

NumberSee9.8.1.
StringReturn the input argument (no conversion)
Object

Apply the following steps:

1. LetprimValue beToPrimitive(input argument, hint String).

2. Return ToString(primValue).

9.8.1 ToString Applied to the Number Type

The abstract operationToString converts a Numberm to String format as follows:

  1. Ifm isNaN, return the String"NaN".
  2. Ifm is+0 or−0, return the String"0".
  3. Ifm is less than zero, return the String concatenation of the String"-" andToString(−m).
  4. Ifm is infinity, return the String"Infinity".
  5. Otherwise, letn,k, ands be integers such thatk ≥ 1, 10k−1 ≤s < 10k, the Number value fors × 10n−k ism, andk is as small as possible. Note thatk is the number of digits in the decimal representation ofs, thats is not divisible by 10, and that the least significant digit ofs is not necessarily uniquely determined by these criteria.
  6. Ifk ≤n ≤ 21, return the String consisting of thek digits of the decimal representation of s (in order, with no leading zeroes), followed byn−k occurrences of the character ‘0’.
  7. If 0 < n ≤ 21, return the String consisting of the most significantn digits of the decimal representation ofs, followed by a decimal point ‘.’, followed by the remainingk−n digits of the decimal representation ofs.
  8. If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a decimal point ‘.’, followed by −n occurrences of the character ‘0’, followed by thek digits of the decimal representation ofs.
  9. Otherwise, ifk = 1, return the String consisting of the single digit ofs, followed by lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whethern−1 is positive or negative, followed by the decimal representation of the integerabs(n−1) (with no leading zeroes).
  10. Return the String consisting of the most significant digit of the decimal representation ofs, followed by a decimal point ‘.’, followed by the remainingk−1 digits of the decimal representation ofs, followed by the lowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whethern−1 is positive or negative, followed by the decimal representation of the integerabs(n−1) (with no leading zeroes).

NOTE 1 The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:

  • If x is any Number value other than−0, thenToNumber(ToString(x)) is exactly the same Number value as x.

  • The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

NOTE 2 For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:

Otherwise, letn,k, ands be integers such thatk ≥ 1, 10k−1 ≤s < 10k, the Number value fors × 10n−k ism, andk is as small as possible. If there are multiple possibilities fors, choose the value ofs for whichs × 10n−k is closest in value tom. If there are two such possible values ofs, choose the one that is even. Note thatk is the number of digits in the decimal representation ofs and thats is not divisible by 10.

NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as
http://cm.bell-labs.com/netlib/fp/dtoa.c.gz and as
http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz and may also be found at the variousnetlib mirror sites.

9.9 ToObject

The abstract operation ToObject converts its argument to a value of type Object according to Table 14:

Table 14 — ToObject
Argument TypeResult
UndefinedThrow aTypeError exception.
NullThrow aTypeError exception.
BooleanCreate a new Boolean object whose [[PrimitiveValue]] internal property is set to the value of the argument. See15.6 for a description of Boolean objects.
NumberCreate a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See15.7 for a description of Number objects.
StringCreate a new String object whose [[PrimitiveValue]] internal property is set to the value of the argument. See15.5 for a description of String objects.
ObjectThe result is the input argument (no conversion).

9.10 CheckObjectCoercible

The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an Object usingToObject. It is defined by Table 15:

Table 15 — CheckObjectCoercible Results
Argument TypeResult
UndefinedThrow aTypeError exception.
NullThrow aTypeError exception.
BooleanReturn
NumberReturn
StringReturn
ObjectReturn

9.11 IsCallable

The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value, is a callable function Object according to Table 16:

Table 16 — IsCallable Results
Argument TypeResult
UndefinedReturnfalse.
NullReturnfalse.
BooleanReturnfalse.
NumberReturnfalse.
StringReturnfalse.
ObjectIf the argument object has a [[Call]] internal method, then returntrue, otherwise returnfalse.

9.12 The SameValue Algorithm

The internal comparison abstract operation SameValue(x,y), wherex andy are ECMAScript language values, producestrue orfalse. Such a comparison is performed as follows:

  1. IfType(x) is different fromType(y), returnfalse.
  2. IfType(x) is Undefined, returntrue.
  3. IfType(x) is Null, returntrue.
  4. IfType(x) is Number, then.
    1. Ifx is NaN andy is NaN, returntrue.
    2. Ifx is +0 andy is -0, returnfalse.
    3. Ifx is -0 andy is +0, returnfalse.
    4. Ifx is the same Number value asy, returntrue.
    5. Returnfalse.
  5. IfType(x) is String, then returntrue ifx andy are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, returnfalse.
  6. IfType(x) is Boolean, returntrue ifx andy are bothtrue or bothfalse; otherwise, returnfalse.
  7. Return true ifx andy refer to the same object. Otherwise, returnfalse.

10 Executable Code and Execution Contexts

10.1 Types of Executable Code

There are three types of ECMAScript executable code:

10.1.1 Strict Mode Code

An ECMAScriptProgram syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. When processed using strict mode the three types of ECMAScript code are referred to as strict global code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following situations:

10.2 Lexical Environments

ALexical Environment is a specification type used to define the association ofIdentifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of anEnvironment Record and a possibly null reference to anouter Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as aFunctionDeclaration, aWithStatement, or aCatch clause of aTryStatement and a new Lexical Environment is created each time such code is evaluated.

AnEnvironment Record records the identifier bindings that are created within the scope of its associated Lexical Environment.

The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical Environments. For example, if aFunctionDeclaration contains two nestedFunctionDeclarations then the Lexical Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the current execution of the surrounding function.

Lexical Environments andEnvironment Record values are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

10.2.1 Environment Records

There are two kinds of Environment Record values used in this specification:declarative environment records andobject environment records. Declarative environment records are used to define the effect of ECMAScript language syntactic elements such asFunctionDeclarations,VariableDeclarations, andCatch clauses that directly associate identifier bindings with ECMAScript language values. Object environment records are used to define the effect of ECMAScript elements such asProgram andWithStatement that associate identifier bindings with the properties of some object.

For specification purposes Environment Record values can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with two concrete subclasses, declarative environment record and object environment record. The abstract class includes the abstract specification methods defined in Table 17. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 17 — Abstract Methods of Environment Records
MethodPurpose
HasBinding(N)Determine if an environment record has a binding for an identifier. Returntrue if it does andfalse if it does not. The String valueN is the text of the identifier.
CreateMutableBinding(N, D)Create a new mutable binding in an environment record. The String valueN is the text of the bound name. If the optional Boolean argumentD istrue the binding is may be subsequently deleted.
SetMutableBinding(N,V, S)Set the value of an already existing mutable binding in an environment record. The String valueN is the text of the bound name.V is the value for the binding and may be a value of any ECMAScript language type.S is a Boolean flag. IfS istrue and the binding cannot be set throw aTypeError exception.S is used to identify strict mode references.
GetBindingValue(N,S)Returns the value of an already existing binding from an environment record. The String valueN is the text of the bound name.S is used to identify strict mode references. IfS istrue and the binding does not exist or is uninitialised throw aReferenceError exception.
DeleteBinding(N)Delete a binding from an environment record. The String valueN is the text of the bound name If a binding forN exists, remove the binding and returntrue. If the binding exists but cannot be removed returnfalse. If the binding does not exist returntrue.
ImplicitThisValue()Returns the value to use as thethis value on calls to function objects that are obtained as binding values from this environment record.

10.2.1.1 Declarative Environment Records

Eachdeclarative environment record is associated with an ECMAScript program scope containing variable and/or function declarations. Adeclarative environment record binds the set of identifiers defined by the declarations contained within its scope.

In addition to the mutable bindings supported by all Environment Records, declarative environment records also provide for immutable bindings. An immutable binding is one where the association between an identifier and a value may not be modified once it has been established. Creation and initialisation of immutable binding are distinct steps so it is possible for such bindings to exist in either an initialised or uninitialised state. Declarative environment records support the methods listed in Table 18 in addition to the Environment Record abstract specification methods:

Table 18 — Additional Methods of Declarative Environment Records
MethodPurpose
CreateImmutableBinding(N)Create a new but uninitialised immutable binding in an environment record. The String valueN is the text of the bound name.
InitializeImmutableBinding(N,V)Set the value of an already existing but uninitialised immutable binding in an environment record. The String valueN is the text of the bound name.V is the value for the binding and is a value of any ECMAScript language type.

The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following algorithms.

10.2.1.1.1 HasBinding(N)

The concrete environment record method HasBinding for declarative environment records simply determines if the argument identifier is one of the identifiers bound by the record:

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. IfenvRec has a binding for the name that is the value ofN, returntrue.
  3. If it does not have such a binding, returnfalse.

10.2.1.1.2 CreateMutableBinding (N, D)

The concreteEnvironment Record method CreateMutableBinding for declarative environment records creates a new mutable binding for the nameN that is initialised to the valueundefined. A binding must not already exist in thisEnvironment Record forN. If Boolean argumentD is provided and has the valuetrue the new binding is marked as being subject to deletion.

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. Assert:envRec does not already have a binding forN.
  3. Create a mutable binding inenvRec forN and set its bound value toundefined. IfD is true record that the newly created binding may be deleted by a subsequent DeleteBinding call.

10.2.1.1.3 SetMutableBinding (N,V,S)

The concreteEnvironment Record method SetMutableBinding for declarative environment records attempts to change the bound value of the current binding of the identifier whose name is the value of the argumentN to the value of argumentV. A binding forN must already exist. If the binding is an immutable binding, aTypeError is thrown ifS istrue.

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. Assert:envRec must have a binding forN.
  3. If the binding forN inenvRec is a mutable binding, change its bound value toV.
  4. Else this must be an attempt to change the value of an immutable binding so ifS iftrue throw aTypeError exception.

10.2.1.1.4 GetBindingValue(N,S)

The concreteEnvironment Record method GetBindingValue for declarative environment records simply returns the value of its bound identifier whose name is the value of the argumentN. The binding must already exist. IfS istrue and the binding is an uninitialised immutable binding throw aReferenceError exception.

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. Assert:envRec has a binding forN.
  3. If the binding forN inenvRec is an uninitialised immutable binding, then
    1. IfS isfalse, return the valueundefined, otherwise throw aReferenceError exception.
  4. Else, return the value currently bound toN inenvRec.

10.2.1.1.5 DeleteBinding (N)

The concreteEnvironment Record method DeleteBinding for declarative environment records can only delete bindings that have been explicitly designated as being subject to deletion.

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. IfenvRec does not have a binding for the name that is the value ofN, returntrue.
  3. If the binding forN inenvRec is cannot be deleted, returnfalse.
  4. Remove the binding forN fromenvRec.
  5. Returntrue.

10.2.1.1.6 ImplicitThisValue()

Declarative Environment Records always returnundefined as their ImplicitThisValue.

  1. Returnundefined.

10.2.1.1.7 CreateImmutableBinding (N)

The concreteEnvironment Record method CreateImmutableBinding for declarative environment records creates a new immutable binding for the nameN that is initialised to the valueundefined. A binding must not already exist in this environment record forN.

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. Assert:envRec does not already have a binding forN.
  3. Create an immutable binding inenvRec forN and record that it is uninitialised.

10.2.1.1.8 InitializeImmutableBinding (N,V)

The concreteEnvironment Record method InitializeImmutableBinding for declarative environment records is used to set the bound value of the current binding of the identifier whose name is the value of the argumentN to the value of argumentV. An uninitialised immutable binding forN must already exist.

  1. LetenvRec be thedeclarative environment record for which the method was invoked.
  2. Assert:envRec must have an uninitialised immutable binding for N.
  3. Set the bound value forN inenvRec toV.
  4. Record that the immutable binding forN inenvRec has been initialised.

10.2.1.2 Object Environment Records

Eachobject environment record is associated with an object called itsbinding object. Anobject environment record binds the set of identifier names that directly correspond to the property names of its binding object. Property names that are not anIdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by anobject environment record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the valuefalse. Immutable bindings do not exist for object environment records.

Object environment records can be configured to provide their binding object as an implicit this value for use in function calls. This capability is used to specify the behaviour of With Statement (12.10) induced bindings. The capability is controlled by aprovideThis Boolean value that is associated with eachobject environment record. By default, the value ofprovideThis isfalse for anyobject environment record.

The behaviour of the concrete specification methods for Object Environment Records is defined by the following algorithms.

10.2.1.2.1 HasBinding(N)

The concreteEnvironment Record method HasBinding for object environment records determines if its associated binding object has a property whose name is the value of the argumentN:

  1. LetenvRec be theobject environment record for which the method was invoked.
  2. Letbindings be the binding object forenvRec.
  3. Return the result of calling the [[HasProperty]] internal method ofbindings, passingN as the property name.

10.2.1.2.2 CreateMutableBinding (N, D)

The concreteEnvironment Record method CreateMutableBinding for object environment records creates in an environment record’s associated binding object a property whose name is the String value and initialises it to the valueundefined. A property namedN must not already exist in the binding object. If Boolean argumentD is provided and has the valuetrue the new property’s [[Configurable]] attribute is set totrue, otherwise it is set tofalse.

  1. LetenvRec be theobject environment record for which the method was invoked.
  2. Letbindings be the binding object forenvRec.
  3. Assert: The result of calling the [[HasProperty]] internal method ofbindings, passingN as the property name, isfalse.
  4. If D istrue then letconfigValue betrue otherwise letconfigValue befalse.
  5. Call the [[DefineOwnProperty]] internal method ofbindings, passingN,Property Descriptor {[[Value]]:undefined, [[Writable]]:true, [[Enumerable]]:true , [[Configurable]]:configValue}, andtrue as arguments.

10.2.1.2.3 SetMutableBinding (N,V,S)

The concreteEnvironment Record method SetMutableBinding for object environment records attempts to set the value of the environment record’s associated binding object’s property whose name is the value of the argumentN to the value of argumentV. A property namedN should already exist but if it does not or is not currently writable, error handling is determined by the value of the Boolean argumentS.

  1. LetenvRec be theobject environment record for which the method was invoked.
  2. Letbindings be the binding object forenvRec.
  3. Call the [[Put]] internal method ofbindings with argumentsN,V, andS.

10.2.1.2.4 GetBindingValue(N,S)

The concreteEnvironment Record method GetBindingValue for object environment records returns the value of its associated binding object’s property whose name is the String value of the argument identifierN. The property should already exist but if it does not the result depends upon the value of theS argument:

  1. LetenvRec be theobject environment record for which the method was invoked.
  2. Letbindings be the binding object forenvRec.
  3. Letvalue be the result of calling the [[HasProperty]] internal method ofbindings, passingN as the property name.
  4. Ifvalue isfalse, then
    1. IfS isfalse, return the valueundefined, otherwise throw aReferenceError exception.
  5. Return the result of calling the [[Get]] internal method ofbindings, passingN for the argument.

10.2.1.2.5 DeleteBinding (N)

The concreteEnvironment Record method DeleteBinding for object environment records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the valuetrue.

  1. LetenvRec be theobject environment record for which the method was invoked.
  2. Letbindings be the binding object forenvRec.
  3. Return the result of calling the [[Delete]] internal method ofbindings, passingN andfalse as arguments.

10.2.1.2.6 ImplicitThisValue()

Object Environment Records returnundefined as their ImplicitThisValue unless theirprovideThis flag istrue.

  1. LetenvRec be theobject environment record for which the method was invoked.
  2. If theprovideThis flag ofenvRec istrue, return the binding object forenvRec.
  3. Otherwise, returnundefined.

10.2.2 Lexical Environment Operations

The following abstract operations are used in this specification to operate upon lexical environments:

10.2.2.1 GetIdentifierReference (lex, name, strict)

The abstract operation GetIdentifierReference is called with aLexical Environmentlex, an identifier Stringname, and a Boolean flagstrict. The value oflex may benull. When called, the following steps are performed:

  1. Iflex is the valuenull, then
    1. Return a value of typeReference whose base value isundefined, whose referenced name isname, and whose strict mode flag isstrict.
  2. LetenvRec belex’s environment record.
  3. Letexists be the result of calling the HasBinding(N) concrete method ofenvRec passingname as the argumentN.
  4. Ifexists istrue, then
    1. Return a value of typeReference whose base value isenvRec, whose referenced name isname, and whose strict mode flag isstrict.
  5. Else
    1. Letouter be the value oflex’souter environment reference.
    2. Return the result of calling GetIdentifierReference passingouter,name, andstrict as arguments.

10.2.2.2 NewDeclarativeEnvironment (E)

When the abstract operation NewDeclarativeEnvironment is called with either aLexical Environment ornull as argumentE the following steps are performed:

  1. Letenv be a newLexical Environment.
  2. LetenvRec be a newdeclarative environment record containing no bindings.
  3. Setenv’s environment record to beenvRec.
  4. Set theouter lexical environment reference ofenv toE.
  5. Returnenv.

10.2.2.3 NewObjectEnvironment (O, E)

When the abstract operation NewObjectEnvironment is called with an ObjectO and aLexical EnvironmentE (ornull) as arguments, the following steps are performed:

  1. Letenv be a newLexical Environment.
  2. LetenvRec be a newobject environment record containingO as the binding object.
  3. Setenv’s environment record to beenvRec.
  4. Set theouter lexical environment reference ofenv toE.
  5. Returnenv.

10.2.3 The Global Environment

Theglobal environment is a uniqueLexical Environment which is created before any ECMAScript code is executed. The global environment’sEnvironment Record is anobject environment record whose binding object is the global object (15.1). The global environment’souter environment reference isnull.

As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

10.3 Execution Contexts

When control is transferred to ECMAScript executable code, control is entering anexecution context. Active execution contexts logically form a stack. The top execution context on this logical stack is the running execution context. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever state is necessary to track the execution progress of its associated code. In addition, each execution context has the state components listed in Table 19.

Table 19 —Execution Context State Components
ComponentPurpose
LexicalEnvironmentIdentifies theLexical Environment used to resolve identifier references made by code within this execution context.
VariableEnvironmentIdentifies theLexical Environment whose environment record holds bindings created byVariableStatements andFunctionDeclarations within this execution context.
ThisBindingThe value associated with thethis keyword within ECMAScript code associated with this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, “VariableEnvironment” and “ThisBinding” are used without qualification they are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to access an execution context.

10.3.1 Identifier Resolution

Identifier resolution is the process of determining the binding of anIdentifier using theLexicalEnvironment of the running execution context. During execution of ECMAScript code, the syntactic productionPrimaryExpression:Identifier is evaluated using the following algorithm:

  1. Letenv be the running execution context’sLexicalEnvironment.
  2. If the syntactic production that is being evaluated is contained in astrict mode code, then letstrict betrue, else letstrict befalse.
  3. Return the result of callingGetIdentifierReference function passingenv,Identifier, andstrict as arguments.

The result of evaluating an identifier is always a value of typeReference with its referenced name component equal to theIdentifier String.

10.4 Establishing an Execution Context

Evaluation of global code or code using the eval function (15.1.2.1) establishes and enters a new execution context. Every invocation of an ECMAScript code function (13.2.1) also establishes and enters a new execution context, even if a function is calling itself recursively. Every return exits an execution context. A thrown exception may also exit one or more execution contexts.

When control enters an execution context, the execution context’sThisBinding is set, itsVariableEnvironment and initialLexicalEnvironment are defined, anddeclaration binding instantiation (10.5) is performed. The exact manner in which these actions occur depend on the type of code being entered.

10.4.1 Entering Global Code

The following steps are performed when control enters the execution context for global code:

  1. Initialise the execution context using the global code as described in10.4.1.1.
  2. PerformDeclaration Binding Instantiation as described in10.5 using the global code.

10.4.1.1 Initial Global Execution Context

The following steps are performed to initialise a global execution context for ECMAScript codeC:

  1. Set theVariableEnvironment tothe Global Environment.
  2. Set theLexicalEnvironment tothe Global Environment.
  3. Set theThisBinding to the global object.

10.4.2 Entering Eval Code

The following steps are performed when control enters the execution context for eval code:

  1. If there is no calling context or if the eval code is not being evaluated by a direct call (15.1.2.1.1) to the eval function then,
    1. Initialise the execution context as if it was a global execution context using the eval code asC as described in10.4.1.1.
  2. Else,
    1. Set theThisBinding to the same value as theThisBinding of the calling execution context.
    2. Set theLexicalEnvironment to the same value as theLexicalEnvironment of the calling execution context.
    3. Set theVariableEnvironment to the same value as theVariableEnvironment of the calling execution context.
  3. If the eval code isstrict code, then
    1. LetstrictVarEnv be the result of callingNewDeclarativeEnvironment passing theLexicalEnvironment as the argument.
    2. Set theLexicalEnvironment tostrictVarEnv.
    3. Set theVariableEnvironment tostrictVarEnv.
  4. PerformDeclaration Binding Instantiation as described in10.5 using the eval code.

10.4.2.1 Strict Mode Restrictions

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code isstrict code. Instead such bindings are instantiated in a newVariableEnvironment that is only accessible to the eval code.

10.4.3 Entering Function Code

The following steps are performed when control enters the execution context for function code contained in function objectF, a caller providedthisArg, and a caller providedargumentsList:

  1. If the function code isstrict code, set theThisBinding tothisArg.
  2. Else ifthisArg isnull orundefined, set theThisBinding to the global object.
  3. Else ifType(thisArg) is not Object, set theThisBinding toToObject(thisArg).
  4. Else set theThisBinding tothisArg.
  5. LetlocalEnv be the result of callingNewDeclarativeEnvironment passing the value of the [[Scope]] internal property ofF as the argument.
  6. Set theLexicalEnvironment tolocalEnv.
  7. Set theVariableEnvironment tolocalEnv.
  8. Letcode be the value ofF’s [[Code]] internal property.
  9. PerformDeclaration Binding Instantiation using the function codecode andargumentsList as described in10.5.

10.5 Declaration Binding Instantiation

Every execution context has an associatedVariableEnvironment. Variables and functions declared in ECMAScript code evaluated in an execution context are added as bindings in thatVariableEnvironment’sEnvironment Record. For function code, parameters are also added as bindings to thatEnvironment Record.

WhichEnvironment Record is used to bind a declaration and its kind depends upon the type of ECMAScript code executed by the execution context, but the remainder of the behaviour is generic. On entering an execution context, bindings are created in theVariableEnvironment as follows using the caller providedcode and, if it is function code, argumentListargs:

  1. Letenv be the environment record component of the running execution context’sVariableEnvironment.
  2. Ifcode is eval code, then letconfigurableBindings betrue else letconfigurableBindings befalse.
  3. Ifcode isstrict mode code, then letstrict betrue else letstrict befalse.
  4. Ifcode is function code, then
    1. Letfunc be the function whose [[Call]] internal method initiated execution ofcode. Letnames be the value offunc’s [[FormalParameters]] internal property.
    2. LetargCount be the number of elements inargs.
    3. Letn be the number 0.
    4. For each StringargName innames, in list order do
      1. Letn be the current value ofn plus 1.
      2. Ifn is greater thanargCount, letv beundefined otherwise letv be the value of then’th element ofargs.
      3. LetargAlreadyDeclared be the result of callingenv’s HasBinding concrete method passingargName as the argument.
      4. IfargAlreadyDeclared isfalse, callenv’s CreateMutableBinding concrete method passingargName as the argument.
      5. Callenv’s SetMutableBinding concrete method passingargName,v, andstrict as the arguments.
  5. For eachFunctionDeclarationf incode, in source text order do
    1. Letfn be theIdentifier inFunctionDeclarationf.
    2. Letfo be the result of instantiatingFunctionDeclaration f as described inClause 13.
    3. LetfuncAlreadyDeclared be the result of callingenv’s HasBinding concrete method passingfn as the argument.
    4. IffuncAlreadyDeclared isfalse, callenv’s CreateMutableBinding concrete method passingfn andconfigurableBindings as the arguments.
    5. Else ifenv is the environment record component ofthe global environment then
      1. Letgo be the global object.
      2. LetexistingProp be the resulting of calling the [[GetProperty]] internal method ofgo with argumentfn.
      3. IfexistingProp .[[Configurable]] istrue, then
        1. Call the [[DefineOwnProperty]] internal method ofgo, passingfn,Property Descriptor {[[Value]]:undefined, [[Writable]]:true, [[Enumerable]]:true , [[Configurable]]:configurableBindings }, andtrue as arguments.
      4. Else ifIsAccessorDescriptor(existingProp) orexistingProp does not have attribute values {[[Writable]]:true, [[Enumerable]]:true}, then
        1. Throw a TypeError exception.
    6. Callenv’s SetMutableBinding concrete method passingfn,fo, andstrict as the arguments.
  6. LetargumentsAlreadyDeclared be the result of callingenv’s HasBinding concrete method passing"arguments" as the argument.
  7. Ifcode is function code andargumentsAlreadyDeclared isfalse, then
    1. LetargsObj be the result of calling the abstract operation CreateArgumentsObject (10.6) passingfunc, names, args,env andstrict as arguments.
    2. Ifstrict istrue, then
      1. Callenv’s CreateImmutableBinding concrete method passing the String "arguments" as the argument.
      2. Callenv’s InitializeImmutableBinding concrete method passing "arguments" andargsObj as arguments.
    3. Else,
      1. Callenv’s CreateMutableBinding concrete method passing the String "arguments" as the argument.
      2. Callenv’s SetMutableBinding concrete method passing "arguments",argsObj, andfalse as arguments.
  8. For eachVariableDeclaration andVariableDeclarationNoInd incode, in source text order do
    1. Letdn be theIdentifier ind.
    2. LetvarAlreadyDeclared be the result of callingenv’s HasBinding concrete method passingdn as the argument.
    3. IfvarAlreadyDeclared isfalse, then
      1. Callenv’s CreateMutableBinding concrete method passingdn andconfigurableBindings as the arguments.
      2. Callenv’s SetMutableBinding concrete method passingdn,undefined, andstrict as the arguments.

10.6 Arguments Object

When control enters an execution context for function code, an arguments object is created unless (as specified in10.5) the identifierarguments occurs as anIdentifier in the function’sFormalParameterList or occurs as theIdentifier of aVariableDeclaration orFunctionDeclaration contained in the function code.

The arguments object is created by calling the abstract operation CreateArgumentsObject with argumentsfunc the function object whose code is to be evaluated, names aList containing the function’s formal parameter names, args the actual arguments passed to the [[Call]] internal method,env the variable environment for the function code, andstrict a Boolean that indicates whether or not the function code isstrict code. When CreateArgumentsObject is called the following steps are performed:

  1. Letlen be the number of elements inargs.
  2. Letobj be the result of creating a new ECMAScript object.
  3. Set all the internal methods ofobj as specified in8.12.
  4. Set the [[Class]] internal property ofobj to"Arguments".
  5. LetObject be the standard built-in Object constructor (15.2.2).
  6. Set the [[Prototype]] internal property ofobj to the standard built-in Object prototype object (15.2.4).
  7. Call the [[DefineOwnProperty]] internal method onobj passing"length", theProperty Descriptor {[[Value]]:len, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}, andfalse as arguments.
  8. Letmap be the result of creating a new object as if by the expressionnew Object() whereObject is the standard built-in constructor with that name
  9. LetmappedNames be an emptyList.
  10. Letindx =len - 1.
  11. Repeat whileindx >= 0,
    1. Letval be the element ofargs at 0-origined list positionindx.
    2. Call the [[DefineOwnProperty]] internal method onobj passingToString(indx), the property descriptor {[[Value]]:val, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse as arguments.
    3. Ifindx is less than the number of elements innames, then
      1. Letname be the element ofnames at 0-origined list positionindx.
      2. Ifstrict isfalse andname is not an element ofmappedNames, then
        1. Addname as an element of the listmappedNames.
        2. Letg be the result of calling theMakeArgGetter abstract operation with argumentsname andenv.
        3. Letp be the result of calling theMakeArgSetter abstract operation with argumentsname andenv.
        4. Call the [[DefineOwnProperty]] internal method ofmap passingToString(indx), theProperty Descriptor {[[Set]]:p, [[Get]]:g, [[Configurable]]:true}, andfalse as arguments.
    4. Letindx =indx - 1
  12. IfmappedNames is not empty, then
    1. Set the [[ParameterMap]] internal property ofobj tomap.
    2. Set the [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]] internal methods ofobj to the definitions provided below.
  13. Ifstrict isfalse, then
    1. Call the [[DefineOwnProperty]] internal method onobj passing "callee", the property descriptor {[[Value]]:func, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}, andfalse as arguments.
  14. Else,strict istrue so
    1. Letthrower be the [[ThrowTypeError]] function Object (13.2.3).
    2. Call the [[DefineOwnProperty]] internal method ofobj with arguments"caller", PropertyDescriptor {[[Get]]:thrower, [[Set]]:thrower, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
    3. Call the [[DefineOwnProperty]] internal method ofobj with arguments"callee", PropertyDescriptor {[[Get]]:thrower, [[Set]]:thrower, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  15. Returnobj

The abstract operationMakeArgGetter called with Stringname and environment recordenv creates a function object that when executed returns the value bound forname inenv. It performs the following steps:

  1. Letbody be the result of concatenating the Strings "return ",name, and ";".
  2. Return the result of creating a function object as described in13.2 using noFormalParameterList,body forFunctionBody,env asScope, andtrue forStrict.

The abstract operationMakeArgSetter called with Stringname and environment recordenv creates a function object that when executed sets the value bound forname inenv. It performs the following steps:

  1. Letparam be the Stringname concatenated with the String "_arg".
  2. Letbody be the String"<name>= <param>;" with<name> replaced by the value ofname and<param> replaced by the value ofparam.
  3. Return the result of creating a function object as described in13.2 using aList containing the single Stringparam asFormalParameterList,body forFunctionBody,env asScope, andtrue forStrict.

The [[Get]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property nameP performs the following steps:

  1. Letmap be the value of the [[ParameterMap]] internal property of the arguments object.
  2. LetisMapped be the result of calling the [[GetOwnProperty]] internal method ofmap passingP as the argument.
  3. If the value ofisMapped isundefined, then
    1. Letv be the result of calling the default [[Get]] internal method (8.12.3) on the arguments object passingP as the argument.
    2. IfP is"caller" andv is a strict mode Function object, throw aTypeError exception.
    3. Returnv.
  4. Else,map contains a formal parameter mapping forP so,
    1. Return the result of calling the [[Get]] internal method ofmap passingP as the argument.

The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property nameP performs the following steps:

  1. Letdesc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on the arguments object passingP as the argument.
  2. Ifdesc isundefined then returndesc.
  3. Letmap be the value of the [[ParameterMap]] internal property of the arguments object.
  4. LetisMapped be the result of calling the [[GetOwnProperty]] internal method ofmap passingP as the argument.
  5. If the value ofisMapped is notundefined, then
    1. Setdesc.[[Value]] to the result of calling the [[Get]] internal method ofmap passingP as the argument.
  6. Returndesc.

The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property nameP,Property DescriptorDesc, and Boolean flagThrow performs the following steps:

  1. Letmap be the value of the [[ParameterMap]] internal property of the arguments object.
  2. LetisMapped be the result of calling the [[GetOwnProperty]] internal method ofmap passingP as the argument.
  3. Letallowed be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on the arguments object passingP,Desc, andfalse as the arguments.
  4. Ifallowed isfalse, then
    1. IfThrow istrue then throw aTypeError exception, otherwise returnfalse.
  5. If the value ofisMapped is notundefined, then
    1. IfIsAccessorDescriptor(Desc) istrue, then
      1. Call the [[Delete]] internal method ofmap passingP, andfalse as the arguments.
    2. Else
      1. IfDesc.[[Value]] is present, then
        1. Call the [[Put]] internal method ofmap passingP,Desc.[[Value]], andThrow as the arguments.
      2. IfDesc.[[Writable]] is present and its value isfalse, then
        1. Call the [[Delete]] internal method ofmap passingP andfalse as arguments.
  6. Returntrue.

The [[Delete]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property nameP and Boolean flagThrow performs the following steps:

  1. Letmap be the value of the [[ParameterMap]] internal property of the arguments object.
  2. LetisMapped be the result of calling the [[GetOwnProperty]] internal method ofmap passingP as the argument.
  3. Letresult be the result of calling the default [[Delete]] internal method (8.12.7) on the arguments object passingP andThrow as the arguments.
  4. Ifresult istrue and the value ofisMapped is notundefined, then
    1. Call the [[Delete]] internal method ofmap passingP, andfalse as the arguments.
  5. Returnresult.

NOTE 1 For non-strict mode functions the array index (defined in15.4) named data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly accessible from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

NOTE 3 Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw aTypeError exception on access. The "callee" property has a more specific meaning for non-strict mode functions and a "caller" property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by conforming ECMAScript implementations.

11 Expressions

11.1 Primary Expressions

Syntax

PrimaryExpression:
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
(Expression)

11.1.1 Thethis Keyword

Thethis keyword evaluates to the value of theThisBinding of the current execution context.

11.1.2 Identifier Reference

AnIdentifier is evaluated by performingIdentifier Resolution as specified in 10.3.1. The result of evaluating anIdentifier is always a value of typeReference.

11.1.3 Literal Reference

ALiteral is evaluated as described in7.8.

11.1.4 Array Initialiser

An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal. It is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initialiser is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by anAssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

ArrayLiteral:
[Elisionopt]
[ElementList]
[ElementList,Elisionopt]
ElementList:
ElisionoptAssignmentExpression
ElementList,ElisionoptAssignmentExpression
Elision:
,
Elision,

Semantics

The productionArrayLiteral:[Elisionopt] is evaluated as follows:

  1. Letarray be the result of creating a new object as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
  2. Letpad be the result of evaluatingElision; if not present, use the numeric value zero.
  3. Call the [[Put]] internal method ofarray with arguments"length",pad, andfalse.
  4. Returnarray.

The productionArrayLiteral:[ElementList] is evaluated as follows:

  1. Return the result of evaluatingElementList.

The productionArrayLiteral:[ElementList,Elisionopt] is evaluated as follows:

  1. Letarray be the result of evaluatingElementList.
  2. Letpad be the result of evaluatingElision; if not present, use the numeric value zero.
  3. Letlen be the result of calling the [[Get]] internal method ofarray with argument"length".
  4. Call the [[Put]] internal method ofarray with arguments"length",ToUint32(pad+len), andfalse.
  5. Returnarray.

The productionElementList:ElisionoptAssignmentExpression is evaluated as follows:

  1. Letarray be the result of creating a new object as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
  2. LetfirstIndex be the result of evaluatingElision; if not present, use the numeric value zero.
  3. LetinitResult be the result of evaluatingAssignmentExpression.
  4. LetinitValue beGetValue(initResult).
  5. Call the [[DefineOwnProperty]] internal method ofarray with argumentsToString(firstIndex), theProperty Descriptor { [[Value]]:initValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  6. Returnarray.

The productionElementList:ElementList,ElisionoptAssignmentExpression is evaluated as follows:

  1. Letarray be the result of evaluatingElementList.
  2. Letpad be the result of evaluatingElision; if not present, use the numeric value zero.
  3. LetinitResult be the result of evaluatingAssignmentExpression.
  4. LetinitValue beGetValue(initResult).
  5. Letlen be the result of calling the [[Get]] internal method ofarray with argument"length".
  6. Call the [[DefineOwnProperty]] internal method ofarray with argumentsToString(ToUint32((pad+len)) and theProperty Descriptor { [[Value]]:initValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  7. Returnarray.

The productionElision:, is evaluated as follows:

  1. Return the numeric value 1.

The productionElision:Elision, is evaluated as follows:

  1. Letpreceding be the result of evaluatingElision.
  2. Returnpreceding+1.

NOTE [[DefineOwnProperty]] is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Put]].

11.1.5 Object Initialiser

An object initialiser is an expression describing the initialisation of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The values need not be literals; they are evaluated each time the object initialiser is evaluated.

Syntax

ObjectLiteral:
{}
{PropertyNameAndValueList}
{PropertyNameAndValueList,}
PropertyNameAndValueList:
PropertyAssignment
PropertyNameAndValueList,PropertyAssignment
PropertyAssignment:
PropertyName:AssignmentExpression
getPropertyName(){FunctionBody}
setPropertyName(PropertySetParameterList){FunctionBody}
PropertyName:
IdentifierName
StringLiteral
NumericLiteral
PropertySetParameterList:
Identifier

Semantics

The productionObjectLiteral:{} is evaluated as follows:

  1. Return a new object created as if by the expressionnew Object() whereObject is the standard built-in constructor with that name.

The productionsObjectLiteral:{PropertyNameAndValueList} and
ObjectLiteral:{PropertyNameAndValueList,} are evaluated as follows:

  1. Return the result of evaluatingPropertyNameAndValueList.

The productionPropertyNameAndValueList:PropertyAssignment is evaluated as follows:

  1. Letobj be the result of creating a new object as if by the expressionnew Object() whereObject is the standard built-in constructor with that name.
  2. LetpropId be the result of evaluatingPropertyAssignment.
  3. Call the [[DefineOwnProperty]] internal method ofobj with argumentspropId.name,propId.descriptor, andfalse.
  4. Returnobj.

The production
PropertyNameAndValueList:PropertyNameAndValueList,PropertyAssignment
is evaluated as follows:

  1. Letobj be the result of evaluatingPropertyNameAndValueList.
  2. LetpropId be the result of evaluatingPropertyAssignment.
  3. Letprevious be the result of calling the [[GetOwnProperty]] internal method ofobj with argumentpropId.name.
  4. Ifprevious is notundefined then throw aSyntaxError exception if any of the following conditions are true
    1. This production is contained instrict code andIsDataDescriptor(previous) istrue andIsDataDescriptor(propId.descriptor) istrue.
    2. IsDataDescriptor(previous) istrue andIsAccessorDescriptor(propId.descriptor) istrue.
    3. IsAccessorDescriptor(previous) istrue andIsDataDescriptor(propId.descriptor) istrue.
    4. IsAccessorDescriptor(previous) istrue andIsAccessorDescriptor(propId.descriptor) istrue and either bothprevious andpropId.descriptor have [[Get]] fields or bothprevious andpropId.descriptor have [[Set]] fields
  5. Call the [[DefineOwnProperty]] internal method ofobj with argumentspropId.name,propId.descriptor, andfalse.
  6. Returnobj.

If the above steps would throw aSyntaxError then an implementation must treat the error as an early error (Clause 16).

The productionPropertyAssignment:PropertyName:AssignmentExpression is evaluated as follows:

  1. LetpropName be the result of evaluatingPropertyName.
  2. LetexprValue be the result of evaluatingAssignmentExpression.
  3. LetpropValue beGetValue(exprValue).
  4. Letdesc be theProperty Descriptor{[[Value]]:propValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}
  5. ReturnProperty Identifier (propName,desc).

The productionPropertyAssignment:getPropertyName(){FunctionBody} is evaluated as follows:

  1. LetpropName be the result of evaluatingPropertyName.
  2. Letclosure be the result of creating a new Function object as specified in13.2 with an empty parameter list and body specified byFunctionBody. Pass in theLexicalEnvironment of the running execution context as theScope. Pass intrue as theStrict flag if thePropertyAssignment is contained instrict code or if itsFunctionBody isstrict code.
  3. Letdesc be theProperty Descriptor{[[Get]]:closure, [[Enumerable]]:true, [[Configurable]]:true}
  4. ReturnProperty Identifier (propName,desc).

The productionPropertyAssignment:setPropertyName(PropertySetParameterList){FunctionBody} is evaluated as follows:

  1. LetpropName be the result of evaluatingPropertyName.
  2. Letclosure be the result of creating a new Function object as specified in13.2 with parameters specified byPropertySetParameterList and body specified byFunctionBody. Pass in theLexicalEnvironment of the running execution context as theScope. Pass intrue as theStrict flag if thePropertyAssignment is contained instrict code or if itsFunctionBody isstrict code.
  3. Letdesc be theProperty Descriptor{[[Set]]:closure, [[Enumerable]]:true, [[Configurable]]:true}
  4. ReturnProperty Identifier (propName,desc).

It is aSyntaxError if theIdentifier"eval" or theIdentifier"arguments" occurs as theIdentifier in aPropertySetParameterList of aPropertyAssignment thatis contained instrict code or if itsFunctionBody isstrict code.

The productionPropertyName:IdentifierName is evaluated as follows:

  1. Return the String value containing the same sequence of characters as theIdentifierName.

The productionPropertyName:StringLiteral is evaluated as follows:

  1. Return the SV of theStringLiteral.

The productionPropertyName:NumericLiteral is evaluated as follows:

  1. Letnbr be the result of forming the value of theNumericLiteral.
  2. ReturnToString(nbr).

11.1.6 The Grouping Operator

The productionPrimaryExpression:(Expression) is evaluated as follows:

  1. Return the result of evaluatingExpression. This may be of typeReference.

NOTE This algorithm does not applyGetValue to the result of evaluating Expression. The principal motivation for this is so that operators such asdelete andtypeof may be applied to parenthesised expressions.

11.2 Left-Hand-Side Expressions

Syntax

MemberExpression:
PrimaryExpression
FunctionExpression
MemberExpression[Expression]
MemberExpression.IdentifierName
newMemberExpressionArguments
NewExpression:
MemberExpression
newNewExpression
CallExpression:
MemberExpressionArguments
CallExpressionArguments
CallExpression[Expression]
CallExpression.IdentifierName
Arguments:
()
(ArgumentList)
ArgumentList:
AssignmentExpression
ArgumentList,AssignmentExpression
LeftHandSideExpression:
NewExpression
CallExpression

11.2.1 Property Accessors

Properties are accessed by name, using either the dot notation:

MemberExpression. IdentifierName
CallExpression. IdentifierName

or the bracket notation:

MemberExpression[ Expression]
CallExpression[ Expression]

The dot notation is explained by the following syntactic conversion:

MemberExpression. IdentifierName

is identical in its behaviour to

MemberExpression[ <identifier-name-string>]

and similarly

CallExpression. IdentifierName

is identical in its behaviour to

CallExpression[ <identifier-name-string>]

where<identifier-name-string> is a string literal containing the same sequence of characters after processing of Unicode escape sequences as theIdentifierName.

The productionMemberExpression:MemberExpression[Expression] is evaluated as follows:

  1. LetbaseReference be the result of evaluatingMemberExpression.
  2. LetbaseValue beGetValue(baseReference).
  3. LetpropertyNameReference be the result of evaluatingExpression.
  4. LetpropertyNameValue beGetValue(propertyNameReference).
  5. CallCheckObjectCoercible(baseValue).
  6. LetpropertyNameString beToString(propertyNameValue).
  7. If the syntactic production that is being evaluated is contained instrict mode code, letstrict betrue, else letstrict befalse.
  8. Return a value of typeReference whose base value isbaseValue and whose referenced name ispropertyNameString, and whose strict mode flag isstrict.

The productionCallExpression:CallExpression[Expression] is evaluated in exactly the same manner, except that the containedCallExpression is evaluated in step 1.

11.2.2 Thenew Operator

The productionNewExpression:newNewExpression is evaluated as follows:

  1. Letref be the result of evaluatingNewExpression.
  2. Letconstructor beGetValue(ref).
  3. IfType(constructor) is not Object, throw aTypeError exception.
  4. Ifconstructor does not implement the [[Construct]] internal method, throw aTypeError exception.
  5. Return the result of calling the [[Construct]] internal method onconstructor, providing no arguments (that is, an empty list of arguments).

The productionMemberExpression:newMemberExpressionArguments is evaluated as follows:

  1. Letref be the result of evaluatingMemberExpression.
  2. Letconstructor beGetValue(ref).
  3. LetargList be the result of evaluatingArguments, producing an internal list of argument values (11.2.4).
  4. IfType(constructor) is not Object, throw aTypeError exception.
  5. Ifconstructor does not implement the [[Construct]] internal method, throw aTypeError exception.
  6. Return the result of calling the [[Construct]] internal method onconstructor, providing the listargList as the argument values.

11.2.3 Function Calls

The productionCallExpression:MemberExpressionArguments is evaluated as follows:

  1. Letref be the result of evaluatingMemberExpression.
  2. Letfunc beGetValue(ref).
  3. LetargList be the result of evaluatingArguments, producing an internal list of argument values (see 11.2.4).
  4. IfType(func) is not Object, throw aTypeError exception.
  5. IfIsCallable(func) isfalse, throw aTypeError exception.
  6. IfType(ref) isReference, then
    1. IfIsPropertyReference(ref) istrue, then
      1. LetthisValue beGetBase(ref).
    2. Else, the base ofref is anEnvironment Record
      1. LetthisValue be the result of calling the ImplicitThisValue concrete method ofGetBase(ref).
  7. Else,Type(ref) is notReference.
    1. LetthisValue beundefined.
  8. Return the result of calling the [[Call]] internal method onfunc, providingthisValue as thethis value and providing the listargList as the argument values.

The productionCallExpression:CallExpressionArguments is evaluated in exactly the same manner, except that the containedCallExpression is evaluated in step 1.

NOTE The returned result will never be of typeReference iffunc is a native ECMAScript object. Whether calling a host object can return a value of typeReference is implementation-dependent. If a value of typeReference is returned, it must be a non-strict PropertyReference.

11.2.4 Argument Lists

The evaluation of an argument list produces aList of values (see 8.8).

The productionArguments:() is evaluated as follows:

  1. Return an emptyList.

The productionArguments:(ArgumentList) is evaluated as follows:

  1. Return the result of evaluatingArgumentList.

The productionArgumentList:AssignmentExpression is evaluated as follows:

  1. Letref be the result of evaluatingAssignmentExpression.
  2. Letarg beGetValue(ref).
  3. Return aList whose sole item isarg.

The productionArgumentList:ArgumentList,AssignmentExpression is evaluated as follows:

  1. LetprecedingArgs be the result of evaluatingArgumentList.
  2. Letref be the result of evaluatingAssignmentExpression.
  3. Letarg beGetValue(ref).
  4. Return aList whose length is one greater than the length ofprecedingArgs and whose items are the items ofprecedingArgs, in order, followed at the end byarg which is the last item of the new list.

11.2.5 Function Expressions

The productionMemberExpression:FunctionExpression is evaluated as follows:

  1. Return the result of evaluatingFunctionExpression.

11.3 Postfix Expressions

Syntax

PostfixExpression:
LeftHandSideExpression
LeftHandSideExpression[noLineTerminator here]++
LeftHandSideExpression[noLineTerminator here]--

11.3.1 Postfix Increment Operator

The productionPostfixExpression:LeftHandSideExpression[noLineTerminator here]++ is evaluated as follows:

  1. Letlhs be the result of evaluatingLeftHandSideExpression.
  2. Throw aSyntaxError exception if the following conditions are all true:
  1. LetoldValue beToNumber(GetValue(lhs)).
  2. LetnewValue be the result of adding the value1 tooldValue, using the same rules as for the+ operator (see 11.6.3).
  3. CallPutValue(lhs,newValue).
  4. ReturnoldValue.

11.3.2 Postfix Decrement Operator

The productionPostfixExpression:LeftHandSideExpression[noLineTerminator here]-- is evaluated as follows:

  1. Letlhs be the result of evaluatingLeftHandSideExpression.
  2. Throw aSyntaxError exception if the following conditions are all true:
  1. LetoldValue beToNumber(GetValue(lhs)).
  2. LetnewValue be the result of subtracting the value1 fromoldValue, using the same rules as for the- operator (11.6.3).
  3. CallPutValue(lhs,newValue).
  4. ReturnoldValue.

11.4 Unary Operators

Syntax

UnaryExpression:
PostfixExpression
deleteUnaryExpression
voidUnaryExpression
typeofUnaryExpression
++UnaryExpression
--UnaryExpression
+UnaryExpression
-UnaryExpression
~UnaryExpression
!UnaryExpression

11.4.1 Thedelete Operator

The productionUnaryExpression:deleteUnaryExpression is evaluated as follows:

  1. Letref be the result of evaluatingUnaryExpression.
  2. IfType(ref) is notReference, returntrue.
  3. IfIsUnresolvableReference(ref) then,
    1. IfIsStrictReference(ref) istrue, throw aSyntaxError exception.
    2. Else, returntrue.
  4. IfIsPropertyReference(ref) istrue, then
    1. Return the result of calling the [[Delete]] internal method onToObject(GetBase(ref)) providingGetReferencedName(ref) andIsStrictReference(ref) as the arguments.
  5. Else,ref is aReference to anEnvironment Record binding, so
    1. IfIsStrictReference(ref) istrue, throw aSyntaxError exception.
    2. Letbindings beGetBase(ref).
    3. Return the result of calling the DeleteBinding concrete method ofbindings, providingGetReferencedName(ref) as the argument.

NOTE When adelete operator occurs withinstrict mode code, aSyntaxError exception is thrown if itsUnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if adelete operator occurs withinstrict mode code and the property to be deleted has the attribute { [[Configurable]]:false }, aTypeError exception is thrown.

11.4.2 Thevoid Operator

The productionUnaryExpression:voidUnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluatingUnaryExpression.
  2. CallGetValue(expr).
  3. Returnundefined.

NOTEGetValue must be called even though its value is not used because it may have observable side-effects.

11.4.3 Thetypeof Operator

The productionUnaryExpression:typeofUnaryExpression is evaluated as follows:

  1. Letval be the result of evaluatingUnaryExpression.
  2. IfType(val) isReference, then
    1. IfIsUnresolvableReference(val) istrue, return"undefined".
    2. Letval beGetValue(val).
  3. Return a String determined byType(val) according to Table 20.
Table 20 — typeof Operator Results
Type ofvalResult
Undefined"undefined"
Null"object"
Boolean"boolean"
Number"number"
String"string"
Object (native and does not implement [[Call]])"object"
Object (native or host and does implement [[Call]])"function"
Object (host and does not implement [[Call]])Implementation-defined except may not be"undefined","boolean","number", or"string".

11.4.4 Prefix Increment Operator

The productionUnaryExpression:++UnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluating UnaryExpression.
  2. Throw aSyntaxError exception if the following conditions are all true:
  1. LetoldValue beToNumber(GetValue(expr)).
  2. LetnewValue be the result of adding the value1 tooldValue, using the same rules as for the+ operator (see 11.6.3).
  3. CallPutValue(expr,newValue).
  4. ReturnnewValue.

11.4.5 Prefix Decrement Operator

The productionUnaryExpression:--UnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluating UnaryExpression.
  2. Throw aSyntaxError exception if the following conditions are all true:
  1. LetoldValue beToNumber(GetValue(expr)).
  2. LetnewValue be the result of subtracting the value1 fromoldValue, using the same rules as for the- operator (see 11.6.3).
  3. CallPutValue(expr,newValue).
  4. ReturnnewValue.

11.4.6 Unary+ Operator

The unary + operator converts its operand to Number type.

The productionUnaryExpression:+UnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluating UnaryExpression.
  2. ReturnToNumber(GetValue(expr)).

11.4.7 Unary- Operator

The unary- operator converts its operand to Number type and then negates it. Note that negating+0 produces−0, and negating−0 produces+0.

The productionUnaryExpression:-UnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluating UnaryExpression.
  2. LetoldValue beToNumber(GetValue(expr)).
  3. IfoldValue isNaN, returnNaN.
  4. Return the result of negatingoldValue; that is, compute a Number with the same magnitude but opposite sign.

11.4.8 Bitwise NOT Operator (~ )

The productionUnaryExpression:~UnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluatingUnaryExpression.
  2. LetoldValue beToInt32(GetValue(expr)).
  3. Return the result of applying bitwise complement tooldValue. The result is a signed 32-bit integer.

11.4.9 Logical NOT Operator (! )

The productionUnaryExpression:!UnaryExpression is evaluated as follows:

  1. Letexpr be the result of evaluatingUnaryExpression.
  2. LetoldValue beToBoolean(GetValue(expr)).
  3. IfoldValue istrue, returnfalse.
  4. Returntrue.

11.5 Multiplicative Operators

Syntax

MultiplicativeExpression:
UnaryExpression
MultiplicativeExpression*UnaryExpression
MultiplicativeExpression/UnaryExpression
MultiplicativeExpression%UnaryExpression

Semantics

The productionMultiplicativeExpression:MultiplicativeExpression@UnaryExpression , where @ stands for one of the operators in the above definitions, is evaluated as follows:

  1. Letleft be the result of evaluating MultiplicativeExpression.
  2. LetleftValue beGetValue(left).
  3. Letright be the result of evaluating UnaryExpression.
  4. LetrightValue beGetValue(right).
  5. LetleftNum beToNumber(leftValue).
  6. LetrightNum beToNumber(rightValue).
  7. Return the result of applying the specified operation (*, /, or %) toleftNum andrightNum. See the Notes below11.5.1,11.5.2, 11.5.3.

11.5.1 Applying the* Operator

The* operator performs multiplication, producing the product of its operands. Multiplication is commutative. Multiplication is not always associative in ECMAScript, because of finite precision.

The result of a floating-point multiplication is governed by the rules of IEEE 754 binary double-precision arithmetic:

  • If either operand isNaN, the result isNaN.

  • The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.

  • Multiplication of an infinity by a zero results inNaN.

  • Multiplication of an infinity by an infinity results in an infinity. The sign is determined by the rule already stated above.

  • Multiplication of an infinity by a finite nonzero value results in a signed infinity. The sign is determined by the rule already stated above.

  • In the remaining cases, where neither an infinity or NaN is involved, the product is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the result is then a zero of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

11.5.2 Applying the/ Operator

The/ operator performs division, producing the quotient of its operands. The left operand is the dividend and the right operand is the divisor. ECMAScript does not perform integer division. The operands and result of all division operations are double-precision floating-point numbers. The result of division is determined by the specification of IEEE 754 arithmetic:

  • If either operand isNaN, the result isNaN.

  • The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.

  • Division of an infinity by an infinity results inNaN.

  • Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.

  • Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.

  • Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.

  • Division of a zero by a zero results inNaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.

  • Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.

  • In the remaining cases, where neither an infinity, nor a zero, norNaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

11.5.3 Applying the% Operator

The% operator yields the remainder of its operands from an implied division; the left operand is the dividend and the right operand is the divisor.

NOTE In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

The result of a floating-point remainder operation as computed by the% operator is not the same as the “remainder” operation defined by IEEE 754. The IEEE 754 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines% on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:

  • If either operand isNaN, the result isNaN.

    • The sign of the result equals the sign of the dividend.

    • If the dividend is an infinity, or the divisor is a zero, or both, the result isNaN.

    • If the dividend is finite and the divisor is an infinity, the result equals the dividend.

    • If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.

    • In the remaining cases, where neither an infinity, nor a zero, norNaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n − (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode.

11.6 Additive Operators

Syntax

AdditiveExpression:
MultiplicativeExpression
AdditiveExpression+MultiplicativeExpression
AdditiveExpression-MultiplicativeExpression

11.6.1 The Addition operator (+ )

The addition operator either performs string concatenation or numeric addition.

The productionAdditiveExpression:AdditiveExpression+MultiplicativeExpression is evaluated as follows:

  1. Letlref be the result of evaluating AdditiveExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluating MultiplicativeExpression.
  4. Letrval beGetValue(rref).
  5. Letlprim beToPrimitive(lval).
  6. Letrprim beToPrimitive(rval).
  7. IfType(lprim) is String orType(rprim) is String, then
    1. Return the String that is the result of concatenatingToString(lprim) followed byToString(rprim)
  8. Return the result of applying the addition operation toToNumber(lprim) andToNumber(rprim). See the Note below11.6.3.

NOTE 1 No hint is provided in the calls toToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.

NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the logical-or operation instead of the logical-and operation.

11.6.2 The Subtraction Operator (- )

The productionAdditiveExpression:AdditiveExpression-MultiplicativeExpression is evaluated as follows:

  1. Letlref be the result of evaluating AdditiveExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluating MultiplicativeExpression.
  4. Letrval beGetValue(rref).
  5. Letlnum beToNumber(lval).
  6. Letrnum beToNumber(rval).
  7. Return the result of applying the subtraction operation tolnum andrnum. See the note below11.6.3.

11.6.3 Applying the Additive Operators to Numbers

The+ operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The- operator performs subtraction, producing the difference of two numeric operands.

Addition is a commutative operation, but not always associative.

The result of an addition is determined using the rules of IEEE 754 binary double-precision arithmetic:

  • If either operand isNaN, the result isNaN.

  • The sum of two infinities of opposite sign isNaN.

  • The sum of two infinities of the same sign is the infinity of that sign.

  • The sum of an infinity and a finite value is equal to the infinite operand.

  • The sum of two negative zeroes is−0. The sum of two positive zeroes, or of two zeroes of opposite sign, is+0.

  • The sum of a zero and a nonzero finite value is equal to the nonzero operand.

  • The sum of two nonzero finite values of the same magnitude and opposite sign is+0.

  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If the magnitude is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754.

The- operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operandsa andb, it is always the case thata–b produces the same result asa+(–b).

11.7 Bitwise Shift Operators

Syntax

ShiftExpression:
AdditiveExpression
ShiftExpression<<AdditiveExpression
ShiftExpression>>AdditiveExpression
ShiftExpression>>>AdditiveExpression

11.7.1 The Left Shift Operator (<< )

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

The productionShiftExpression:ShiftExpression<<AdditiveExpression is evaluated as follows:

  1. Letlref be the result of evaluatingShiftExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingAdditiveExpression.
  4. Letrval beGetValue(rref).
  5. Letlnum beToInt32(lval).
  6. Letrnum beToUint32(rval).
  7. LetshiftCount be the result of masking out all but the least significant 5 bits ofrnum, that is, computernum & 0x1F.
  8. Return the result of left shiftinglnum byshiftCount bits. The result is a signed 32-bit integer.

11.7.2 The Signed Right Shift Operator (>> )

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

The productionShiftExpression:ShiftExpression>>AdditiveExpression is evaluated as follows:

  1. Letlref be the result of evaluatingShiftExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingAdditiveExpression.
  4. Letrval beGetValue(rref).
  5. Letlnum beToInt32(lval).
  6. Letrnum beToUint32(rval).
  7. LetshiftCount be the result of masking out all but the least significant 5 bits ofrnum, that is, computernum & 0x1F.
  8. Return the result of performing a sign-extending right shift oflnum byshiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.

11.7.3 The Unsigned Right Shift Operator (>>> )

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

The productionShiftExpression:ShiftExpression>>>AdditiveExpression is evaluated as follows:

  1. Letlref be the result of evaluatingShiftExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingAdditiveExpression.
  4. Letrval beGetValue(rref).
  5. Letlnum beToUint32(lval).
  6. Letrnum beToUint32(rval).
  7. LetshiftCount be the result of masking out all but the least significant 5 bits ofrnum, that is, computernum & 0x1F.
  8. Return the result of performing a zero-filling right shift oflnum byshiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.

11.8 Relational Operators

Syntax

RelationalExpression:
ShiftExpression
RelationalExpression<ShiftExpression
RelationalExpression>ShiftExpression
RelationalExpression<=ShiftExpression
RelationalExpression>=ShiftExpression
RelationalExpressioninstanceofShiftExpression
RelationalExpressioninShiftExpression
RelationalExpressionNoIn:
ShiftExpression
RelationalExpressionNoIn<ShiftExpression
RelationalExpressionNoIn>ShiftExpression
RelationalExpressionNoIn<=ShiftExpression
RelationalExpressionNoIn>=ShiftExpression
RelationalExpressionNoIninstanceofShiftExpression

NOTE The “NoIn” variants are needed to avoid confusing thein operator in a relational expression with thein operator in afor statement.

Semantics

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

TheRelationalExpressionNoIn productions are evaluated in the same manner as theRelationalExpression productions except that the containedRelationalExpressionNoIn is evaluated instead of the containedRelationalExpression.

11.8.1 The Less-than Operator (< )

The productionRelationalExpression:RelationalExpression<ShiftExpression is evaluated as follows:

  1. Letlref be the result of evaluatingRelationalExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingShiftExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of performing abstract relational comparisonlval <rval. (see 11.8.5)
  6. Ifr isundefined, returnfalse. Otherwise, returnr.

11.8.2 The Greater-than Operator (> )

The production RelationalExpression:RelationalExpression>ShiftExpression is evaluated as follows:

  1. Letlref be the result of evaluatingRelationalExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingShiftExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of performing abstract relational comparisonrval <lval withLeftFirst equal tofalse. (see 11.8.5).
  6. Ifr isundefined, returnfalse. Otherwise, returnr.

11.8.3 The Less-than-or-equal Operator (<= )

The production RelationalExpression:RelationalExpression<=ShiftExpression is evaluated as follows:

  1. Letlref be the result of evaluatingRelationalExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingShiftExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of performing abstract relational comparisonrval <lval withLeftFirst equal tofalse. (see 11.8.5).
  6. Ifr istrue orundefined, returnfalse. Otherwise, returntrue.

11.8.4 The Greater-than-or-equal Operator (>= )

The production RelationalExpression:RelationalExpression>=ShiftExpression is evaluated as follows:

  1. Letlref be the result of evaluatingRelationalExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingShiftExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of performing abstract relational comparisonlval <rval. (see 11.8.5)
  6. Ifr istrue orundefined, returnfalse. Otherwise, returntrue.

11.8.5 The Abstract Relational Comparison Algorithm

The comparisonx <y, wherex andy are values, producestrue,false, orundefined (which indicates that at least one operand isNaN). In addition tox andy the algorithm takes a Boolean flag namedLeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed uponx andy. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value ofLeftFirst istrue and indicates that thex parameter corresponds to an expression that occurs to the left of they parameter’s corresponding expression. IfLeftFirst isfalse, the reverse is the case and operations must be performed upony beforex. Such a comparison is performed as follows:

  1. If theLeftFirst flag istrue, then
    1. Letpx be the result of callingToPrimitive(x, hint Number).
    2. Letpy be the result of callingToPrimitive(y, hint Number).
  2. Else the order of evaluation needs to be reversed to preserve left to right evaluation
    1. Letpy be the result of callingToPrimitive(y, hint Number).
    2. Letpx be the result of callingToPrimitive(x, hint Number).
  3. If it is not the case that bothType(px) is String andType(py) is String, then
    1. Letnx be the result of callingToNumber(px). Becausepx andpy are primitive values evaluation order is not important.
    2. Letny be the result of callingToNumber(py).
    3. Ifnx isNaN, returnundefined.
    4. Ifny isNaN, returnundefined.
    5. Ifnx andny are the same Number value, returnfalse.
    6. Ifnx is+0 andny is−0, returnfalse.
    7. Ifnx is−0 andny is+0, returnfalse.
    8. Ifnx is+∞, returnfalse.
    9. Ifny is+∞, returntrue.
    10. Ifny is−∞, returnfalse.
    11. Ifnx is−∞, returntrue.
    12. If the mathematical value ofnx is less than the mathematical value ofny —note that these mathematical values are both finite and not both zero—returntrue. Otherwise, returnfalse.
  4. Else, bothpx andpy are Strings
    1. Ifpy is a prefix ofpx, returnfalse. (A String valuep is a prefix of String valueq ifq can be the result of concatenatingp and some other Stringr. Note that any String is a prefix of itself, becauser may be the empty String.)
    2. Ifpx is a prefix ofpy, returntrue.
    3. Letk be the smallest nonnegative integer such that the character at positionk withinpx is different from the character at positionk withinpy. (There must be such ak, for neither String is a prefix of the other.)
    4. Letm be the integer that is the code unit value for the character at positionk withinpx.
    5. Letn be the integer that is the code unit value for the character at positionk withinpy.
    6. Ifm <n, returntrue. Otherwise, returnfalse.

NOTE 1 Step 3 differs from step 7 in the algorithm for the addition operator+ (11.6.1) in using and instead of or.

NOTE 2 The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.

11.8.6 The instanceof operator

The productionRelationalExpression:RelationalExpressioninstanceofShiftExpression is evaluated as follows:

  1. Letlref be the result of evaluatingRelationalExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingShiftExpression.
  4. Letrval beGetValue(rref).
  5. IfType(rval) is not Object, throw aTypeError exception.
  6. Ifrval does not have a [[HasInstance]] internal method, throw aTypeError exception.
  7. Return the result of calling the [[HasInstance]] internal method ofrval with argumentlval.

11.8.7 The in operator

The productionRelationalExpression:RelationalExpressioninShiftExpression is evaluated as follows:

  1. Letlref be the result of evaluatingRelationalExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingShiftExpression.
  4. Letrval beGetValue(rref).
  5. IfType(rval) is not Object, throw aTypeError exception.
  6. Return the result of calling the [[HasProperty]] internal method ofrval with argumentToString(lval).

11.9 Equality Operators

Syntax

EqualityExpression:
RelationalExpression
EqualityExpression==RelationalExpression
EqualityExpression!=RelationalExpression
EqualityExpression===RelationalExpression
EqualityExpression!==RelationalExpression
EqualityExpressionNoIn:
RelationalExpressionNoIn
EqualityExpressionNoIn==RelationalExpressionNoIn
EqualityExpressionNoIn!=RelationalExpressionNoIn
EqualityExpressionNoIn===RelationalExpressionNoIn
EqualityExpressionNoIn!==RelationalExpressionNoIn

Semantics

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

TheEqualityExpressionNoIn productions are evaluated in the same manner as theEqualityExpression productions except that the containedEqualityExpressionNoIn andRelationalExpressionNoIn are evaluated instead of the containedEqualityExpression andRelationalExpression, respectively.

11.9.1 The Equals Operator (== )

The production EqualityExpression :EqualityExpression==RelationalExpression is evaluated as follows:

  1. Letlref be the result of evaluatingEqualityExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingRelationalExpression.
  4. Letrval beGetValue(rref).
  5. Return the result of performing abstract equality comparisonrval ==lval. (see 11.9.3).

11.9.2 The Does-not-equals Operator (!= )

The production EqualityExpression:EqualityExpression!=RelationalExpression is evaluated as follows:

  1. Letlref be the result of evaluatingEqualityExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingRelationalExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of performing abstract equality comparisonrval ==lval. (see 11.9.3).
  6. Ifr istrue, returnfalse. Otherwise, returntrue.

11.9.3 The Abstract Equality Comparison Algorithm

The comparisonx ==y, wherex andy are values, producestrue orfalse. Such a comparison is performed as follows:

  1. IfType(x) is the same asType(y), then
    1. IfType(x) is Undefined, returntrue.
    2. IfType(x) is Null, returntrue.
    3. IfType(x) is Number, then
      1. Ifx isNaN, returnfalse.
      2. Ify isNaN, returnfalse.
      3. Ifx is the same Number value asy, returntrue.
      4. Ifx is+0 andy is−0, returntrue.
      5. Ifx is−0 andy is+0, returntrue.
      6. Returnfalse.
    4. IfType(x) is String, then returntrue ifx andy are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, returnfalse.
    5. IfType(x) is Boolean, returntrue ifx andy are bothtrue or bothfalse. Otherwise, returnfalse.
    6. Returntrue ifx andy refer to the same object. Otherwise, returnfalse.
  2. Ifx isnull andy isundefined, returntrue.
  3. Ifx isundefined andy isnull, returntrue.
  4. IfType(x) is Number andType(y) is String,
    return the result of the comparisonx ==ToNumber(y).
  5. IfType(x) is String andType(y) is Number,
    return the result of the comparisonToNumber(x) ==y.
  6. IfType(x) is Boolean, return the result of the comparisonToNumber(x) ==y.
  7. IfType(y) is Boolean, return the result of the comparisonx ==ToNumber(y).
  8. IfType(x) is either String or Number andType(y) is Object,
    return the result of the comparisonx ==ToPrimitive(y).
  9. IfType(x) is Object andType(y) is either String or Number,
    return the result of the comparisonToPrimitive(x) ==y.
  10. Returnfalse.

NOTE 1 Given the above definition of equality:

  • String comparison can be forced by:"" + a == "" + b.
  • Numeric comparison can be forced by:+a == +b.
  • Boolean comparison can be forced by:!a == !b.

NOTE 2 The equality operators maintain the following invariants:

  • A!=B is equivalent to!(A==B).
  • A==B is equivalent toB==A, except in the order of evaluation ofA andB.

NOTE 3 The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the== operator, but the two String objects would not be equal to each other. For Example:

  • new String("a")=="a" and"a"==new String("a")are bothtrue.
  • new String("a")==new String("a") isfalse.

NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form.

11.9.4 The Strict Equals Operator (=== )

The production EqualityExpression:EqualityExpression===RelationalExpression is evaluated as follows:

  1. Letlref be the result of evaluatingEqualityExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingRelationalExpression.
  4. Letrval beGetValue(rref).
  5. Return the result of performing the strict equality comparisonrval ===lval. (See11.9.6)

11.9.5 The Strict Does-not-equal Operator (!== )

The production EqualityExpression:EqualityExpression!==RelationalExpression is evaluated as follows:

  1. Letlref be the result of evaluatingEqualityExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingRelationalExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of performing strict equality comparisonrval ===lval. (See11.9.6)
  6. Ifr istrue, returnfalse. Otherwise, returntrue.

11.9.6 The Strict Equality Comparison Algorithm

The comparisonx ===y, wherex andy are values, producestrue orfalse. Such a comparison is performed as follows:

  1. IfType(x) is different fromType(y), returnfalse.
  2. IfType(x) is Undefined, returntrue.
  3. IfType(x) is Null, returntrue.
  4. IfType(x) is Number, then
    1. Ifx isNaN, returnfalse.
    2. Ify isNaN, returnfalse.
    3. Ifx is the same Number value asy, returntrue.
    4. Ifx is+0 andy is−0, returntrue.
    5. Ifx is−0 andy is+0, returntrue.
    6. Returnfalse.
  5. IfType(x) is String, then returntrue ifx andy are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, returnfalse.
  6. IfType(x) is Boolean, returntrue ifx andy are bothtrue or bothfalse; otherwise, returnfalse.
  7. Returntrue ifx andy refer to the same object. Otherwise, returnfalse.

NOTE This algorithm differs fromthe SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.

11.10 Binary Bitwise Operators

Syntax

BitwiseANDExpression:
EqualityExpression
BitwiseANDExpression&EqualityExpression
BitwiseANDExpressionNoIn:
EqualityExpressionNoIn
BitwiseANDExpressionNoIn&EqualityExpressionNoIn
BitwiseXORExpression:
BitwiseANDExpression
BitwiseXORExpression^BitwiseANDExpression
BitwiseXORExpressionNoIn:
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn^BitwiseANDExpressionNoIn
BitwiseORExpression:
BitwiseXORExpression
BitwiseORExpression|BitwiseXORExpression
BitwiseORExpressionNoIn:
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn|BitwiseXORExpressionNoIn

Semantics

The productionA:A @B, where @ is one of the bitwise operators in the productions above, is evaluated as follows:

  1. Letlref be the result of evaluatingA.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingB.
  4. Letrval beGetValue(rref).
  5. Letlnum beToInt32(lval).
  6. Letrnum beToInt32(rval).
  7. Return the result of applying the bitwise operator @ tolnum andrnum. The result is a signed 32 bit integer.

11.11 Binary Logical Operators

Syntax

LogicalANDExpression:
BitwiseORExpression
LogicalANDExpression&&BitwiseORExpression
LogicalANDExpressionNoIn:
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn&&BitwiseORExpressionNoIn
LogicalORExpression:
LogicalANDExpression
LogicalORExpression||LogicalANDExpression
LogicalORExpressionNoIn:
LogicalANDExpressionNoIn
LogicalORExpressionNoIn||LogicalANDExpressionNoIn

Semantics

The productionLogicalANDExpression:LogicalANDExpression&&BitwiseORExpression is evaluated as follows:

  1. Letlref be the result of evaluatingLogicalANDExpression.
  2. Letlval beGetValue(lref).
  3. IfToBoolean(lval) isfalse, returnlval.
  4. Letrref be the result of evaluatingBitwiseORExpression.
  5. ReturnGetValue(rref).

The productionLogicalORExpression:LogicalORExpression||LogicalANDExpression is evaluated as follows:

  1. Letlref be the result of evaluatingLogicalORExpression.
  2. Letlval beGetValue(lref).
  3. IfToBoolean(lval) istrue, returnlval.
  4. Letrref be the result of evaluatingLogicalANDExpression.
  5. ReturnGetValue(rref).

TheLogicalANDExpressionNoIn andLogicalORExpressionNoIn productions are evaluated in the same manner as theLogicalANDExpression andLogicalORExpression productions except that the containedLogicalANDExpressionNoIn,BitwiseORExpressionNoIn andLogicalORExpressionNoIn are evaluated instead of the containedLogicalANDExpression,BitwiseORExpression andLogicalORExpression, respectively.

NOTE The value produced by a&& or|| operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

11.12 Conditional Operator (? : )

Syntax

ConditionalExpression:
LogicalORExpression
LogicalORExpression?AssignmentExpression:AssignmentExpression
ConditionalExpressionNoIn:
LogicalORExpressionNoIn
LogicalORExpressionNoIn?AssignmentExpression:AssignmentExpressionNoIn

Semantics

The productionConditionalExpression:LogicalORExpression?AssignmentExpression:AssignmentExpression is evaluated as follows:

  1. Letlref be the result of evaluatingLogicalORExpression.
  2. IfToBoolean(GetValue(lref)) istrue, then
    1. LettrueRef be the result of evaluating the firstAssignmentExpression.
    2. ReturnGetValue(trueRef).
  3. Else
    1. LetfalseRef be the result of evaluating the secondAssignmentExpression.
    2. ReturnGetValue(falseRef).

TheConditionalExpressionNoIn production is evaluated in the same manner as theConditionalExpression production except that the containedLogicalORExpressionNoIn,AssignmentExpression andAssignmentExpressionNoIn are evaluated instead of the containedLogicalORExpression, firstAssignmentExpression and secondAssignmentExpression, respectively.

NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

11.13 Assignment Operators

Syntax

AssignmentExpression:
ConditionalExpression
LeftHandSideExpression=AssignmentExpression
LeftHandSideExpressionAssignmentOperatorAssignmentExpression
AssignmentExpressionNoIn:
ConditionalExpressionNoIn
LeftHandSideExpression=AssignmentExpressionNoIn
LeftHandSideExpressionAssignmentOperatorAssignmentExpressionNoIn
AssignmentOperator:one of
*=/=%=+=-=<<=>>=>>>=&=^=|=

Semantics

TheAssignmentExpressionNoIn productions are evaluated in the same manner as theAssignmentExpression productions except that the containedConditionalExpressionNoIn andAssignmentExpressionNoIn are evaluated instead of the containedConditionalExpression andAssignmentExpression, respectively.

11.13.1 Simple Assignment (= )

The productionAssignmentExpression:LeftHandSideExpression=AssignmentExpression is evaluated as follows:

  1. Letlref be the result of evaluatingLeftHandSideExpression.
  2. Letrref be the result of evaluatingAssignmentExpression.
  3. Letrval beGetValue(rref).
  4. Throw aSyntaxError exception if the following conditions are all true:
  1. CallPutValue(lref,rval).
  2. Returnrval.

NOTE When an assignment occurs withinstrict mode code, itsLeftHandSide must not evaluate to an unresolvable reference. If it does aReferenceError exception is thrown upon assignment. TheLeftHandSide also may not be a reference to a data property with the attribute value{[[Writable]]:false}, to an accessor property with the attribute value{[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal property has the valuefalse. In these cases aTypeError exception is thrown.

11.13.2 Compound Assignment (op= )

The productionAssignmentExpression:LeftHandSideExpressionAssignmentOperatorAssignmentExpression , whereAssignmentOperator is @= and @ represents one of the operators indicated above, is evaluated as follows:

  1. Letlref be the result of evaluatingLeftHandSideExpression.
  2. Letlval beGetValue(lref).
  3. Letrref be the result of evaluatingAssignmentExpression.
  4. Letrval beGetValue(rref).
  5. Letr be the result of applying operator @ tolval andrval.
  6. Throw aSyntaxError exception if the following conditions are all true:
  1. CallPutValue(lref,r).
  2. Returnr.

NOTE See NOTE 11.13.1.

11.14 Comma Operator (, )

Syntax

Expression:
AssignmentExpression
Expression,AssignmentExpression
ExpressionNoIn:
AssignmentExpressionNoIn
ExpressionNoIn,AssignmentExpressionNoIn

Semantics

The productionExpression:Expression,AssignmentExpression is evaluated as follows:

  1. Letlref be the result of evaluatingExpression.
  2. CallGetValue(lref).
  3. Letrref be the result of evaluatingAssignmentExpression.
  4. ReturnGetValue(rref).

TheExpressionNoIn production is evaluated in the same manner as theExpression production except that the containedExpressionNoIn andAssignmentExpressionNoIn are evaluated instead of the containedExpression andAssignmentExpression, respectively.

NOTEGetValue must be called even though its value is not used because it may have observable side-effects.

12 Statements

Syntax

Statement:
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement

Semantics

AStatement can be part of aLabelledStatement, which itself can be part of aLabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements. ALabelledStatement has no semantic meaning other than the introduction of a label to alabel set. The label set of anIterationStatement or aSwitchStatement initially contains the single elementempty. The label set of any other statement is initially empty.

The result of evaluating aStatement is always aCompletion value.

NOTE Several widely used implementations of ECMAScript are known to support the use ofFunctionDeclaration as aStatement. However there are significant and irreconcilable variations among the implementations in the semantics applied to suchFunctionDeclarations. Because of these irreconcilable differences, the use of aFunctionDeclaration as aStatement results in code that is not reliably portable among implementations. It is recommended that ECMAScript implementations either disallow this usage ofFunctionDeclaration or issue a warning when such a usage is encountered. Future editions of ECMAScript may define alternative portable means for declaring functions in aStatement context.

12.1 Block

Syntax

Block:
{StatementListopt}
StatementList:
Statement
StatementListStatement

Semantics

The productionBlock:{} is evaluated as follows:

  1. Return (normal,empty,empty).

The productionBlock:{StatementList} is evaluated as follows:

  1. Return the result of evaluatingStatementList.

The productionStatementList:Statement is evaluated as follows:

  1. Lets be the result of evaluatingStatement.
  2. If an exception was thrown, return (throw,V,empty) whereV is the exception. (Execution now proceeds as if no exception were thrown.)
  3. Returns.

The productionStatementList:StatementListStatement is evaluated as follows:

  1. Letsl be the result of evaluatingStatementList.
  2. Ifsl is anabrupt completion, returnsl.
  3. Lets be the result of evaluatingStatement.
  4. If an exception was thrown, return (throw,V,empty) whereV is the exception. (Execution now proceeds as if no exception were thrown.)
  5. Ifs.value isempty, letV =sl.value, otherwise letV =s.value.
  6. Return (s.type,V,s.target).

NOTE Steps 5 and 6 of the above algoritm ensure that the value of aStatementList is the value of the last value producingStatement in theStatementList. For example, the following calls to theeval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

12.2 Variable Statement

Syntax

VariableStatement:
varVariableDeclarationList;
VariableDeclarationList:
VariableDeclaration
VariableDeclarationList,VariableDeclaration
VariableDeclarationListNoIn:
VariableDeclarationNoIn
VariableDeclarationListNoIn,VariableDeclarationNoIn
VariableDeclaration:
IdentifierInitialiseropt
VariableDeclarationNoIn:
IdentifierInitialiserNoInopt
Initialiser:
=AssignmentExpression
InitialiserNoIn:
=AssignmentExpressionNoIn

A variable statement declares variables that are created as defined in10.5. Variables are initialised toundefined when created. A variable with anInitialiser is assigned the value of itsAssignmentExpression when theVariableStatement is executed, not when the variable is created.

Semantics

The productionVariableStatement:varVariableDeclarationList; is evaluated as follows:

  1. EvaluateVariableDeclarationList.
  2. Return (normal,empty,empty).

The productionVariableDeclarationList:VariableDeclaration is evaluated as follows:

  1. EvaluateVariableDeclaration.

The productionVariableDeclarationList:VariableDeclarationList,VariableDeclaration is evaluated as follows:

  1. EvaluateVariableDeclarationList.
  2. EvaluateVariableDeclaration.

The productionVariableDeclaration:Identifier is evaluated as follows:

  1. Return a String value containing the same sequence of characters as in theIdentifier.

The productionVariableDeclaration:IdentifierInitialiser is evaluated as follows:

  1. Letlhs be the result of evaluatingIdentifier as described in11.1.2.
  2. Letrhs be the result of evaluatingInitialiser.
  3. Letvalue beGetValue(rhs).
  4. CallPutValue(lhs,value).
  5. Return a String value containing the same sequence of characters as in theIdentifier.

NOTE The String value of aVariableDeclaration is used in the evaluation of for-in statements (12.6.4).

If aVariableDeclaration is nested within a with statement and the Identifier in theVariableDeclaration is the same as a property name of the binding object of the with statement’sobject environment record, then step 4 will assign value to the property instead of to theVariableEnvironment binding of theIdentifier.

The productionInitialiser:=AssignmentExpression is evaluated as follows:

  1. Return the result of evaluatingAssignmentExpression.

TheVariableDeclarationListNoIn,VariableDeclarationNoIn andInitialiserNoIn productions are evaluated in the same manner as theVariableDeclarationList,VariableDeclaration andInitialiser productions except that the containedVariableDeclarationListNoIn,VariableDeclarationNoIn,InitialiserNoIn andAssignmentExpressionNoIn are evaluated instead of the containedVariableDeclarationList,VariableDeclaration,Initialiser andAssignmentExpression, respectively.

12.2.1 Strict Mode Restrictions

It is aSyntaxError if aVariableDeclaration orVariableDeclarationNoIn occurs withinstrict code and itsIdentifier is either"eval"or"arguments".

12.3 Empty Statement

Syntax

EmptyStatement:
;

Semantics

The productionEmptyStatement:; is evaluated as follows:

  1. Return (normal,empty,empty).

12.4 Expression Statement

Syntax

ExpressionStatement:
[lookahead ∉ {{,function}]Expression;

NOTE AnExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with aBlock. Also, anExpressionStatement cannot start with thefunction keyword because that might make it ambiguous with aFunctionDeclaration.

Semantics

The productionExpressionStatement:[lookahead ∉ {{,function}]Expression; is evaluated as follows:

  1. LetexprRef be the result of evaluatingExpression.
  2. Return (normal,GetValue(exprRef),empty).

12.5 Theif Statement

Syntax

IfStatement:
if(Expression)StatementelseStatement
if(Expression)Statement

Eachelse for which the choice of associatedif is ambiguous shall be associated with the nearest possibleif that would otherwise have no correspondingelse.

Semantics

The productionIfStatement:if(Expression)StatementelseStatement is evaluated as follows:

  1. LetexprRef be the result of evaluatingExpression.
  2. IfToBoolean(GetValue(exprRef)) istrue, then
    1. Return the result of evaluating the firstStatement.
  3. Else,
    1. Return the result of evaluating the secondStatement.

The productionIfStatement:if(Expression)Statement is evaluated as follows:

  1. LetexprRef be the result of evaluatingExpression.
  2. IfToBoolean(GetValue(exprRef)) isfalse, return (normal,empty,empty).
  3. Return the result of evaluatingStatement.

12.6 Iteration Statements

Syntax

IterationStatement:
doStatementwhile(Expression);
while(Expression)Statement
for(ExpressionNoInopt;Expressionopt;Expressionopt)Statement
for(varVariableDeclarationListNoIn;Expressionopt;Expressionopt)Statement
for(LeftHandSideExpressioninExpression)Statement
for(varVariableDeclarationNoIninExpression)Statement

12.6.1 Thedo-while Statement

The productiondoStatementwhile(Expression); is evaluated as follows:

  1. LetV =empty.
  2. Letiterating betrue.
  3. Repeat, whileiterating istrue
    1. Letstmt be the result of evaluatingStatement.
    2. Ifstmt.value is notempty, letV =stmt.value.
    3. Ifstmt.type is notcontinue ||stmt.target is not in the current label set, then
      1. Ifstmt.type isbreak andstmt.target is in the current label set, return (normal,V,empty).
      2. Ifstmt is anabrupt completion, returnstmt.
    4. LetexprRef be the result of evaluatingExpression.
    5. IfToBoolean(GetValue(exprRef)) isfalse, setiterating tofalse.
  4. Return (normal,V,empty);

12.6.2 Thewhile Statement

The productionIterationStatement:while(Expression)Statement is evaluated as follows:

  1. LetV =empty.
  2. Repeat
    1. LetexprRef be the result of evaluatingExpression.
    2. IfToBoolean(GetValue(exprRef)) isfalse, return (normal,V,empty).
    3. Letstmt be the result of evaluatingStatement.
    4. Ifstmt.value is notempty, letV =stmt.value.
    5. Ifstmt.type is notcontinue ||stmt.target is not in the current label set, then
      1. Ifstmt.type isbreak andstmt.target is in the current label set, then
        1. Return (normal,V,empty).
      2. Ifstmt is anabrupt completion, returnstmt.

12.6.3 Thefor Statement

The production
IterationStatement:for(ExpressionNoInopt;Expressionopt;Expressionopt)Statement
is evaluated as follows:

  1. IfExpressionNoIn is present, then.
    1. LetexprRef be the result of evaluatingExpressionNoIn.
    2. CallGetValue(exprRef). (This value is not used but the call may have side-effects.)
  2. LetV =empty.
  3. Repeat
    1. If the firstExpression is present, then
      1. LettestExprRef be the result of evaluating the firstExpression.
      2. IfToBoolean(GetValue(testExprRef)) isfalse, return (normal,V,empty).
    2. Letstmt be the result of evaluatingStatement.
    3. Ifstmt.value is notempty, letV =stmt.value
    4. Ifstmt.type isbreak andstmt.target is in the current label set, return (normal,V,empty).
    5. Ifstmt.type is notcontinue ||stmt.target is not in the current label set, then
      1. Ifstmt is anabrupt completion, returnstmt.
    6. If the secondExpression is present, then
      1. LetincExprRef be the result of evaluating the secondExpression.
      2. CallGetValue(incExprRef). (This value is not used.)

The production
IterationStatement:for(varVariableDeclarationListNoIn;Expressionopt;Expressionopt)Statement
is evaluated as follows:

  1. EvaluateVariableDeclarationListNoIn.
  2. Let V =empty.
  3. Repeat
    1. If the firstExpression is present, then
      1. LettestExprRef be the result of evaluating the firstExpression.
      2. IfToBoolean(GetValue(testExprRef)) isfalse, then return (normal, V,empty).
    2. Letstmt be the result of evaluatingStatement.
    3. Ifstmt.value is notempty, let V =stmt.value.
    4. Ifstmt.type isbreak andstmt.target is in the current label set, return (normal, V,empty).
    5. Ifstmt.type is notcontinue ||stmt.target is not in the current label set, then
      1. Ifstmt is anabrupt completion, returnstmt.
    6. If the secondExpression is present, then.
      1. LetincExprRef be the result of evaluating the secondExpression.
      2. CallGetValue(incExprRef). (This value is not used.)

12.6.4 Thefor-in Statement

The productionIterationStatement:for(LeftHandSideExpressioninExpression)Statement is evaluated as follows:

  1. LetexprRef be the result of evaluating theExpression.
  2. LetexperValue beGetValue(exprRef).
  3. IfexperValue isnull orundefined, return (normal,empty,empty).
  4. Letobj beToObject(experValue).
  5. LetV =empty.
  6. Repeat
    1. LetP be the name of the next property ofobj whose [[Enumerable]] attribute istrue. If there is no such property, return (normal,V,empty).
    2. LetlhsRef be the result of evaluating theLeftHandSideExpression ( it may be evaluated repeatedly).
    3. CallPutValue(lhsRef,P).
    4. Letstmt be the result of evaluatingStatement.
    5. Ifstmt.value is notempty, letV =stmt.value.
    6. Ifstmt.type isbreak andstmt.target is in the current label set, return (normal,V,empty).
    7. Ifstmt.type is notcontinue ||stmt.target is not in the current label set, then
      1. Ifstmt is anabrupt completion, returnstmt.

The production
IterationStatement:for(varVariableDeclarationNoIninExpression)Statement
is evaluated as follows:

  1. LetvarName be the result of evaluatingVariableDeclarationNoIn.
  2. LetexprRef be the result of evaluating theExpression.
  3. LetexperValue beGetValue(exprRef).
  4. IfexperValue isnull orundefined, return (normal,empty,empty).
  5. Letobj beToObject(experValue).
  6. LetV =empty.
  7. Repeat
    1. LetP be the name of the next property ofobj whose [[Enumerable]] attribute istrue. If there is no such property, return (normal,V,empty).
    2. LetvarRef be the result of evaluatingvarName as if it were an IdentifierReference (11.1.2); it may be evaluated repeatedly.
    3. CallPutValue(varRef,P).
    4. Letstmt be the result of evaluatingStatement.
    5. Ifstmt.value is notempty, letV =stmt.value.
    6. Ifstmt.type isbreak andstmt.target is in the current label set, return (normal,V,empty).
    7. Ifstmt.type is notcontinue ||stmt.target is not in the current label set, then
      1. Ifstmt is anabrupt completion, returnstmt.

The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in the second) is not specified. Properties of the object being enumerated may be deleted during enumeration. If a property that has not yet been visited during enumeration is deleted, then it will not be visited. If new properties are added to the object being enumerated during enumeration, the newly added properties are not guaranteed to be visited in the active enumeration. A property name must not be visited more than once in any enumeration.

Enumerating the properties of an object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not enumerated if it is “shadowed” because some previous object in the prototype chain has a property with the same name. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object is shadowed by a previous object on the prototype chain.

NOTE See NOTE 11.13.1.

12.7 Thecontinue Statement

Syntax

ContinueStatement:
continue;
continue[noLineTerminator here]Identifier;

Semantics

A program is considered syntactically incorrect if either of the following is true:

AContinueStatement without anIdentifier is evaluated as follows:

  1. Return (continue,empty,empty).

AContinueStatement with the optionalIdentifier is evaluated as follows:

  1. Return (continue,empty,Identifier).

12.8 Thebreak Statement

Syntax

BreakStatement:
break;
break[noLineTerminator here]Identifier;

Semantics

A program is considered syntactically incorrect if either of the following is true:

ABreakStatement without anIdentifier is evaluated as follows:

  1. Return (break,empty,empty).

ABreakStatement with anIdentifier is evaluated as follows:

  1. Return (break,empty,Identifier).

12.9 Thereturn Statement

Syntax

ReturnStatement:
return;
return[noLineTerminator here]Expression;

Semantics

An ECMAScript program is considered syntactically incorrect if it contains areturn statement that is not within aFunctionBody. Areturn statement causes a function to cease execution and return a value to the caller. IfExpression is omitted, the return value isundefined. Otherwise, the return value is the value ofExpression.

AReturnStatement is evaluated as follows:

  1. If theExpression is not present, return (return,undefined,empty).
  2. LetexprRef be the result of evaluatingExpression.
  3. Return (return,GetValue(exprRef),empty).

12.10 Thewith Statement

Syntax

WithStatement:
with(Expression)Statement

Thewith statement adds anobject environment record for a computed object to thelexical environment of the current execution context. It then executes a statement using this augmentedlexical environment. Finally, it restores the originallexical environment.

Semantics

The productionWithStatement:with(Expression)Statement is evaluated as follows:

  1. Letval be the result of evaluatingExpression.
  2. Letobj beToObject(GetValue(val)).
  3. LetoldEnv be the running execution context’sLexicalEnvironment.
  4. LetnewEnv be the result of callingNewObjectEnvironment passingobj andoldEnv as the arguments.
  5. Set theprovideThis flag ofnewEnv totrue.
  6. Set the running execution context’sLexicalEnvironment tonewEnv.
  7. LetC be the result of evaluatingStatement but if an exception is thrown during the evaluation, letC be (throw,V,empty), whereV is the exception. (Execution now proceeds as if no exception were thrown.)
  8. Set the running execution context’sLexical Environment tooldEnv.
  9. ReturnC.

NOTE No matter how control leaves the embeddedStatement, whether normally or by some form ofabrupt completion or exception, theLexicalEnvironment is always restored to its former state.

12.10.1 Strict Mode Restrictions

Strict mode code may not include aWithStatement. The occurrence of aWithStatement in such a context is treated as aSyntaxError.

12.11 Theswitch Statement

Syntax

SwitchStatement:
switch(Expression)CaseBlock
CaseBlock:
{CaseClausesopt}
{CaseClausesoptDefaultClauseCaseClausesopt}
CaseClauses:
CaseClause
CaseClausesCaseClause
CaseClause:
caseExpression:StatementListopt
DefaultClause:
default:StatementListopt

Semantics

The productionSwitchStatement:switch(Expression)CaseBlock is evaluated as follows:

  1. LetexprRef be the result of evaluatingExpression.
  2. LetR be the result of evaluatingCaseBlock, passing itGetValue(exprRef) as a parameter.
  3. IfR.type isbreak andR.target is in the current label set, return (normal,R.value,empty).
  4. ReturnR.

The productionCaseBlock:{CaseClausesopt} is given an input parameter,input, and is evaluated as follows:

  1. LetV =empty.
  2. LetA be the list ofCaseClause items in source text order.
  3. Letsearching betrue.
  4. Repeat, whilesearching istrue
    1. LetC be the nextCaseClause inA. If there is no suchCaseClause, return (normal,V,empty).
    2. LetclauseSelector be the result of evaluatingC.
    3. Ifinput is equal toclauseSelector as defined by the=== operator, then
      1. Setsearching tofalse.
      2. IfC has aStatementList, then
        1. EvaluateC’sStatementList and letR be the result.
        2. IfR is anabrupt completion, then returnR.
        3. LetV =R.value.
  5. Repeat
    1. LetC be the nextCaseClause inA. If there is no suchCaseClause, return(normal,V,empty).
    2. IfC has aStatementList, then
      1. EvaluateC’sStatementList and letR be the result.
      2. IfR.value is notempty, then letV =R.value.
      3. IfR is anabrupt completion, then return(R.type,V,R.target).

The productionCaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt} is given an input parameter,input, and is evaluated as follows:

  1. LetV =empty.
  2. LetA be the list ofCaseClause items in the firstCaseClauses, in source text order.
  3. LetB be the list ofCaseClause items in the secondCaseClauses, in source text order.
  4. Letfound befalse.
  5. Repeat lettingC be in order eachCaseClause inA
    1. Iffound isfalse, then
      1. LetclauseSelector be the result of evaluatingC.
      2. Ifinput is equal toclauseSelector as defined by the=== operator, then setfound totrue.
    2. Iffound istrue, then
      1. IfC has aStatementList, then
        1. EvaluateC’sStatementList and letR be the result.
        2. IfR.value is notempty, then letV =R.value.
        3. R is anabrupt completion, then return(R.type,V,R.target).
  6. LetfoundInB befalse.
  7. Iffound isfalse, then
    1. Repeat, whilefoundInB isfalse and all elements ofB have not been processed
      1. LetC be the nextCaseClause inB.
      2. LetclauseSelector be the result of evaluatingC.
      3. Ifinput is equal toclauseSelector as defined by the=== operator, then
        1. SetfoundInB totrue.
        2. IfC has aStatementList, then
          1. EvaluateC’sStatementList and letR be the result.
          2. IfR.value is notempty, then letV =R.value.
          3. R is anabrupt completion, then return(R.type,V,R.target).
  8. IffoundInB isfalse and theDefaultClause has aStatementList, then
    1. Evaluate theDefaultClause’sStatementList and letR be the result.
    2. IfR.value is notempty, then letV =R.value.
    3. IfR is anabrupt completion, then return (R.type,V,R.target).
  9. Repeat (Note that if step 7.a.i has been performed this loop does not start at the beginning ofB)
    1. LetC be the nextCaseClause inB. If there is no suchCaseClause, return (normal,V,empty).
    2. IfC has aStatementList, then
      1. EvaluateC’sStatementList and letR be the result.
      2. IfR.value is notempty, then letV =R.value.
      3. IfR is anabrupt completion, then return (R.type,V,R.target).

The productionCaseClause:caseExpression:StatementListopt is evaluated as follows:

  1. LetexprRef be the result of evaluatingExpression.
  2. ReturnGetValue(exprRef).

NOTE EvaluatingCaseClause does not execute the associatedStatementList. It simply evaluates theExpression and returns the value, which theCaseBlock algorithm uses to determine whichStatementList to start executing.

12.12 Labelled Statements

Syntax

LabelledStatement:
Identifier:Statement

Semantics

AStatement may be prefixed by a label. Labelled statements are only used in conjunction with labelledbreak andcontinue statements. ECMAScript has nogoto statement.

An ECMAScript program is considered syntactically incorrect if it contains aLabelledStatement that is enclosed by aLabelledStatement with the sameIdentifier as label. This does not apply to labels appearing within the body of aFunctionDeclaration that is nested, directly or indirectly, within a labelled statement.

The productionIdentifier:Statement is evaluated by addingIdentifier to the label set ofStatement and then evaluatingStatement. If theLabelledStatement itself has a non-empty label set, these labels are also added to the label set ofStatement before evaluating it. If the result of evaluatingStatement is (break,V,L) whereL is equal toIdentifier, the production results in (normal,V,empty).

Prior to the evaluation of aLabelledStatement, the containedStatement is regarded as possessing an empty label set, unless it is anIterationStatement or aSwitchStatement, in which case it is regarded as possessing a label set consisting of the single element,empty.

12.13 Thethrow Statement

Syntax

ThrowStatement:
throw[noLineTerminator here]Expression;

Semantics

The productionThrowStatement:throw[noLineTerminator here]Expression; is evaluated as follows:

  1. LetexprRef be the result of evaluatingExpression.
  2. Return (throw,GetValue(exprRef),empty).

12.14 Thetry Statement

Syntax

TryStatement:
tryBlockCatch
tryBlockFinally
tryBlockCatchFinally
Catch:
catch(Identifier)Block
Finally:
finallyBlock

Thetry statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or athrow statement. Thecatch clause provides the exception-handling code. When a catch clause catches an exception, itsIdentifier is bound to that exception.

Semantics

The productionTryStatement:tryBlockCatch is evaluated as follows:

  1. LetB be the result of evaluatingBlock.
  2. IfB.type is notthrow, returnB.
  3. Return the result of evaluatingCatch with parameterB.value.

The productionTryStatement:tryBlockFinally is evaluated as follows:

  1. LetB be the result of evaluatingBlock.
  2. LetF be the result of evaluatingFinally.
  3. IfF.type isnormal, returnB.
  4. ReturnF.

The productionTryStatement:tryBlockCatchFinally is evaluated as follows:

  1. LetB be the result of evaluatingBlock.
  2. IfB.type isthrow, then
    1. LetC be the result of evaluatingCatch with parameterB.value.
  3. Else,B.type is notthrow,
    1. LetC beB.
  4. LetF be the result of evaluatingFinally.
  5. IfF.type isnormal, returnC.
  6. ReturnF.

The productionCatch:catch(Identifier)Block is evaluated as follows:

  1. LetC be the parameter that has been passed to this production.
  2. LetoldEnv be the running execution context’sLexicalEnvironment.
  3. LetcatchEnv be the result of callingNewDeclarativeEnvironment passingoldEnv as the argument.
  4. Call the CreateMutableBinding concrete method ofcatchEnv passing theIdentifier String value as the argument.
  5. Call the SetMutableBinding concrete method ofcatchEnv passing theIdentifier,C, andfalse as arguments. Note that the last argument is immaterial in this situation.
  6. Set the running execution context’sLexicalEnvironment tocatchEnv.
  7. LetB be the result of evaluatingBlock.
  8. Set the running execution context’sLexicalEnvironment tooldEnv.
  9. ReturnB.

NOTE No matter how control leaves theBlock theLexicalEnvironment is always restored to its former state.

The productionFinally:finallyBlock is evaluated as follows:

  1. Return the result of evaluatingBlock.

12.14.1 Strict Mode Restrictions

It is aSyntaxError if aTryStatement with aCatch occurs withinstrict code and theIdentifier of theCatch production is either"eval"or"arguments".

12.15 Thedebugger statement

Syntax

DebuggerStatement:
debugger;

Semantics

Evaluating theDebuggerStatement production may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

The productionDebuggerStatement:debugger; is evaluated as follows:

  1. If an implementation defined debugging facility is available and enabled, then
    1. Perform an implementation defined debugging action.
    2. Letresult be an implementation definedCompletion value.
  2. Else
    1. Letresult be (normal,empty,empty).
  3. Returnresult.

13 Function Definition

Syntax

FunctionDeclaration:
functionIdentifier(FormalParameterListopt){FunctionBody}
FunctionExpression:
functionIdentifieropt(FormalParameterListopt){FunctionBody}
FormalParameterList:
Identifier
FormalParameterList,Identifier
FunctionBody:
SourceElementsopt

Semantics

The production
FunctionDeclaration:functionIdentifier(FormalParameterListopt){FunctionBody}
is instantiated as follows during Declaration Binding instantiation (10.5):

  1. Return the result of creating a new Function object as specified in13.2 with parameters specified byFormalParameterListopt, and body specified byFunctionBody. Pass in theVariableEnvironment of the running execution context as theScope. Pass intrue as theStrict flag if theFunctionDeclaration is contained instrict code or if itsFunctionBody isstrict code.

The production
FunctionExpression:function(FormalParameterListopt){FunctionBody}
is evaluated as follows:

  1. Return the result of creating a new Function object as specified in13.2 with parameters specified byFormalParameterListopt and body specified byFunctionBody. Pass in theLexicalEnvironment of the running execution context as theScope. Pass intrue as theStrict flag if theFunctionExpression is contained instrict code or if itsFunctionBody isstrict code.

The production
FunctionExpression:functionIdentifier(FormalParameterListopt){FunctionBody}
is evaluated as follows:

  1. LetfuncEnv be the result of callingNewDeclarativeEnvironment passing the running execution context’sLexical Environment as the argument
  2. LetenvRec befuncEnv’s environment record.
  3. Call the CreateImmutableBinding concrete method ofenvRec passing the String value ofIdentifier as the argument.
  4. Letclosure be the result of creating a new Function object as specified in13.2 with parameters specified byFormalParameterListopt and body specified byFunctionBody. Pass infuncEnv as theScope. Pass intrue as theStrict flag if theFunctionExpression is contained instrict code or if itsFunctionBody isstrict code.
  5. Call the InitializeImmutableBinding concrete method ofenvRec passing the String value ofIdentifier andclosure as the arguments.
  6. Returnclosure.

NOTE The Identifier in aFunctionExpression can be referenced from inside theFunctionExpression'sFunctionBody to allow the function to call itself recursively. However, unlike in aFunctionDeclaration, theIdentifier in aFunctionExpression cannot be referenced from and does not affect the scope enclosing theFunctionExpression.

The productionFunctionBody:SourceElementsopt is evaluated as follows:

  1. The code of thisFunctionBody isstrict mode code if it is part of aFunctionDeclaration orFunctionExpression that is contained instrict mode code or if theDirective Prologue (14.1) of itsSourceElements contains aUse Strict Directive or if any of the conditions in10.1.1 apply. If the code of thisFunctionBody isstrict mode code,SourceElements is evaluated in the following steps asstrict mode code. Otherwise,SourceElements is evaluated in the following steps as non-strict mode code.
  2. IfSourceElements is present return the result of evaluatingSourceElements.
  3. Else return (normal,undefined,empty).

13.1 Strict Mode Restrictions

It is aSyntaxError if anyIdentifier value occurs more than once within aFormalParameterList of a strict modeFunctionDeclaration orFunctionExpression.

It is aSyntaxError if theIdentifier"eval" or theIdentifier"arguments" occurs within aFormalParameterList of a strict modeFunctionDeclaration orFunctionExpression.

It is aSyntaxError if theIdentifier"eval" or theIdentifier"arguments" occurs as theIdentifier of a strict modeFunctionDeclaration orFunctionExpression.

13.2 Creating Function Objects

Given an optional parameter list specified byFormalParameterList, a body specified byFunctionBody, aLexical Environment specified byScope, and a Boolean flagStrict, a Function object is constructed as follows:

  1. Create a new native ECMAScript object and letF be that object.
  2. Set all the internal methods, except for [[Get]], ofF as described in8.12.
  3. Set the [[Class]] internal property ofF to"Function".
  4. Set the [[Prototype]] internal property ofF to the standard built-in Function prototype object as specified in15.3.3.1.
  5. Set the [[Get]] internal property ofF as described in15.3.5.4.
  6. Set the [[Call]] internal property ofF as described in13.2.1.
  7. Set the [[Construct]] internal property ofF as described in13.2.2.
  8. Set the [[HasInstance]] internal property ofF as described in15.3.5.3.
  9. Set the [[Scope]] internal property ofF to the value ofScope.
  10. Letnames be aList containing, in left to right textual order, the Strings corresponding to the identifiers ofFormalParameterList. If no parameters are specified, letnames be the empty list.
  11. Set the [[FormalParameters]] internal property ofF tonames.
  12. Set the [[Code]] internal property ofF toFunctionBody.
  13. Set the [[Extensible]] internal property ofF totrue.
  14. Letlen be the number of formal parameters specified inFormalParameterList. If no parameters are specified, letlen be 0.
  15. Call the [[DefineOwnProperty]] internal method ofF with arguments"length",Property Descriptor {[[Value]]:len, [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  16. Letproto be the result of creating a new object as would be constructed by the expressionnew Object()whereObject is the standard built-in constructor with that name.
  17. Call the [[DefineOwnProperty]] internal method ofproto with arguments"constructor",Property Descriptor {[[Value]]:F, { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}, andfalse.
  18. Call the [[DefineOwnProperty]] internal method ofF with arguments"prototype",Property Descriptor {[[Value]]:proto, { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  19. IfStrict istrue, then
    1. Letthrower be the [[ThrowTypeError]] function Object (13.2.3).
    2. Call the [[DefineOwnProperty]] internal method ofF with arguments"caller", PropertyDescriptor {[[Get]]:thrower, [[Set]]:thrower, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
    3. Call the [[DefineOwnProperty]] internal method ofF with arguments"arguments", PropertyDescriptor {[[Get]]:thrower, [[Set]]:thrower, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  20. ReturnF.

NOTE Aprototype property is automatically created for every function, to allow for the possibility that the function will be used as a constructor.

13.2.1 [[Call]]

When the [[Call]] internal method for a Function objectF is called with a this value and a list of arguments, the following steps are taken:

  1. LetfuncCtx be the result of establishing a new execution context for function code using the value ofF's [[FormalParameters]] internal property, the passed argumentsListargs, and thethis value as described in10.4.3.
  2. Letresult be the result of evaluating theFunctionBody that is the value ofF's [[Code]] internal property. IfF does not have a [[Code]] internal property or if its value is an emptyFunctionBody, thenresult is (normal,undefined,empty).
  3. Exit the execution contextfuncCtx, restoring the previous execution context.
  4. Ifresult.type isthrow then throwresult.value.
  5. Ifresult.type isreturn then returnresult.value.
  6. Otherwiseresult.type must benormal. Returnundefined.

13.2.2 [[Construct]]

When the [[Construct]] internal method for a Function objectF is called with a possibly empty list of arguments, the following steps are taken:

  1. Letobj be a newly created native ECMAScript object.
  2. Set all the internal methods ofobj as specified in8.12.
  3. Set the [[Class]] internal property ofobj to"Object".
  4. Set the [[Extensible]] internal property ofobj totrue.
  5. Letproto be the value of calling the [[Get]] internal property ofF with argument"prototype".
  6. IfType(proto) is Object, set the [[Prototype]] internal property ofobj toproto.
  7. IfType(proto) is not Object, set the [[Prototype]] internal property ofobj to the standard built-in Object prototype object as described in15.2.4.
  8. Letresult be the result of calling the [[Call]] internal property ofF, providingobj as thethis value and providing the argument list passed into [[Construct]] asargs.
  9. IfType(result) is Object then returnresult.
  10. Returnobj.

13.2.3 The [[ThrowTypeError]] Function Object

The [[ThrowTypeError]] object is a unique function object that is defined once as follows:

  1. Create a new native ECMAScript object and letF be that object.
  2. Set all the internal methods ofF as described in8.12.
  3. Set the [[Class]] internal property ofF to"Function".
  4. Set the [[Prototype]] internal property ofF to the standard built-in Function prototype object as specified in15.3.3.1.
  5. Set the [[Call]] internal property ofF as described in13.2.1.
  6. Set the [[Scope]] internal property ofF tothe Global Environment.
  7. Set the [[FormalParameters]] internal property ofF to an emptyList.
  8. Set the [[Code]] internal property ofF to be aFunctionBody that unconditionally throws aTypeError exception and performs no other action.
  9. Call the [[DefineOwnProperty]] internal method ofF with arguments"length",Property Descriptor {[[Value]]:0, [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  10. Set the [[Extensible]] internal property ofF tofalse.
  11. Let [[ThrowTypeError]] beF.

14 Program

Syntax

Program:
SourceElementsopt
SourceElements:
SourceElement
SourceElementsSourceElement
SourceElement:
Statement
FunctionDeclaration

Semantics

The productionProgram:SourceElementsopt is evaluated as follows:

  1. The code of thisProgram isstrict mode code if theDirective Prologue (14.1) of itsSourceElements contains aUse Strict Directive or if any of the conditions of10.1.1 apply. If the code of thisProgram isstrict mode code,SourceElements is evaluated in the following steps asstrict mode code. OtherwiseSourceElements is evaluated in the following steps as non-strict mode code.
  2. IfSourceElements is not present, return (normal,empty,empty).
  3. LetprogCxt be a new execution context for global code as described in10.4.1.
  4. Letresult be the result of evaluatingSourceElements.
  5. Exit the execution contextprogCxt.
  6. Returnresult.

NOTE The processes for initiating the evaluation of aProgram and for dealing with the result of such an evaluation are defined by an ECMAScript implementation and not by this specification.

The productionSourceElements:SourceElementsSourceElement is evaluated as follows:

  1. LetheadResult be the result of evaluatingSourceElements.
  2. IfheadResult is anabrupt completion, returnheadResult.
  3. LettailResult be result of evaluatingSourceElement.
  4. IftailResult.value isempty, letV =headResult.value, otherwise letV =tailResult.value.
  5. Return (tailResult.type, V, tailResult.target)

The productionSourceElement:Statement is evaluated as follows:

  1. Return the result of evaluatingStatement.

The productionSourceElement:FunctionDeclaration is evaluated as follows:

  1. Return (normal,empty,empty).

14.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence ofExpressionStatement productions occurring as the initialSourceElement productions of aProgram orFunctionBody and where eachExpressionStatement in the sequence consists entirely of aStringLiteral token followed a semicolon. The semicolon may appear explicitly or may be inserted byautomatic semicolon insertion. A Directive Prologue may be an empty sequence.

A Use Strict Directive is anExpressionStatement in a Directive Prologue whoseStringLiteral is either the exact character sequences"use strict" or'use strict'. A Use Strict Directive may not contain anEscapeSequence orLineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

NOTE TheExpressionStatement productions of a Directive Prologue are evaluated normally during evaluation of the containingSourceElements production. Implementations may define implementation specific meanings forExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue anExpressionStatement that is not a Use Strict Directive or which does not have a meaning defined by the implementation.

15 Standard Built-in ECMAScript Objects

There are certain built-in objects available whenever an ECMAScript program begins execution. One, the global object, is part of thelexical environment of the executing program. Others are accessible as initial properties of the global object.

Unless specified otherwise, the [[Class]] internal property of a built-in object is"Function" if that built-in object has a [[Call]] internal property, or"Object" if that built-in object does not have a [[Call]] internal property. Unless specified otherwise, the [[Extensible]] internal property of a built-in object initially has the valuetrue.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with thenew operator. For each built-in function, this specification describes the arguments required by that function and properties of the Function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by anew expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being theundefined value.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of aTypeError exception that is predicated simply on the presence of an extra argument.

NOTE Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expressionFunction.prototype (15.3.4), as the value of its [[Prototype]] internal property.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expressionObject.prototype (15.2.4), as the value of its [[Prototype]] internal property, except the Object prototype object itself.

None of the built-in functions described in this clause that are not constructors shall implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. None of the built-in functions described in this clause shall have aprototype property unless otherwise specified in the description of a particular function.

This clause generally describes distinct behaviours for when a constructor is “called as a function” and for when it is “called as part of anew expression”. The “called as a function” behaviour corresponds to the invocation of the constructor’s [[Call]] internal method and the “called as part of a new expression” behaviour corresponds to the invocation of the constructor’s [[Construct]] internal method.

Every built-in Function object described in this clause—whether as a constructor, an ordinary function, or both—has alength property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description, including optional parameters.

NOTE For example, the Function object that is the initial value of theslice property of the String prototype object is described under the subclause heading “String.prototype.slice (start, end)” which shows the two named arguments start and end; therefore the value of thelength property of that Function object is2.

In every case, thelength property of a built-in Function object described in this clause has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }. Every other property described in this clause has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true } unless otherwise specified.

15.1 The Global Object

The uniqueglobal object is created before control enters any execution context.

Unless otherwise specified, the standard built-in properties of the global object have attributes {[[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.

The global object does not have a [[Construct]] internal property; it is not possible to use the global object as a constructor with thenew operator.

The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as a function.

The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.

In addition to the properties defined in this specification the global object may have additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model thewindow property of the global object is the global object itself.

15.1.1 Value Properties of the Global Object

15.1.1.1 NaN

The value ofNaN isNaN (see 8.5). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.1.1.2 Infinity

The value ofInfinity is+∞ (see 8.5). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.1.1.3 undefined

The value ofundefined isundefined (see 8.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.1.2 Function Properties of the Global Object

15.1.2.1 eval (x)

When theeval function is called with one argumentx, the following steps are taken:

  1. IfType(x) is not String, returnx.
  2. Letprog be the ECMAScript code that is the result of parsingx as aProgram. If the parse fails, throw aSyntaxError exception (butsee also clause 16).
  3. LetevalCtx be the result of establishing a new execution context (10.4.2) for the eval codeprog.
  4. Letresult be the result of evaluating the programprog.
  5. Exit the running execution contextevalCtx, restoring the previous execution context.
  6. Ifresult.type isnormal and its completion value is a valueV, then return the valueV.
  7. Ifresult.type isnormal and its completion value isempty, then return the valueundefined.
  8. Otherwise,result.type must bethrow. Throwresult.value as an exception.

15.1.2.1.1 Direct Call to Eval

A direct call to the eval function is one that is expressed as aCallExpression that meets the following two conditions:

TheReference that is the result of evaluating theMemberExpression in theCallExpression has an environment record as its base value and its reference name is"eval".

The result of calling the abstract operationGetValue with thatReference as the argument is the standard built-in function defined in15.1.2.1.

15.1.2.2 parseInt (string , radix)

TheparseInt function produces an integer value dictated by interpretation of the contents of thestring argument according to the specifiedradix. Leading white space instring is ignored. Ifradix isundefined or 0, it is assumed to be10 except when the number begins with the character pairs0x or0X, in which case a radix of 16 is assumed. Ifradix is16, the number may also optionally begin with the character pairs0x or0X.

When theparseInt function is called, the following steps are taken:

  1. LetinputString beToString(string).
  2. LetS be a newly created substring ofinputString consisting of the first character that is not aStrWhiteSpaceChar and all characters following that character. (In other words, remove leading white space.) IfinputString does not contain any such characters, letS be the empty string.
  3. Letsign be 1.
  4. IfS is not empty and the first character ofS is a minus sign-, letsign be −1.
  5. IfS is not empty and the first character ofS is a plus sign+ or a minus sign-, then remove the first character fromS.
  6. LetR =ToInt32(radix).
  7. LetstripPrefix betrue.
  8. IfR ≠ 0, then
    1. IfR < 2 orR > 36, then returnNaN.
    2. IfR ≠ 16, letstripPrefix befalse.
  9. Else,R = 0
    1. LetR = 10.
  10. IfstripPrefix istrue, then
    1. If the length ofS is at least 2 and the first two characters ofS are either “0x” or “0X”, then remove the first two characters fromS and letR = 16.
  11. IfS contains any character that is not a radix-R digit, then letZ be the substring ofS consisting of all characters before the first such character; otherwise, letZ beS.
  12. IfZ is empty, returnNaN.
  13. LetmathInt be the mathematical integer value that is represented byZ in radix-R notation, using the lettersA-Z anda-z for digits with values 10 through 35. (However, ifR is 10 andZ contains more than 20 significant digits, every significant digit after the 20th may be replaced by a0 digit, at the option of the implementation; and ifR is not 2, 4, 8, 10, 16, or 32, thenmathInt may be an implementation-dependent approximation to the mathematical integer value that is represented byZ in radix-R notation.)
  14. Letnumber be the Number value formathInt.
  15. Returnsign ×number.

NOTEparseInt may interpret only a leading portion ofstring as an integer value; it ignores any characters that cannot be interpreted as part of the notation of an integer, and no indication is given that any such characters were ignored.

15.1.2.3 parseFloat (string)

TheparseFloat function produces a Number value dictated by interpretation of the contents of thestring argument as a decimal literal.

When theparseFloat function is called, the following steps are taken:

  1. LetinputString beToString(string).
  2. LettrimmedString be a substring ofinputString consisting of the leftmost character that is not aStrWhiteSpaceChar and all characters to the right of that character. (In other words, remove leading white space.) IfinputString does not contain any such characters, lettrimmedString be the empty string.
  3. If neithertrimmedString nor any prefix oftrimmedString satisfies the syntax of aStrDecimalLiteral (see 9.3.1), returnNaN.
  4. LetnumberString be the longest prefix oftrimmedString, which might betrimmedString itself, that satisfies the syntax of aStrDecimalLiteral.
  5. Return the Number value for the MV ofnumberString.

NOTEparseFloat may interpret only a leading portion ofstring as a Number value; it ignores any characters that cannot be interpreted as part of the notation of an decimal literal, and no indication is given that any such characters were ignored.

15.1.2.4 isNaN (number)

Returnstrue if the argument coerces toNaN, and otherwise returnsfalse.

  1. IfToNumber(number) isNaN, returntrue.
  2. Otherwise, returnfalse.

NOTE A reliable way for ECMAScript code to test if a valueX is aNaN is an expression of the formX !== X. The result will betrue if and only ifX is aNaN.

15.1.2.5 isFinite (number)

Returnsfalse if the argument coerces toNaN,+∞, or−∞, and otherwise returnstrue.

  1. IfToNumber(number) isNaN,+∞, or−∞, returnfalse.
  2. Otherwise, returntrue.

15.1.3 URI Handling Function Properties

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in15.1.3.1,15.1.3.2,15.1.3.3 and15.1.3.4.

NOTE Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme:First/Second;Third?Fourth

where the italicised names represent components and “:”, “/”, “;” and “?” are reserved characters used as separators. TheencodeURI anddecodeURI functions are intended to work with complete URIs; they assume that any reserved characters in the URI are intended to have special meaning and so are not encoded. TheencodeURIComponent anddecodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved characters represent text and so must be encoded so that they are not interpreted as reserved characters when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax

uri:::
uriCharactersopt
uriCharacters:::
uriCharacteruriCharactersopt
uriCharacter:::
uriReserved
uriUnescaped
uriEscaped
uriReserved:::one of
;/?:@&=+$,
uriUnescaped:::
uriAlpha
DecimalDigit
uriMark
uriEscaped:::
%HexDigitHexDigit
uriAlpha:::one of
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
uriMark:::one of
-_.!~*'()

NOTE The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.

When a character to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved characters, that character must be encoded. The character is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form “%xx”.

The encoding and escaping process is described by the abstract operation Encode taking two String argumentsstring andunescapedSet.

  1. LetstrLen be the number of characters instring.
  2. LetR be the empty String.
  3. Letk be 0.
  4. Repeat
    1. Ifk equalsstrLen, returnR.
    2. LetC be the character at positionk withinstring.
    3. IfC is inunescapedSet, then
      1. LetS be a String containing only the characterC.
      2. LetR be a new String value computed by concatenating the previous value ofR andS.
    4. Else,C is not inunescapedSet
      1. If the code unit value ofC is not less than 0xDC00 and not greater than 0xDFFF, throw aURIError exception.
      2. If the code unit value ofC is less than 0xD800 or greater than 0xDBFF, then
        1. LetV be the code unit value ofC.
      3. Else,
        1. Increasek by 1.
        2. Ifk equalsstrLen, throw aURIError exception.
        3. LetkChar be the code unit value of the character at positionk withinstring.
        4. IfkChar is less than 0xDC00 or greater than 0xDFFF, throw aURIError exception.
        5. LetV be (((the code unit value ofC) – 0xD800) × 0x400 + (kChar – 0xDC00) + 0x10000).
      4. LetOctets be the array of octets resulting by applying the UTF-8 transformation toV, and letL be the array size.
      5. Letj be 0.
      6. Repeat, whilej <L
        1. LetjOctet be the value at positionj withinOctets.
        2. LetS be a String containing three characters “%XY” whereXY are two uppercase hexadecimal digits encoding the value ofjOctet.
        3. LetR be a new String value computed by concatenating the previous value ofR andS.
        4. Increasej by 1.
    5. Increasek by 1.

The unescaping and decoding process is described by the abstract operation Decode taking two String argumentsstring andreservedSet.

  1. LetstrLen be the number of characters instring.
  2. LetR be the empty String.
  3. Letk be 0.
  4. Repeat
    1. Ifk equalsstrLen, returnR.
    2. LetC be the character at positionk withinstring.
    3. IfC is not ‘%’, then
      1. LetS be the String containing only the characterC.
    4. Else,C is ‘%’
      1. Letstart bek.
      2. Ifk + 2 is greater than or equal tostrLen, throw aURIError exception.
      3. If the characters at position (k+1) and (k + 2) withinstring do not represent hexadecimal digits, throw aURIError exception.
      4. LetB be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
      5. Incrementk by 2.
      6. If the most significant bit inB is 0, then
        1. LetC be the character with code unit valueB.
        2. IfC is not inreservedSet, then
          1. LetS be the String containing only the characterC.
        3. Else,C is inreservedSet
          1. LetS be the substring ofstring from positionstart to positionk included.
      7. Else, the most significant bit inB is 1
        1. Letn be the smallest non-negative number such that (B <<n) & 0x80 is equal to 0.
        2. Ifn equals 1 orn is greater than 4, throw aURIError exception.
        3. LetOctets be an array of 8-bit integers of sizen.
        4. PutB intoOctets at position 0.
        5. Ifk + (3 × (n – 1)) is greater than or equal tostrLen, throw aURIError exception.
        6. Letj be 1.
        7. Repeat, whilej <n
          1. Incrementk by 1.
          2. If the character at positionk is not ‘%’, throw aURIError exception.
          3. If the characters at position (k +1) and (k + 2) withinstring do not represent hexadecimal digits, throw aURIError exception.
          4. LetB be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
          5. If the two most significant bits inB are not 10, throw aURIError exception.
          6. Incrementk by 2.
          7. PutB intoOctets at positionj.
          8. Incrementj by 1.
        8. LetV be the value obtained by applying the UTF-8 transformation toOctets, that is, from an array of octets into a 21-bit value. IfOctets does not contain a valid UTF-8 encoding of a Unicode code point throw anURIError exception.
        9. IfV is less than 0x10000, then
          1. LetC be the character with code unit valueV.
          2. IfC is not inreservedSet, then
            1. LetS be the String containing only the characterC.
          3. Else,C is inreservedSet
            1. LetS be the substring ofstring from positionstart to positionk included.
        10. Else,V is ≥ 0x10000
          1. LetL be (((V – 0x10000) & 0x3FF) + 0xDC00).
          2. LetH be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).
          3. LetS be the String containing the two characters with code unit valuesH andL.
    5. LetR be a new String value computed by concatenating the previous value ofR andS.
    6. Increasek by 1.

NOTE This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a "sequence" of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 21.

Table 21 — UTF-8 Encodings
Code Unit ValueRepresentation1st Octet2nd Octet3rd Octet4th Octet
0x0000 - 0x007F000000000zzzzzzz0zzzzzzz
0x0080 - 0x07FF00000yyy yyzzzzzz110yyyyy10zzzzzz
0x0800 - 0xD7FFxxxxyyyy yyzzzzzz1110xxxx10yyyyyy10zzzzzz

0xD800 - 0xDBFF

followed by

0xDC00 – 0xDFFF

110110vv vvwwwwxx

followed by

110111yy yyzzzzzz

11110uuu10uuwwww10xxyyyy10zzzzzz

0xD800 - 0xDBFF

not followed by

0xDC00 – 0xDFFF

causes URIError
0xDC00 – 0xDFFFcauses URIError
0xE000 - 0xFFFFxxxxyyyy yyzzzzzz1110xxxx10yyyyyy10zzzzzz

Where

uuuuu =vvvv + 1

to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.

The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the character U+0000. Implementations of the Decode algorithm are required to throw aURIError when encountering such invalid sequences.

15.1.3.1 decodeURI (encodedURI)

ThedecodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by theencodeURI function is replaced with the character that it represents. Escape sequences that could not have been introduced byencodeURI are not replaced.

When thedecodeURI function is called with one argumentencodedURI, the following steps are taken:

  1. LeturiString beToString(encodedURI).
  2. LetreservedURISet be a String containing one instance of each character valid inuriReserved plus “#”.
  3. Return the result of calling Decode(uriString,reservedURISet)

NOTE The character “#” is not decoded from escape sequences even though it is not a reserved URI character.

15.1.3.2 decodeURIComponent (encodedURIComponent)

ThedecodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by theencodeURIComponent function is replaced with the character that it represents.

When thedecodeURIComponent function is called with one argumentencodedURIComponent, the following steps are taken:

  1. LetcomponentString beToString(encodedURIComponent).
  2. LetreservedURIComponentSet be the empty String.
  3. Return the result of calling Decode(componentString,reservedURIComponentSet)

15.1.3.3 encodeURI (uri)

TheencodeURI function computes a new version of a URI in which each instance of certain characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.

When theencodeURI function is called with one argumenturi, the following steps are taken:

  1. LeturiString beToString(uri).
  2. LetunescapedURISet be a String containing one instance of each character valid inuriReserved anduriUnescaped plus “#”.
  3. Return the result of calling Encode(uriString,unescapedURISet)

NOTE The character “#” is not encoded to an escape sequence even though it is not a reserved or unescaped URI character.

15.1.3.4 encodeURIComponent (uriComponent)

TheencodeURIComponent function computes a new version of a URI in which each instance of certain characters is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the character.

When theencodeURIComponent function is called with one argumenturiComponent, the following steps are taken:

  1. LetcomponentString beToString(uriComponent).
  2. LetunescapedURIComponentSet be a String containing one instance of each character valid inuriUnescaped.
  3. Return the result of calling Encode(componentString,unescapedURIComponentSet)

15.1.4 Constructor Properties of the Global Object

15.1.4.1 Object ( . . . )

See15.2.1 and15.2.2.

15.1.4.2 Function ( . . . )

See15.3.1 and15.3.2.

15.1.4.3 Array ( . . . )

See15.4.1 and15.4.2.

15.1.4.4 String ( . . . )

See15.5.1 and15.5.2.

15.1.4.5 Boolean ( . . . )

See15.6.1 and15.6.2.

15.1.4.6 Number ( . . . )

See15.7.1 and15.7.2.

15.1.4.7 Date ( . . . )

See15.9.2.

15.1.4.8 RegExp ( . . . )

See15.10.3 and15.10.4.

15.1.4.9 Error ( . . . )

See15.11.1 and15.11.2.

15.1.4.10 EvalError ( . . . )

See15.11.6.1.

15.1.4.11 RangeError ( . . . )

See15.11.6.2.

15.1.4.12 ReferenceError ( . . . )

See15.11.6.3.

15.1.4.13 SyntaxError ( . . . )

See15.11.6.4.

15.1.4.14 TypeError ( . . . )

See15.11.6.5.

15.1.4.15 URIError ( . . . )

See15.11.6.6.

15.1.5 Other Properties of the Global Object

15.1.5.1 Math

See15.8.

15.1.5.2 JSON

See15.12.

15.2 Object Objects

15.2.1 The Object Constructor Called as a Function

WhenObject is called as a function rather than as a constructor, it performs a type conversion.

15.2.1.1 Object ( [ value ] )

When theObject function is called with no arguments or with one argumentvalue, the following steps are taken:

  1. Ifvalue isnull,undefined or not supplied, create and return a new Object object exactly as if the standard built-in Object constructor had been called with the same arguments (15.2.2.1).
  2. ReturnToObject(value).

15.2.2 The Object Constructor

WhenObject is called as part of anew expression, it is a constructor that may create an object.

15.2.2.1 new Object ( [ value ] )

When theObject constructor is called with no arguments or with one argumentvalue, the following steps are taken:

  1. Ifvalue is supplied, then
    1. IfType(value) is Object, then
      1. If thevalue is a native ECMAScript object, do not create a new object but simply returnvalue.
      2. If thevalue is a host object, then actions are taken and a result is returned in an implementation-dependent manner that may depend on the host object.
    2. IfType(value) is String, returnToObject(value).
    3. IfType(value) is Boolean, returnToObject(value).
    4. IfType(value) is Number, returnToObject(value).
  2. Assert: The argumentvalue was not supplied or its type was Null or Undefined.
  3. Letobj be a newly created native ECMAScript object.
  4. Set the [[Prototype]] internal property ofobj to the standard built-in Object prototype object (15.2.4).
  5. Set the [[Class]] internal property ofobj to"Object".
  6. Set the [[Extensible]] internal property ofobj totrue.
  7. Set all the internal methods ofobj as specified in8.12.
  8. Returnobj.

15.2.3 Properties of the Object Constructor

The value of the [[Prototype]] internal property of the Object constructor is the standard built-in Function prototype object.

Besides the internal properties and thelength property (whose value is1), the Object constructor has the following properties:

15.2.3.1 Object.prototype

The initial value ofObject.prototype is the standard built-in Object prototype object (15.2.4).

This property has the attributes {[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.2.3.2 Object.getPrototypeOf ( O )

When thegetPrototypeOf function is called with argumentO, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Return the value of the [[Prototype]] internal property ofO.

15.2.3.3 Object.getOwnPropertyDescriptor ( O, P )

When thegetOwnPropertyDescriptor function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Letname beToString(P).
  3. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO with argumentname.
  4. Return the result of callingFromPropertyDescriptor(desc) (8.10.4).

15.2.3.4 Object.getOwnPropertyNames ( O )

When thegetOwnPropertyNames function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Letarray be the result of creating a new object as if by the expressionnew Array () whereArray is the standard built-in constructor with that name.
  3. Letn be 0.
  4. For each named own propertyP ofO
    1. Letname be the String value that is the name ofP.
    2. Call the [[DefineOwnProperty]] internal method ofarray with argumentsToString(n), the PropertyDescriptor {[[Value]]:name, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    3. Incrementn by 1.
  5. Returnarray.

NOTE IfO is a String instance, the set of own properties processed in step 4 includes the implicit properties defined in15.5.5.2 that correspond to character positions within the object’s [[PrimitiveValue]] String.

15.2.3.5 Object.create ( O [, Properties] )

Thecreate function creates a new object with a specified prototype. When thecreate function is called, the following steps are taken:

  1. IfType(O) is not Object or Null throw aTypeError exception.
  2. Letobj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name
  3. Set the [[Prototype]] internal property ofobj toO.
  4. If the argumentProperties is present and notundefined, add own properties toobj as if by calling the standard built-in functionObject.defineProperties with argumentsobj andProperties.
  5. Returnobj.

15.2.3.6 Object.defineProperty ( O, P, Attributes )

ThedefineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When thedefineProperty function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Letname beToString(P).
  3. Letdesc be the result of callingToPropertyDescriptor withAttributes as the argument.
  4. Call the [[DefineOwnProperty]] internal method ofO with argumentsname,desc, andtrue.
  5. ReturnO.

15.2.3.7 Object.defineProperties ( O, Properties )

ThedefineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When thedefineProperties function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Letprops beToObject(Properties).
  3. Letnames be an internal list containing the names of each enumerable own property ofprops.
  4. Letdescriptors be an empty internalList.
  5. For each elementP ofnames in list order,
    1. LetdescObj be the result of calling the [[Get]] internal method ofprops withP as the argument.
    2. Letdesc be the result of callingToPropertyDescriptor withdescObj as the argument.
    3. Append the pair (a two elementList) consisting ofP anddesc to the end ofdescriptors.
  6. For eachpair fromdescriptors in list order,
    1. LetP be the first element ofpair.
    2. Letdesc be the second element ofpair.
    3. Call the [[DefineOwnProperty]] internal method ofO with argumentsP,desc, andtrue.
  7. ReturnO.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must be used to order the list elements in step 3 of this algorithm.

15.2.3.8 Object.seal ( O )

When theseal function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. For each named own property nameP ofO,
    1. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO withP.
    2. Ifdesc.[[Configurable]] istrue, setdesc.[[Configurable]] tofalse.
    3. Call the [[DefineOwnProperty]] internal method ofO withP,desc, andtrue as arguments.
  3. Set the [[Extensible]] internal property ofO tofalse.
  4. ReturnO.

15.2.3.9 Object.freeze ( O )

When thefreeze function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. For each named own property nameP ofO,
    1. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO withP.
    2. IfIsDataDescriptor(desc) istrue, then
      1. Ifdesc.[[Writable]] istrue, setdesc.[[Writable]] tofalse.
    3. Ifdesc.[[Configurable]] istrue, setdesc.[[Configurable]] tofalse.
    4. Call the [[DefineOwnProperty]] internal method ofO withP,desc, andtrue as arguments.
  3. Set the [[Extensible]] internal property ofO tofalse.
  4. ReturnO.

15.2.3.10 Object.preventExtensions ( O )

When thepreventExtensions function is called, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Set the [[Extensible]] internal property ofO tofalse.
  3. ReturnO.

15.2.3.11 Object.isSealed ( O )

When theisSealed function is called with argumentO, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. For each named own property nameP ofO,
    1. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO withP.
    2. Ifdesc.[[Configurable]] istrue, then returnfalse.
  3. If the [[Extensible]] internal property ofO isfalse, then returntrue.
  4. Otherwise, returnfalse.

15.2.3.12 Object.isFrozen ( O )

When theisFrozen function is called with argumentO, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. For each named own property nameP ofO,
    1. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO withP.
    2. IfIsDataDescriptor(desc) istrue then
      1. Ifdesc.[[Writable]] istrue, returnfalse.
    3. Ifdesc.[[Configurable]] istrue, then returnfalse.
  3. If the [[Extensible]] internal property ofO isfalse, then returntrue.
  4. Otherwise, returnfalse.

15.2.3.13 Object.isExtensible ( O )

When theisExtensible function is called with argumentO, the following steps are taken:

  1. IfType(O) is not Object throw aTypeError exception.
  2. Return the Boolean value of the [[Extensible]] internal property ofO.

15.2.3.14 Object.keys ( O )

When thekeys function is called with argumentO, the following steps are taken:

  1. If theType(O) is not Object, throw aTypeError exception.
  2. Letn be the number of own enumerable properties ofO
  3. Letarray be the result of creating a new Object as if by the expressionnew Array(n) whereArray is the standard built-in constructor with that name.
  4. Letindex be 0.
  5. For each own enumerable property ofO whose name String isP
    1. Call the [[DefineOwnProperty]] internal method ofarray with argumentsToString(index), the PropertyDescriptor {[[Value]]:P, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    2. Incrementindex by 1.
  6. Returnarray.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must be used in step 5 of this algorithm.

15.2.4 Properties of the Object Prototype Object

The value of the [[Prototype]] internal property of the Object prototype object isnull, the value of the [[Class]] internal property is"Object", and the initial value of the [[Extensible]] internal property istrue.

15.2.4.1 Object.prototype.constructor

The initial value ofObject.prototype.constructor is the standard built-inObject constructor.

15.2.4.2 Object.prototype.toString ( )

When thetoString method is called, the following steps are taken:

  1. If thethis value isundefined, return"[object Undefined]".
  2. If thethis value isnull, return"[object Null]".
  3. LetO be the result of callingToObject passing thethis value as the argument.
  4. Letclass be the value of the [[Class]] internal property ofO.
  5. Return the String value that is the result of concatenating the three Strings"[object ",class, and"]".

15.2.4.3 Object.prototype.toLocaleString ( )

When thetoLocaleString method is called, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LettoString be the result of calling the [[Get]] internal method ofO passing"toString" as the argument.
  3. IfIsCallable(toString) isfalse, throw aTypeError exception.
  4. Return the result of calling the [[Call]] internal method oftoString passingO as thethis value and no arguments.

NOTE 1 This function is provided to give all Objects a generictoLocaleString interface, even though not all may use it. Currently,Array,Number, andDate provide their own locale-sensitivetoLocaleString methods.

NOTE 2 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.2.4.4 Object.prototype.valueOf ( )

When thevalueOf method is called, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. IfO is the result of calling the Object constructor with a host object (15.2.2.1), then
    1. Return eitherO or another value such as the host object originally passed to the constructor. The specific result that is returned is implementation-defined.
  3. ReturnO.

15.2.4.5 Object.prototype.hasOwnProperty (V)

When thehasOwnProperty method is called with argumentV, the following steps are taken:

  1. LetP beToString(V).
  2. LetO be the result of callingToObject passing thethis value as the argument.
  3. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO passingP as the argument.
  4. Ifdesc isundefined, returnfalse.
  5. Returntrue.

NOTE 1 Unlike [[HasProperty]] (8.12.6), this method does not consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if thethis value isundefined ornull.

15.2.4.6 Object.prototype.isPrototypeOf (V)

When theisPrototypeOf method is called with argumentV, the following steps are taken:

  1. IfV is not an object, returnfalse.
  2. LetO be the result of callingToObject passing thethis value as the argument.
  3. Repeat
    1. LetV be the value of the [[Prototype]] internal property ofV.
    2. ifV isnull, returnfalse
    3. IfO andV refer to the same object, returntrue.

NOTE The ordering of steps 1 and 2 is chosen to preserve the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.

15.2.4.7 Object.prototype.propertyIsEnumerable (V)

When thepropertyIsEnumerable method is called with argumentV, the following steps are taken:

  1. LetP beToString(V).
  2. LetO be the result of callingToObject passing thethis value as the argument.
  3. Letdesc be the result of calling the [[GetOwnProperty]] internal method ofO passingP as the argument.
  4. Ifdesc isundefined, returnfalse.
  5. Return the value ofdesc.[[Enumerable]].

NOTE 1 This method does not consider objects in the prototype chain.

NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if thethis value isundefined ornull.

15.2.5 Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

15.3 Function Objects

15.3.1 The Function Constructor Called as a Function

WhenFunction is called as a function rather than as a constructor, it creates and initialises a new Function object. Thus the function callFunction(…) is equivalent to the object creation expressionnew Function(…) with the same arguments.

15.3.1.1 Function (p1, p2, … , pn, body)

When theFunction function is called with some argumentsp1,p2, … ,pn,body (wheren might be0, that is, there are no “p” arguments, and wherebody might also not be provided), the following steps are taken:

  1. Create and return a new Function object as if the standard built-in constructor Function was used in anew expression with the same arguments (15.3.2.1).

15.3.2 The Function Constructor

WhenFunction is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.3.2.1 new Function (p1, p2, … , pn, body)

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When theFunction constructor is called with some argumentsp1,p2, … ,pn,body (wheren might be0, that is, there are no “p” arguments, and wherebody might also not be provided), the following steps are taken:

  1. LetargCount be the total number of arguments passed to this function invocation.
  2. LetP be the empty String.
  3. IfargCount = 0, letbody be the empty String.
  4. Else ifargCount = 1, letbody be that argument.
  5. Else,argCount > 1
    1. LetfirstArg be the first argument.
    2. LetP beToString(firstArg).
    3. Letk be 2.
    4. Repeat, whilek <argCount
      1. LetnextArg be thek’th argument.
      2. LetP be the result of concatenating the previous value ofP, the String"," (a comma), andToString(nextArg).
      3. Increasek by 1.
    5. Letbody be thek’th argument.
  6. Letbody beToString(body).
  7. IfP is not parsable as aFormalParameterListopt then throw aSyntaxError exception.
  8. Ifbody is not parsable asFunctionBody then throw aSyntaxError exception.
  9. Ifbody isstrict mode code (see 10.1.1) then letstrict betrue, else letstrict befalse.
  10. Ifstrict istrue, throw any exceptions specified in13.1 that apply.
  11. Return a new Function object created as specified in13.2 passingP as theFormalParameterListopt andbody as theFunctionBody. Pass inthe Global Environment as theScope parameter andstrict as theStrict flag.

Aprototype property is automatically created for every function, to provide for the possibility that the function will be used as a constructor.

NOTE It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")

15.3.3 Properties of the Function Constructor

The Function constructor is itself a Function object and its [[Class]] is"Function". The value of the [[Prototype]] internal property of the Function constructor is the standard built-in Function prototype object (15.3.4).

The value of the [[Extensible]] internal property of the Function constructor istrue.

The Function constructor has the following properties:

15.3.3.1 Function.prototype

The initial value ofFunction.prototype is the standard built-in Function prototype object (15.3.4).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.3.3.2 Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.3.4 Properties of the Function Prototype Object

The Function prototype object is itself a Function object (its [[Class]] is"Function") that, when invoked, accepts any arguments and returnsundefined.

The value of the [[Prototype]] internal property of the Function prototype object is the standard built-in Object prototype object (15.2.4). The initial value of the [[Extensible]] internal property of the Function prototype object istrue.

The Function prototype object does not have avalueOf property of its own; however, it inherits thevalueOf property from the Object prototype Object.

Thelength property of the Function prototype object is0.

15.3.4.1 Function.prototype.constructor

The initial value ofFunction.prototype.constructor is the built-inFunction constructor.

15.3.4.2 Function.prototype.toString ( )

An implementation-dependent representation of the function is returned. This representation has the syntax of aFunctionDeclaration. Note in particular that the use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.

ThetoString function is not generic; it throws aTypeError exception if itsthis value is not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.3.4.3 Function.prototype.apply (thisArg, argArray)

When theapply method is called on an objectfunc with argumentsthisArg andargArray, the following steps are taken:

  1. IfIsCallable(func) isfalse, then throw aTypeError exception.
  2. IfargArray isnull orundefined, then
    1. Return the result of calling the [[Call]] internal method offunc, providingthisArg as thethis value and an empty list of arguments.
  3. IfType(argArray) is not Object, then throw aTypeError exception.
  4. Letlen be the result of calling the [[Get]] internal method ofargArray with argument"length".
  5. Letn beToUint32(len).
  6. LetargList be an emptyList.
  7. Letindex be 0.
  8. Repeat whileindex <n
    1. LetindexName beToString(index).
    2. LetnextArg be the result of calling the [[Get]] internal method ofargArray withindexName as the argument.
    3. AppendnextArg as the last element ofargList.
    4. Setindex toindex + 1.
  9. Return the result of calling the [[Call]] internal method offunc, providingthisArg as thethis value andargList as the list of arguments.

Thelength property of theapply method is2.

NOTE The thisArg value is passed without modification as thethis value. This is a change from Edition 3, where aundefined ornull thisArg is replaced with the global object andToObject is applied to all other values and that result is passed as thethis value.

15.3.4.4 Function.prototype.call (thisArg [ , arg1 [ , arg2, … ] ] )

When thecall method is called on an objectfunc with argumentthisArg and optional argumentsarg1,arg2 etc, the following steps are taken:

  1. IfIsCallable(func) isfalse, then throw aTypeError exception.
  2. LetargList be an emptyList.
  3. If this method was called with more than one argument then in left to right order starting witharg1 append each argument as the last element ofargList
  4. Return the result of calling the [[Call]] internal method offunc, providingthisArg as thethis value andargList as the list of arguments.

Thelength property of thecall method is1.

NOTE The thisArg value is passed without modification as thethis value. This is a change from Edition 3, where aundefined ornull thisArg is replaced with the global object andToObject is applied to all other values and that result is passed as thethis value.

15.3.4.5 Function.prototype.bind (thisArg [, arg1 [, arg2, …]])

The bind method takes one or more arguments,thisArg and (optionally)arg1,arg2, etc, and returns a new function object by performing the following steps:

  1. LetTarget be thethis value.
  2. IfIsCallable(Target) isfalse, throw aTypeError exception.
  3. LetA be a new (possibly empty) internal list of all of the argument values provided afterthisArg (arg1,arg2 etc), in order.
  4. LetF be a new native ECMAScript object .
  5. Set all the internal methods, except for [[Get]], ofF as specified in8.12.
  6. Set the [[Get]] internal property ofF as specified in15.3.5.4.
  7. Set the [[TargetFunction]] internal property ofF toTarget.
  8. Set the [[BoundThis]] internal property ofF to the value ofthisArg.
  9. Set the [[BoundArgs]] internal property ofF toA.
  10. Set the [[Class]] internal property ofF to"Function".
  11. Set the [[Prototype]] internal property ofF to the standard built-in Function prototype object as specified in15.3.3.1.
  12. Set the [[Call]] internal property ofF as described in15.3.4.5.1.
  13. Set the [[Construct]] internal property ofF as described in15.3.4.5.2.
  14. Set the [[HasInstance]] internal property ofF as described in15.3.4.5.3.
  15. If the [[Class]] internal property ofTarget is"Function", then
    1. LetL be thelength property ofTarget minus the length ofA.
    2. Set thelength own property ofF to either 0 orL, whichever is larger.
  16. Else set thelength own property ofF to 0.
  17. Set the attributes of thelength own property ofF to the values specified in15.3.5.1.
  18. Set the [[Extensible]] internal property ofF totrue.
  19. Letthrower be the [[ThrowTypeError]] function Object (13.2.3).
  20. Call the [[DefineOwnProperty]] internal method ofF with arguments"caller", PropertyDescriptor {[[Get]]:thrower, [[Set]]:thrower, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  21. Call the [[DefineOwnProperty]] internal method ofF with arguments"arguments", PropertyDescriptor {[[Get]]:thrower, [[Set]]:thrower, [[Enumerable]]:false, [[Configurable]]:false}, andfalse.
  22. ReturnF.

Thelength property of thebind method is1.

NOTE Function objects created usingFunction.prototype.bind do not have aprototype property or the [[Code]], [[FormalParameters]], and [[Scope]] internal properties.

15.3.4.5.1 [[Call]]

When the [[Call]] internal method of a function object,F, which was created using the bind function is called with athis value and a list of argumentsExtraArgs, the following steps are taken:

  1. LetboundArgs be the value ofF’s [[BoundArgs]] internal property.
  2. LetboundThis be the value ofF’s [[BoundThis]] internal property.
  3. Lettarget be the value ofF’s [[TargetFunction]] internal property.
  4. Letargs be a new list containing the same values as the listboundArgs in the same order followed by the same values as the listExtraArgs in the same order.
  5. Return the result of calling the [[Call]] internal method oftarget providingboundThis as thethis value and providingargs as the arguments.

15.3.4.5.2 [[Construct]]

When the [[Construct]] internal method of a function object,F that was created using the bind function is called with a list of argumentsExtraArgs, the following steps are taken:

  1. Lettarget be the value ofF’s [[TargetFunction]] internal property.
  2. Iftarget has no [[Construct]] internal method, aTypeError exception is thrown.
  3. LetboundArgs be the value ofF’s [[BoundArgs]] internal property.
  4. Letargs be a new list containing the same values as the listboundArgs in the same order followed by the same values as the listExtraArgs in the same order.
  5. Return the result of calling the [[Construct]] internal method oftarget providingargs as the arguments.

15.3.4.5.3 [[HasInstance]] (V)

When the [[HasInstance]] internal method of a function objectF, that was created using the bind function is called with argumentV, the following steps are taken:

  1. Lettarget be the value ofF’s [[TargetFunction]] internal property.
  2. Iftarget has no [[HasInstance]] internal method, aTypeError exception is thrown.
  3. Return the result of calling the [[HasInstance]] internal method oftarget providingV as the argument.

15.3.5 Properties of Function Instances

In addition to the required internal properties, every function instance has a [[Call]] internal property and in most cases uses a different version of the [[Get]] internal property. Depending on how they are created (see8.6.2,13.2, 15, and15.3.4.5), function instances may have a [[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property, a [[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]] internal property, a [[BoundThis]] internal property, and a [[BoundArgs]] internal property.

The value of the [[Class]] internal property is"Function".

Function instances that correspond to strict mode functions (13.2) and function instances created using theFunction.prototype.bind method (15.3.4.5) have properties named “caller” and “arguments” that throw aTypeError exception. An ECMAScript implementation must not associate any implementation specific behaviour with accesses of these properties from strict mode function code.

15.3.5.1 length

The value of thelength property is an integer that indicates the “typical” number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by itslength property depends on the function. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.3.5.2 prototype

The value of theprototype property is used to initialise the [[Prototype]] internal property of a newly created object before the Function object is invoked as a constructor for that newly created object. This property has the attribute { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false }.

NOTE Function objects created usingFunction.prototype.bind do not have aprototype property.

15.3.5.3 [[HasInstance]] (V)

AssumeF is a Function object.

When the [[HasInstance]] internal method ofF is called with valueV, the following steps are taken:

  1. IfV is not an object, returnfalse.
  2. LetO be the result of calling the [[Get]] internal method ofF with property name"prototype".
  3. IfType(O) is not Object, throw aTypeError exception.
  4. Repeat
    1. LetV be the value of the [[Prototype]] internal property ofV.
    2. IfV isnull, returnfalse.
    3. IfO andV refer to the same object, returntrue.

NOTE Function objects created usingFunction.prototype.bind have a different implementation of [[HasInstance]] defined in15.3.4.5.3.

15.3.5.4 [[Get]] (P)

Function objects use a variation of the [[Get]] internal method used for other native ECMAScript objects (8.12.3).

AssumeF is a Function object. When the [[Get]] internal method ofF is called with property nameP, the following steps are taken:

  1. Letv be the result of calling the default [[Get]] internal method (8.12.3) onF passingP as the property name argument.
  2. IfP is"caller" andv is a strict mode Function object, throw aTypeError exception.
  3. Returnv.

NOTE Function objects created usingFunction.prototype.bind use the default [[Get]] internal method.

15.4 Array Objects

Array objects give special treatment to a certain class of property names. A property nameP (in the form of a String value) is anarray index if and only ifToString(ToUint32(P)) is equal toP andToUint32(P) is not equal to232−1. A property whose property name is an array index is also called anelement. Every Array object has alength property whose value is always a nonnegative integer less than232. The value of thelength property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, thelength property is changed, if necessary, to be one more than the numeric value of that array index; and whenever thelength property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own properties of an Array object and is unaffected bylength or array index properties that may be inherited from its prototypes.

An object,O, is said to besparse if the following algorithm returnstrue:

  1. Letlen be the result of calling the [[Get]] internal method ofO with argument"length".
  2. For each integeri in the range 0≤i<ToUint32(len)
    1. Letelem be the result of calling the [[GetOwnProperty]] internal method ofO with argumentToString(i).
    2. Ifelem isundefined, returntrue.
  3. Returnfalse.

15.4.1 The Array Constructor Called as a Function

WhenArray is called as a function rather than as a constructor, it creates and initialises a new Array object. Thus the function callArray(…) is equivalent to the object creation expressionnew Array(…) with the same arguments.

15.4.1.1 Array ( [ item1 [ , item2 [ , … ] ] ] )

When theArray function is called the following steps are taken:

  1. Create and return a new Array object exactly as if the standard built-in constructorArray was used in anew expression with the same arguments (15.4.2).

15.4.2 The Array Constructor

WhenArray is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.4.2.1 new Array ( [ item0 [ , item1 [ , … ] ] ] )

This description applies if and only if the Array constructor is given no arguments or at least two arguments.

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value ofArray.prototype (15.4.3.1).

The [[Class]] internal property of the newly constructed object is set to"Array".

The [[Extensible]] internal property of the newly constructed object is set totrue.

Thelength property of the newly constructed object is set to the number of arguments.

The0 property of the newly constructed object is set toitem0 (if supplied); the1 property of the newly constructed object is set toitem1 (if supplied); and, in general, for as many arguments as there are, thek property of the newly constructed object is set to argumentk, where the first argument is considered to be argument number0. These properties all have the attributes {[[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.

15.4.2.2 new Array (len)

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value ofArray.prototype (15.4.3.1). The [[Class]] internal property of the newly constructed object is set to"Array". The [[Extensible]] internal property of the newly constructed object is set totrue.

If the argumentlen is a Number andToUint32(len) is equal tolen, then thelength property of the newly constructed object is set toToUint32(len). If the argumentlen is a Number andToUint32(len) is not equal tolen, aRangeError exception is thrown.

If the argumentlen is not a Number, then thelength property of the newly constructed object is set to1 and the0 property of the newly constructed object is set tolen with attributes {[[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.

15.4.3 Properties of the Array Constructor

The value of the [[Prototype]] internal property of the Array constructor is the Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is1), the Array constructor has the following properties:

15.4.3.1 Array.prototype

The initial value ofArray.prototype is the Array prototype object (15.4.4).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.4.3.2 Array.isArray ( arg )

The isArray function takes one argumentarg, and returns the Boolean valuetrue if the argument is an object whose class internal property is"Array"; otherwise it returnsfalse. The following steps are taken:

  1. IfType(arg) is not Object, returnfalse.
  2. If the value of the [[Class]] internal property ofarg is"Array", then returntrue.
  3. Returnfalse.

15.4.4 Properties of the Array Prototype Object

The value of the [[Prototype]] internal property of the Array prototype object is the standard built-in Object prototype object (15.2.4).

The Array prototype object is itself an array; its [[Class]] is"Array", and it has alength property (whose initial value is+0) and the special [[DefineOwnProperty]] internal method described in15.4.5.1.

In following descriptions of functions that are properties of the Array prototype object, the phrase “this object” refers to the object that is thethis value for the invocation of the function. It is permitted for thethis to be an object for which the value of the [[Class]] internal property is not"Array".

NOTE The Array prototype object does not have avalueOf property of its own; however, it inherits thevalueOf property from the standard built-in Object prototype Object.

15.4.4.1 Array.prototype.constructor

The initial value ofArray.prototype.constructor is the standard built-inArray constructor.

15.4.4.2 Array.prototype.toString ( )

When thetoString method is called, the following steps are taken:

  1. Letarray be the result of callingToObject on thethis value.
  2. Letfunc be the result of calling the [[Get]] internal method ofarray with argument"join".
  3. IfIsCallable(func) isfalse, then letfunc be the standard built-in method Object.prototype.toString (15.2.4.2).
  4. Return the result of calling the [[Call]] internal method offunc providingarray as thethis value and an empty arguments list.

NOTE ThetoString function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thetoString function can be applied successfully to a host object is implementation-dependent.

15.4.4.3 Array.prototype.toLocaleString ( )

The elements of the array are converted to Strings using theirtoLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result oftoString, except that the result of this function is intended to be locale-specific.

The result is calculated as follows:

  1. Letarray be the result of callingToObject passing thethis value as the argument.
  2. LetarrayLen be the result of calling the [[Get]] internal method ofarray with argument"length".
  3. Letlen beToUint32(arrayLen).
  4. Letseparator be the String value for the list-separator String appropriate for the host environment’s current locale (this is derived in an implementation-defined way).
  5. Iflen is zero, return the empty String.
  6. LetfirstElement be the result of calling the [[Get]] internal method ofarray with argument"0".
  7. IffirstElement isundefined ornull, then
    1. LetR be the empty String.
  8. Else
    1. LetelementObj beToObject(firstElement).
    2. Letfunc be the result of calling the [[Get]] internal method ofelementObj with argument"toLocaleString".
    3. IfIsCallable(func) isfalse, throw aTypeError exception.
    4. LetR be the result of calling the [[Call]] internal method offunc providingelementObj as thethis value and an empty arguments list.
  9. Letk be1.
  10. Repeat, whilek <len
    1. LetS be a String value produced by concatenatingR andseparator.
    2. LetnextElement be the result of calling the [[Get]] internal method ofarray with argumentToString(k).
    3. IfnextElement isundefined ornull, then
      1. LetR be the empty String.
    4. Else
      1. LetelementObj beToObject(nextElement).
      2. Letfunc be the result of calling the [[Get]] internal method ofelementObj with argument"toLocaleString".
      3. IfIsCallable(func) isfalse, throw aTypeError exception.
      4. LetR be the result of calling the [[Call]] internal method offunc providingelementObj as thethis value and an empty arguments list.
    5. LetR be a String value produced by concatenatingS andR.
    6. Increasek by 1.
  11. ReturnR.

NOTE 1 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 ThetoLocaleString function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thetoLocaleString function can be applied successfully to a host object is implementation-dependent.

15.4.4.4 Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )

When theconcat method is called with zero or more argumentsitem1,item2, etc., it returns an array containing the array elements of the object followed by the array elements of each argument in order.

The following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetA be a new array created as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
  3. Letn be 0.
  4. Letitems be an internalList whose first element isO and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
  5. Repeat, whileitems is not empty
    1. Remove the first element fromitems and letE be the value of the element.
    2. If the value of the [[Class]] internal property ofE is"Array", then
      1. Letk be 0.
      2. Letlen be the result of calling the [[Get]] internal method ofE with argument"length".
      3. Repeat, whilek <len
        1. LetP beToString(k).
        2. Letexists be the result of calling the [[HasProperty]] internal method ofE withP.
        3. Ifexists istrue, then
          1. LetsubElement be the result of calling the [[Get]] internal method ofE with argumentP.
          2. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(n),Property Descriptor {[[Value]]:subElement, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
        4. Increasen by 1.
        5. Increasek by 1.
    3. Else,E is not an Array
      1. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(n),Property Descriptor {[[Value]]:E, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
      2. Increasen by 1.
  6. ReturnA.

Thelength property of theconcat method is1.

NOTE Theconcat function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theconcat function can be applied successfully to a host object is implementation-dependent.

15.4.4.5 Array.prototype.join (separator)

The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of theseparator. If no separator is provided, a single comma is used as the separator.

Thejoin method takes one argument,separator, and performs the following steps:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  3. Letlen beToUint32(lenVal).
  4. Ifseparator isundefined, letseparator be the single-character String",".
  5. Letsep beToString(separator).
  6. Iflen is zero, return the empty String.
  7. Letelement0 be the result of calling the [[Get]] internal method ofO with argument"0".
  8. Ifelement0 isundefined ornull, letR be the empty String; otherwise, LetR beToString(element0).
  9. Letk be1.
  10. Repeat, whilek <len
    1. LetS be the String value produced by concatenatingR andsep.
    2. Letelement be the result of calling the [[Get]] internal method ofO with argumentToString(k).
    3. Ifelement isundefined ornull, Letnext be the empty String; otherwise, letnext beToString(element).
    4. LetR be a String value produced by concatenatingS andnext.
    5. Increasek by 1.
  11. ReturnR.

Thelength property of thejoin method is1.

NOTE Thejoin function is intentionally generic; it does not require that itsthis value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether thejoin function can be applied successfully to a host object is implementation-dependent.

15.4.4.6 Array.prototype.pop ( )

The last element of the array is removed from the array and returned.

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  3. Letlen beToUint32(lenVal).
  4. Iflen is zero,
    1. Call the [[Put]] internal method ofO with arguments"length", 0, andtrue.
    2. Returnundefined.
  5. Else,len > 0
    1. Letindx beToString(len–1).
    2. Letelement be the result of calling the [[Get]] internal method ofO with argumentindx.
    3. Call the [[Delete]] internal method ofO with argumentsindx andtrue.
    4. Call the [[Put]] internal method ofO with arguments"length",indx, andtrue.
    5. Returnelement.

NOTE Thepop function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thepop function can be applied successfully to a host object is implementation-dependent.

15.4.4.7 Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )

The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.

When thepush method is called with zero or more argumentsitem1,item2, etc., the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  3. Letn beToUint32(lenVal).
  4. Letitems be an internalList whose elements are, in left to right order, the arguments that were passed to this function invocation.
  5. Repeat, whileitems is not empty
    1. Remove the first element fromitems and letE be the value of the element.
    2. Call the [[Put]] internal method ofO with argumentsToString(n),E, andtrue.
    3. Increasen by 1.
  6. Call the [[Put]] internal method ofO with arguments"length",n, andtrue.
  7. Returnn.

Thelength property of thepush method is1.

NOTE Thepush function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thepush function can be applied successfully to a host object is implementation-dependent.

15.4.4.8 Array.prototype.reverse ( )

The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  3. Letlen beToUint32(lenVal).
  4. Letmiddle befloor(len/2).
  5. Letlower be0.
  6. Repeat, whilelower ≠middle
    1. Letupper belen−lower −1.
    2. LetupperP beToString(upper).
    3. LetlowerP beToString(lower).
    4. LetlowerValue be the result of calling the [[Get]] internal method ofO with argumentlowerP.
    5. LetupperValue be the result of calling the [[Get]] internal method ofO with argumentupperP .
    6. LetlowerExists be the result of calling the [[HasProperty]] internal method ofO with argumentlowerP.
    7. LetupperExists be the result of calling the [[HasProperty]] internal method ofO with argumentupperP.
    8. IflowerExists istrue andupperExists istrue, then
      1. Call the [[Put]] internal method ofO with argumentslowerP,upperValue, andtrue .
      2. Call the [[Put]] internal method ofO with argumentsupperP,lowerValue, andtrue .
    9. Else iflowerExists isfalse andupperExists istrue, then
      1. Call the [[Put]] internal method ofO with argumentslowerP,upperValue, andtrue .
      2. Call the [[Delete]] internal method ofO, with argumentsupperP andtrue.
    10. Else iflowerExists istrue andupperExists isfalse, then
      1. Call the [[Delete]] internal method ofO, with argumentslowerP andtrue .
      2. Call the [[Put]] internal method ofO with argumentsupperP,lowerValue, andtrue .
    11. Else, bothlowerExists andupperExists arefalse
      1. No action is required.
    12. Increaselower by 1.
  7. ReturnO .

NOTE Thereverse function is intentionally generic; it does not require that itsthis value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether thereverse function can be applied successfully to a host object is implementation-dependent.

15.4.4.9 Array.prototype.shift ( )

The first element of the array is removed from the array and returned.

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  3. Letlen beToUint32(lenVal).
  4. Iflen is zero, then
    1. Call the [[Put]] internal method ofO with arguments"length", 0, andtrue.
    2. Returnundefined.
  5. Letfirst be the result of calling the [[Get]] internal method ofO with argument"0".
  6. Letk be 1.
  7. Repeat, whilek <len
    1. Letfrom beToString(k).
    2. Letto beToString(k–1).
    3. LetfromPresent be the result of calling the [[HasProperty]] internal method ofO with argumentfrom.
    4. IffromPresent istrue, then
      1. LetfromVal be the result of calling the [[Get]] internal method ofO with argumentfrom.
      2. Call the [[Put]] internal method ofO with argumentsto,fromVal, andtrue.
    5. Else,fromPresent isfalse
      1. Call the [[Delete]] internal method ofO with argumentsto andtrue.
    6. Increasek by 1.
  8. Call the [[Delete]] internal method ofO with argumentsToString(len–1) andtrue.
  9. Call the [[Put]] internal method ofO with arguments"length", (len–1) , andtrue.
  10. Returnfirst.

NOTE Theshift function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theshift function can be applied successfully to a host object is implementation-dependent.

15.4.4.10 Array.prototype.slice (start, end)

Theslice method takes two arguments,start andend, and returns an array containing the elements of the array from elementstart up to, but not including, elementend (or through the end of the array ifend isundefined). Ifstart is negative, it is treated aslength+start wherelength is the length of the array. Ifend is negative, it is treated aslength+end wherelength is the length of the array. The following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetA be a new array created as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
  3. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  4. Letlen beToUint32(lenVal).
  5. LetrelativeStart beToInteger(start).
  6. IfrelativeStart is negative, letk be max((len +relativeStart),0); else letk be min(relativeStart,len).
  7. Ifend isundefined, letrelativeEnd belen; else letrelativeEnd beToInteger(end).
  8. IfrelativeEnd is negative, letfinal be max((len +relativeEnd),0); else letfinal be min(relativeEnd,len).
  9. Letn be 0.
  10. Repeat, whilek <final
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(n),Property Descriptor {[[Value]]:kValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    4. Increasek by 1.
    5. Increasen by 1.
  11. ReturnA.

Thelength property of theslice method is2.

NOTE Theslice function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theslice function can be applied successfully to a host object is implementation-dependent.

15.4.4.11 Array.prototype.sort (comparefn)

The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not necessarily remain in their original order). Ifcomparefn is notundefined, it should be a function that accepts two argumentsx andy and returns a negative value ifx<y, zero ifx=y, or a positive value ifx>y.

Letobj be the result of callingToObject passing thethis value as the argument.

Letlen be the result of applying Uint32 to the result of calling the [[Get]] internal method ofobj with argument "length".

Ifcomparefn is notundefined and is not a consistent comparison function for the elements of this array (see below), the behaviour ofsort is implementation-defined.

Letproto be the value of the [[Prototype]] internal property ofobj. Ifproto is notnull and there exists an integerj such that all of the conditions below are satisfied then the behaviour ofsort is implementation-defined:

  • obj is sparse (15.4)
  • 0 ≤j <len
  • The result of calling the [[HasProperty]] internal method ofproto with argumentToString(j) istrue.

The behaviour ofsort is also implementation defined ifobj is sparse and any of the following conditions are true:

  • The [[Extensible]] internal property ofobj isfalse.

  • Any array index property ofobj whose name is a nonnegative integer less thanlen is a data property whose [[Configurable]] attribute isfalse.

The behaviour ofsort is also implementation defined if any array index property ofobj whose name is a nonnegative integer less thanlen is an accessor property or is a data property whose [[Writable]] attribute isfalse.

Otherwise, the following steps are taken.

  1. Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] internal methods ofobj and to SortCompare (described below), where the first argument for each call to [[Get]], [[Put]], or [[Delete]] is a nonnegative integer less thanlen and where the arguments for calls to SortCompare are results of previous calls to the [[Get]] internal method. The throw argument to the [[Put]] and [[Delete]] internal methods will be the valuetrue. Ifobj is not sparse then [[Delete]] must not be called.
  2. Returnobj.

The returned object must have the following two properties.

  • There must be some mathematical permutationπ of the nonnegative integers less thanlen, such that for every nonnegative integerj less thanlen, if propertyold[j] existed, thennew[π(j)] is exactly the same value asold[j],. But if propertyold[j] did not exist, thennew[π(j)] does not exist.

  • Then for all nonnegative integersj andk, each less thanlen, ifSortCompare(j,k) < 0 (see SortCompare below), thenπ(j) <π(k).

Here the notationold[j] is used to refer to the hypothetical result of calling the [[Get]] internal method ofobj with argumentj before this function is executed, and the notationnew[j] to refer to the hypothetical result of calling the [[Get]] internal method ofobj with argumentj after this function has been executed.

A functioncomparefn is a consistent comparison function for a set of valuesS if all of the requirements below are met for all valuesa,b, andc (possibly the same value) in the setS: The notationa <CF b meanscomparefn(a,b) < 0;a =CF b meanscomparefn(a,b) = 0 (of either sign); anda >CF b meanscomparefn(a,b) > 0.

  • Callingcomparefn(a,b) always returns the same valuev when given a specific pair of valuesa andb as its two arguments. Furthermore,Type(v) is Number, andv is not NaN. Note that this implies that exactly one ofa <CF b,a =CF b, anda >CF b will be true for a given pair ofa andb.

  • Callingcomparefn(a,b) does not modify thethis object.

  • a =CF a (reflexivity)

  • Ifa =CF b, thenb =CF a (symmetry)

  • Ifa =CF b andb =CF c, thena =CF c (transitivity of =CF)

  • Ifa <CF b andb <CF c, thena <CF c (transitivity of <CF)

  • Ifa >CF b andb >CF c, thena >CF c (transitivity of >CF)

NOTE The above conditions are necessary and sufficient to ensure thatcomparefn divides the setS into equivalence classes and that these equivalence classes are totally ordered.

When the SortCompare abstract operation is called with two argumentsj andk, the following steps are taken:

  1. LetjString beToString(j).
  2. LetkString beToString(k).
  3. Lethasj be the result of calling the [[HasProperty]] internal method ofobj with argumentjString.
  4. Lethask be the result of calling the [[HasProperty]] internal method ofobj with argumentkString.
  5. Ifhasj andhask are bothfalse, then return+0.
  6. Ifhasj isfalse, then return 1.
  7. Ifhask isfalse, then return –1.
  8. Letx be the result of calling the [[Get]] internal method ofobj with argumentjString.
  9. Lety be the result of calling the [[Get]] internal method ofobj with argumentkString.
  10. Ifx andy are bothundefined, return+0.
  11. Ifx isundefined, return 1.
  12. Ify isundefined, return −1.
  13. If the argumentcomparefn is notundefined, then
    1. IfIsCallable(comparefn) isfalse, throw aTypeError exception.
    2. Return the result of calling the [[Call]] internal method ofcomparefn passingundefined as thethis value and with argumentsx andy.
  14. LetxString beToString(x).
  15. LetyString beToString(y).
  16. IfxString <yString, return −1.
  17. IfxString >yString, return 1.
  18. Return+0.

NOTE 1 Because non-existent property values always compare greater thanundefined property values, andundefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.

NOTE 2 Thesort function is intentionally generic; it does not require that itsthis value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method. Whether thesort function can be applied successfully to a host object is implementation-dependent.

15.4.4.12 Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )

When thesplice method is called with two or more argumentsstart,deleteCount and (optionally)item1,item2, etc., thedeleteCount elements of the array starting at array indexstart are replaced by the argumentsitem1,item2, etc. An Array object containing the deleted elements (if any) is returned. The following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetA be a new array created as if by the expressionnew Array()whereArray is the standard built-in constructor with that name.
  3. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  4. Letlen beToUint32(lenVal).
  5. LetrelativeStart beToInteger(start).
  6. IfrelativeStart is negative, letactualStart be max((len +relativeStart),0); else letactualStart be min(relativeStart,len).
  7. LetactualDeleteCount be min(max(ToInteger(deleteCount),0),len –actualStart).
  8. Letk be 0.
  9. Repeat, whilek <actualDeleteCount
    1. Letfrom beToString(actualStart+k).
    2. LetfromPresent be the result of calling the [[HasProperty]] internal method ofO with argumentfrom.
    3. IffromPresent istrue, then
      1. LetfromValue be the result of calling the [[Get]] internal method ofO with argumentfrom.
      2. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(k),Property Descriptor {[[Value]]:fromValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    4. Incrementk by 1.
  10. Letitems be an internalList whose elements are, in left to right order, the portion of the actual argument list starting withitem1. The list will be empty if no such items are present.
  11. LetitemCount be the number of elements initems.
  12. IfitemCount <actualDeleteCount, then
    1. Letk beactualStart.
    2. Repeat, whilek < (len –actualDeleteCount)
      1. Letfrom beToString(k+actualDeleteCount).
      2. Letto beToString(k+itemCount).
      3. LetfromPresent be the result of calling the [[HasProperty]] internal method ofO with argumentfrom.
      4. IffromPresent istrue, then
        1. LetfromValue be the result of calling the [[Get]] internal method ofO with argumentfrom.
        2. Call the [[Put]] internal method ofO with argumentsto,fromValue, andtrue.
      5. Else,fromPresent isfalse
        1. Call the [[Delete]] internal method ofO with argumentsto andtrue.
      6. Increase k by 1.
    3. Letk belen.
    4. Repeat, whilek > (len –actualDeleteCount +itemCount)
      1. Call the [[Delete]] internal method ofO with argumentsToString(k–1) andtrue.
      2. Decreasek by 1.
  13. Else ifitemCount >actualDeleteCount, then
    1. Letk be (len –actualDeleteCount).
    2. Repeat, whilek >actualStart
      1. Letfrom beToString(k +actualDeleteCount – 1).
      2. Letto beToString(k +itemCount – 1)
      3. LetfromPresent be the result of calling the [[HasProperty]] internal method ofO with argumentfrom.
      4. IffromPresent istrue, then
        1. LetfromValue be the result of calling the [[Get]] internal method ofO with argumentfrom.
        2. Call the [[Put]] internal method ofO with argumentsto,fromValue, andtrue.
      5. Else,fromPresent isfalse
        1. Call the [[Delete]] internal method ofO with argumentto andtrue.
      6. Decreasek by 1.
  14. Letk beactualStart.
  15. Repeat, whileitems is not empty
    1. Remove the first element fromitems and letE be the value of that element.
    2. Call the [[Put]] internal method ofO with argumentsToString(k),E, andtrue.
    3. Increasek by 1.
  16. Call the [[Put]] internal method ofO with arguments"length", (len –actualDeleteCount +itemCount), andtrue.
  17. ReturnA.

Thelength property of thesplice method is2.

NOTE Thesplice function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thesplice function can be applied successfully to a host object is implementation-dependent.

15.4.4.13 Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )

The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

When theunshift method is called with zero or more argumentsitem1,item2, etc., the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenVal be the result of calling the [[Get]] internal method ofO with argument"length".
  3. Letlen beToUint32(lenVal).
  4. LetargCount be the number of actual arguments.
  5. Letk belen.
  6. Repeat, whilek > 0,
    1. Letfrom beToString(k–1).
    2. Letto beToString(k+argCount –1).
    3. LetfromPresent be the result of calling the [[HasProperty]] internal method ofO with argumentfrom.
    4. IffromPresent istrue, then
      1. LetfromValue be the result of calling the [[Get]] internal method ofO with argumentfrom.
      2. Call the [[Put]] internal method ofO with argumentsto,fromValue, andtrue.
    5. Else,fromPresent isfalse
      1. Call the [[Delete]] internal method ofO with argumentsto, andtrue.
    6. Decreasek by 1.
  7. Letj be 0.
  8. Letitems be an internalList whose elements are, in left to right order, the arguments that were passed to this function invocation.
  9. Repeat, whileitems is not empty
    1. Remove the first element fromitems and letE be the value of that element.
    2. Call the [[Put]] internal method ofO with argumentsToString(j),E, andtrue.
    3. Increasej by 1.
  10. Call the [[Put]] internal method ofO with arguments"length",len+argCount, andtrue.
  11. Returnlen+argCount.

Thelength property of theunshift method is1.

NOTE Theunshift function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theunshift function can be applied successfully to a host object is implementation-dependent.

15.4.4.14 Array.prototype.indexOf ( searchElement [ , fromIndex ] )

indexOf comparessearchElement to the elements of the array, in ascending order, using the internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the first such position; otherwise, -1 is returned.

The optional second argumentfromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to computefromIndex. If the computed index is less than 0, the whole array will be searched.

When theindexOf method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. Iflen is 0, return -1.
  5. If argumentfromIndex was passed letn beToInteger(fromIndex); else letn be 0.
  6. Ifn ≥len, return -1.
  7. Ifn ≥ 0, then
    1. Letk ben.
  8. Else,n<0
    1. Letk belen -abs(n).
    2. Ifk is less than 0, then letk be 0.
  9. Repeat, whilek<len
    1. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentToString(k).
    2. IfkPresent istrue, then
      1. LetelementK be the result of calling the [[Get]] internal method ofO with the argumentToString(k).
      2. Letsame be the result of applying the Strict Equality Comparison Algorithm tosearchElement andelementK.
      3. Ifsame istrue, returnk.
    3. Increasek by 1.
  10. Return -1.

Thelength property of theindexOf method is1.

NOTE TheindexOf function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theindexOf function can be applied successfully to a host object is implementation-dependent.

15.4.4.15 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

lastIndexOf comparessearchElement to the elements of the array in descending order using the internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the last such position; otherwise, -1 is returned.

The optional second argumentfromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset from the end of the array to computefromIndex. If the computed index is less than 0, -1 is returned.

When thelastIndexOf method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. Iflen is 0, return -1.
  5. If argumentfromIndex was passed letn beToInteger(fromIndex); else letn belen-1.
  6. Ifn ≥0, then letk be min(n,len – 1).
  7. Else,n < 0
    1. Letk belen -abs(n).
  8. Repeat, whilek≥0
    1. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentToString(k).
    2. IfkPresent istrue, then
      1. LetelementK be the result of calling the [[Get]] internal method ofO with the argumentToString(k).
      2. Letsame be the result of applying the Strict Equality Comparison Algorithm tosearchElement andelementK.
      3. Ifsame istrue, returnk.
    3. Decreasek by 1.
  9. Return -1.

Thelength property of thelastIndexOf method is1.

NOTE ThelastIndexOf function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thelastIndexOf function can be applied successfully to a host object is implementation-dependent.

15.4.4.16 Array.prototype.every ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean valuetrue orfalse.every callscallbackfn once for each element present in the array, in ascending order, until it finds one wherecallbackfn returnsfalse. If such an element is found,every immediately returnsfalse. Otherwise, ifcallbackfn returnedtrue for all elements,every will returntrue.callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If athisArg parameter is provided, it will be used as thethis value for each invocation ofcallbackfn. If it is not provided,undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed byevery is set before the first call tocallbackfn. Elements which are appended to the array after the call toevery begins will not be visited bycallbackfn. If existing elements of the array are changed, their value as passed tocallbackfn will be the value at the timeevery visits them; elements that are deleted after the call toevery begins and before being visited are not visited.every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returnstrue.

When theevery method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. IfthisArg was supplied, letT bethisArg; else letT beundefined.
  6. Letk be 0.
  7. Repeat, whilek <len
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. LettestResult be the result of calling the [[Call]] internal method ofcallbackfn withT as thethis value and argument list containingkValue,k, andO.
      3. IfToBoolean(testResult) isfalse, returnfalse.
    4. Increasek by 1.
  8. Returntrue.

Thelength property of theevery method is1.

NOTE Theevery function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theevery function can be applied successfully to a host object is implementation-dependent.

15.4.4.17 Array.prototype.some ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean valuetrue orfalse.some callscallbackfn once for each element present in the array, in ascending order, until it finds one wherecallbackfn returnstrue. If such an element is found,some immediately returnstrue. Otherwise,some returnsfalse.callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If athisArg parameter is provided, it will be used as thethis value for each invocation ofcallbackfn. If it is not provided,undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed bysome is set before the first call tocallbackfn. Elements that are appended to the array after the call tosome begins will not be visited bycallbackfn. If existing elements of the array are changed, their value as passed tocallbackfn will be the value at the time thatsome visits them; elements that are deleted after the call tosome begins and before being visited are not visited.some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returnsfalse.

When thesome method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. IfthisArg was supplied, letT bethisArg; else letT beundefined.
  6. Letk be 0.
  7. Repeat, whilek <len
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. LettestResult be the result of calling the [[Call]] internal method ofcallbackfn withT as thethis value and argument list containingkValue,k, andO.
      3. IfToBoolean(testResult) istrue, returntrue.
    4. Increasek by 1.
  8. Returnfalse.

Thelength property of thesome method is1.

NOTE Thesome function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thesome function can be applied successfully to a host object is implementation-dependent.

15.4.4.18 Array.prototype.forEach ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments.forEach callscallbackfn once for each element present in the array, in ascending order.callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If athisArg parameter is provided, it will be used as thethis value for each invocation ofcallbackfn. If it is not provided,undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed byforEach is set before the first call tocallbackfn. Elements which are appended to the array after the call toforEach begins will not be visited bycallbackfn. If existing elements of the array are changed, their value as passed to callback will be the value at the timeforEach visits them; elements that are deleted after the call toforEach begins and before being visited are not visited.

When theforEach method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. IfthisArg was supplied, letT bethisArg; else letT beundefined.
  6. Letk be 0.
  7. Repeat, whilek <len
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. Call the [[Call]] internal method ofcallbackfn withT as thethis value and argument list containingkValue,k, andO.
    4. Increasek by 1.
  8. Returnundefined.

Thelength property of theforEach method is1.

NOTE TheforEach function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether theforEach function can be applied successfully to a host object is implementation-dependent.

15.4.4.19 Array.prototype.map ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments.map callscallbackfn once for each element in the array, in ascending order, and constructs a new Array from the results.callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If athisArg parameter is provided, it will be used as thethis value for each invocation ofcallbackfn. If it is not provided,undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed bymap is set before the first call tocallbackfn. Elements which are appended to the array after the call tomap begins will not be visited bycallbackfn. If existing elements of the array are changed, their value as passed tocallbackfn will be the value at the timemap visits them; elements that are deleted after the call tomap begins and before being visited are not visited.

When themap method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. IfthisArg was supplied, letT bethisArg; else letT beundefined.
  6. LetA be a new array created as if by the expressionnew Array(len) whereArray is the standard built-in constructor with that name andlen is the value oflen.
  7. Letk be 0.
  8. Repeat, whilek <len
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. LetmappedValue be the result of calling the [[Call]] internal method ofcallbackfn withT as thethis value and argument list containingkValue,k, andO.
      3. Call the [[DefineOwnProperty]] internal method ofA with argumentsPk,Property Descriptor {[[Value]]:mappedValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    4. Increasek by 1.
  9. ReturnA.

Thelength property of themap method is1.

NOTE Themap function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether themap function can be applied successfully to a host object is implementation-dependent.

15.4.4.20 Array.prototype.filter ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean valuetrue orfalse.filter callscallbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for whichcallbackfn returnstrue.callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If athisArg parameter is provided, it will be used as thethis value for each invocation ofcallbackfn. If it is not provided,undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed byfilter is set before the first call tocallbackfn. Elements which are appended to the array after the call tofilter begins will not be visited bycallbackfn. If existing elements of the array are changed their value as passed tocallbackfn will be the value at the timefilter visits them; elements that are deleted after the call tofilter begins and before being visited are not visited.

When thefilter method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. IfthisArg was supplied, letT bethisArg; else letT beundefined.
  6. LetA be a new array created as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
  7. Letk be 0.
  8. Letto be 0.
  9. Repeat, whilek <len
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. Letselected be the result of calling the [[Call]] internal method ofcallbackfn withT as thethis value and argument list containingkValue,k, andO.
      3. IfToBoolean(selected) istrue, then
        1. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(to),Property Descriptor {[[Value]]:kValue, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
        2. Increaseto by 1.
    4. Increasek by 1.
  10. ReturnA.

Thelength property of thefilter method is1.

NOTE Thefilter function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thefilter function can be applied successfully to a host object is implementation-dependent.

15.4.4.21 Array.prototype.reduce ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments.reduce calls the callback, as a function, once for each element present in the array, in ascending order.

callbackfn is called with four arguments: thepreviousValue (or value from the previous call tocallbackfn), thecurrentValue (value of the current element), thecurrentIndex, and the object being traversed. The first time that callback is called, thepreviousValue andcurrentValue can be one of two values. If aninitialValue was provided in the call toreduce, thenpreviousValue will be equal toinitialValue andcurrentValue will be equal to the first value in the array. If noinitialValue was provided, thenpreviousValue will be equal to the first value in the array andcurrentValue will be equal to the second. It is aTypeError if the array contains no elements andinitialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed byreduce is set before the first call tocallbackfn. Elements that are appended to the array after the call toreduce begins will not be visited bycallbackfn. If existing elements of the array are changed, their value as passed tocallbackfn will be the value at the timereduce visits them; elements that are deleted after the call toreduce begins and before being visited are not visited.

When thereduce method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. If len is 0 andinitialValue is not present, throw aTypeError exception.
  6. Letk be 0.
  7. IfinitialValue is present, then
    1. Setaccumulator toinitialValue.
  8. Else,initialValue is not present
    1. LetkPresent befalse.
    2. Repeat, whilekPresent isfalse andk <len
      1. LetPk beToString(k).
      2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
      3. IfkPresent istrue, then
        1. Letaccumulator be the result of calling the [[Get]] internal method ofO with argumentPk.
      4. Increasek by 1.
    3. IfkPresent isfalse, throw aTypeError exception.
  9. Repeat, whilek <len
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. Letaccumulator be the result of calling the [[Call]] internal method ofcallbackfn withundefined as thethis value and argument list containingaccumulator,kValue,k, andO.
    4. Increasek by 1.
  10. Returnaccumulator.

Thelength property of thereduce method is1.

NOTE Thereduce function is intentionally generic; it does not require that itsthis value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thereduce function can be applied successfully to a host object is implementation-dependent.

15.4.4.22 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments.reduceRight calls the callback, as a function, once for each element present in the array, in descending order.

callbackfn is called with four arguments: thepreviousValue (or value from the previous call tocallbackfn), thecurrentValue (value of the current element), thecurrentIndex, and the object being traversed. The first time the function is called, thepreviousValue andcurrentValue can be one of two values. If aninitialValue was provided in the call toreduceRight, thenpreviousValue will be equal toinitialValue andcurrentValue will be equal to the last value in the array. If noinitialValue was provided, thenpreviousValue will be equal to the last value in the array andcurrentValue will be equal to the second-to-last value. It is aTypeError if the array contains no elements andinitialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls tocallbackfn.

The range of elements processed byreduceRight is set before the first call tocallbackfn. Elements that are appended to the array after the call toreduceRight begins will not be visited bycallbackfn. If existing elements of the array are changed bycallbackfn, their value as passed tocallbackfn will be the value at the timereduceRight visits them; elements that are deleted after the call toreduceRight begins and before being visited are not visited.

When thereduceRight method is called with one or two arguments, the following steps are taken:

  1. LetO be the result of callingToObject passing thethis value as the argument.
  2. LetlenValue be the result of calling the [[Get]] internal method ofO with the argument"length".
  3. Letlen beToUint32(lenValue).
  4. IfIsCallable(callbackfn) isfalse, throw aTypeError exception.
  5. If len is 0 andinitialValue is not present, throw aTypeError exception.
  6. Letk belen-1.
  7. IfinitialValue is present, then
    1. Setaccumulator toinitialValue.
  8. Else,initialValue is not present
    1. LetkPresent befalse.
    2. Repeat, whilekPresent isfalse andk ≥ 0
      1. LetPk beToString(k).
      2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
      3. IfkPresent istrue, then
        1. Letaccumulator be the result of calling the [[Get]] internal method ofO with argumentPk.
      4. Decreasek by 1.
    3. IfkPresent isfalse, throw aTypeError exception.
  9. Repeat, whilek ≥ 0
    1. LetPk beToString(k).
    2. LetkPresent be the result of calling the [[HasProperty]] internal method ofO with argumentPk.
    3. IfkPresent istrue, then
      1. LetkValue be the result of calling the [[Get]] internal method ofO with argumentPk.
      2. Letaccumulator be the result of calling the [[Call]] internal method ofcallbackfn withundefined as thethis value and argument list containingaccumulator,kValue,k, andO.
    4. Decreasek by 1.
  10. Returnaccumulator.

Thelength property of thereduceRight method is1.

NOTE ThereduceRight function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether thereduceRight function can be applied successfully to a host object is implementation-dependent.

15.4.5 Properties of Array Instances

Array instances inherit properties from the Array prototype object and their [[Class]] internal property value is"Array". Array instances also have the following properties.

15.4.5.1 [[DefineOwnProperty]] ( P, Desc, Throw )

Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native ECMAScript objects (8.12.9).

AssumeA is an Array object,Desc is aProperty Descriptor, andThrow is a Boolean flag.

In the following algorithm, the term “Reject” means “IfThrow istrue, then throw aTypeError exception, otherwise returnfalse.”

When the [[DefineOwnProperty]] internal method ofA is called with propertyP,Property DescriptorDesc, and Boolean flagThrow, the following steps are taken:

  1. LetoldLenDesc be the result of calling the [[GetOwnProperty]] internal method ofA passing"length" as the argument. The result will never beundefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
  2. LetoldLen beoldLenDesc.[[Value]].
  3. IfP is"length", then
    1. If the [[Value]] field ofDesc is absent, then
      1. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) onA passing"length",Desc, andThrow as arguments.
    2. LetnewLenDesc be a copy ofDesc.
    3. LetnewLen beToUint32(Desc.[[Value]]).
    4. IfnewLen is not equal toToNumber(Desc.[[Value]]), throw aRangeError exception.
    5. SetnewLenDesc.[[Value] tonewLen.
    6. IfnewLen ≥oldLen, then
      1. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) onA passing"length",newLenDesc, andThrow as arguments.
    7. Reject ifoldLenDesc.[[Writable]] isfalse.
    8. IfnewLenDesc.[[Writable]] is absent or has the valuetrue, letnewWritable betrue.
    9. Else,
      1. Need to defer setting the [[Writable]] attribute tofalse in case any elements cannot be deleted.
      2. LetnewWritable befalse.
      3. SetnewLenDesc.[[Writable] totrue.
    10. Letsucceeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) onA passing"length",newLenDesc, andThrow as arguments.
    11. Ifsucceeded isfalse, returnfalse.
    12. WhilenewLen <oldLen repeat,
      1. SetoldLen tooldLen – 1.
      2. LetdeleteSucceeded be the result of calling the [[Delete]] internal method ofA passingToString(oldLen) andfalse as arguments.
      3. IfdeleteSucceeded isfalse, then
        1. SetnewLenDesc.[[Value] tooldLen+1.
        2. IfnewWritable isfalse, setnewLenDesc.[[Writable] tofalse.
        3. Call the default [[DefineOwnProperty]] internal method (8.12.9) onA passing"length",newLenDesc, andfalse as arguments.
        4. Reject.
    13. IfnewWritable isfalse, then
      1. Call the default [[DefineOwnProperty]] internal method (8.12.9) onA passing"length",Property Descriptor{[[Writable]]:false}, andfalse as arguments. This call will always returntrue.
    14. Returntrue.
  4. Else ifP is an array index (15.4), then
    1. Letindex beToUint32(P).
    2. Reject ifindex ≥oldLen andoldLenDesc.[[Writable]] isfalse.
    3. Letsucceeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) onA passingP,Desc, andfalse as arguments.
    4. Reject ifsucceeded isfalse.
    5. Ifindex ≥oldLen
      1. SetoldLenDesc.[[Value]] toindex + 1.
      2. Call the default [[DefineOwnProperty]] internal method (8.12.9) onA passing"length",oldLenDesc, andfalse as arguments. This call will always returntrue.
    6. Returntrue.
  5. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) onA passingP,Desc, andThrow as arguments.

15.4.5.2 length

Thelength property of this Array object is a data property whose value is always numerically greater than the name of every deletable property whose name is an array index.

Thelength property initially has the attributes{ [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false }.

NOTE Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array will result in the length being set to a numeric value that is one greater than that largest numeric property name. See15.4.5.1.

15.5 String Objects

15.5.1 The String Constructor Called as a Function

WhenString is called as a function rather than as a constructor, it performs a type conversion.

15.5.1.1 String ( [ value ] )

Returns a String value (not a String object) computed byToString(value). Ifvalue is not supplied, the empty String"" is returned.

15.5.2 The String Constructor

WhenString is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.5.2.1 new String ( [ value ] )

The[[Prototype]] internal property of the newly constructed object is set to the standard built-in String prototype object that is the initial value ofString.prototype (15.5.3.1).

The[[Class]] internal property of the newly constructed object is set to"String".

The[[Extensible]] internal property of the newly constructed object is set totrue.

The[[PrimitiveValue]] internal property of the newly constructed object is set toToString(value), or to the empty String ifvalue is not supplied.

15.5.3 Properties of the String Constructor

The value of the [[Prototype]] internal property of the String constructor is the standard built-in Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is1), the String constructor has the following properties:

15.5.3.1 String.prototype

The initial value ofString.prototype is the standard built-in String prototype object (15.5.4).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.5.3.2 String.fromCharCode ( [ char0 [ , char1 [ , … ] ] ] )

Returns a String value containing as many characters as the number of arguments. Each argument specifies one character of the resulting String, with the first argument specifying the first character, and so on, from left to right. An argument is converted to a character by applying the operationToUint16 (9.7) and regarding the resulting 16-bit integer as the code unit value of a character. If no arguments are supplied, the result is the empty String.

Thelength property of thefromCharCode function is1.

15.5.4 Properties of the String Prototype Object

The String prototype object is itself a String object (its [[Class]] is"String") whose value is an empty String.

The value of the [[Prototype]] internal property of the String prototype object is the standard built-in Object prototype object (15.2.4).

15.5.4.1 String.prototype.constructor

The initial value ofString.prototype.constructor is the built-inString constructor.

15.5.4.2 String.prototype.toString ( )

Returns this String value. (Note that, for a String object, thetoString method happens to return the same thing as thevalueOf method.)

ThetoString function is not generic; it throws aTypeError exception if itsthis value is not a String or a String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.3 String.prototype.valueOf ( )

Returns this String value.

ThevalueOf function is not generic; it throws aTypeError exception if itsthis value is not a String or String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.4 String.prototype.charAt (pos)

Returns a String containing the character at positionpos in the String resulting from converting this object to a String. If there is no character at that position, the result is the empty String. The result is a String value, not a String object.

Ifpos is a value of Number type that is an integer, then the result ofx.charAt(pos) is equal to the result ofx.substring(pos,pos+1).

When thecharAt method is called with one argumentpos, the following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. Letposition beToInteger(pos).
  4. Letsize be the number of characters inS.
  5. Ifposition < 0 orposition ≥size, return the empty String.
  6. Return a String of length 1, containing one character fromS, namely the character at positionposition, where the first (leftmost) character inS is considered to be at position 0, the next one at position 1, and so on.

NOTE ThecharAt function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.5 String.prototype.charCodeAt (pos)

Returns a Number (a nonnegative integer less than216) representing the code unit value of the character at positionpos in the String resulting from converting this object to a String. If there is no character at that position, the result isNaN.

When thecharCodeAt method is called with one argumentpos, the following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. Letposition beToInteger(pos).
  4. Letsize be the number of characters inS.
  5. Ifposition < 0 orposition ≥size, returnNaN.
  6. Return a value of Number type, whose value is the code unit value of the character at positionposition in the StringS, where the first (leftmost) character inS is considered to be at position 0, the next one at position 1, and so on.

NOTE ThecharCodeAt function is intentionally generic; it does not require that itsthis value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.6 String.prototype.concat ( [ string1 [ , string2 [ , … ] ] ] )

When theconcat method is called with zero or more argumentsstring1,string2, etc., it returns a String consisting of the characters of this object (converted to a String) followed by the characters of each ofstring1,string2, etc. (where each argument is converted to a String). The result is a String value, not a String object. The following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. Letargs be an internal list that is a copy of the argument list passed to this function.
  4. LetR beS.
  5. Repeat, whileargs is not empty
    1. Remove the first element fromargs and letnext be the value of that element.
    2. LetR be the String value consisting of the characters in the previous value ofR followed by the characters ofToString(next).
  6. ReturnR.

Thelength property of theconcat method is1.

NOTE Theconcat function is intentionally generic; it does not require that itsthis value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.7 String.prototype.indexOf (searchString, position)

IfsearchString appears as a substring of the result of converting this object to a String, at one or more positions that are greater than or equal toposition, then the index of the smallest such position is returned; otherwise,‑1 is returned. Ifposition isundefined, 0 is assumed, so as to search all of the String.

TheindexOf method takes two arguments,searchString andposition, and performs the following steps:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. LetsearchStr beToString(searchString).
  4. Letpos beToInteger(position). (Ifposition isundefined, this step produces the value0).
  5. Letlen be the number of characters inS.
  6. Letstart be min(max(pos, 0),len).
  7. LetsearchLen be the number of characters insearchStr.
  8. Return the smallest possible integerk not smaller thanstart such thatk+searchLen is not greater thanlen, and for all nonnegative integersj less thansearchLen, the character at positionk+j ofS is the same as the character at positionj ofsearchStr; but if there is no such integerk, then return the value-1.

Thelength property of theindexOf method is1.

NOTE TheindexOf function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.8 String.prototype.lastIndexOf (searchString, position)

IfsearchString appears as a substring of the result of converting this object to a String at one or more positions that are smaller than or equal toposition, then the index of the greatest such position is returned; otherwise,‑1 is returned. Ifposition isundefined, the length of the String value is assumed, so as to search all of the String.

ThelastIndexOf method takes two arguments,searchString andposition, and performs the following steps:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. LetsearchStr beToString(searchString).
  4. LetnumPos beToNumber(position). (Ifposition isundefined, this step produces the valueNaN).
  5. IfnumPos isNaN, letpos be+∞; otherwise, letpos beToInteger(numPos).
  6. Letlen be the number of characters inS.
  7. Letstart min(max(pos, 0),len).
  8. LetsearchLen be the number of characters insearchStr.
  9. Return the largest possible nonnegative integerk not larger thanstart such thatk+searchLen is not greater thanlen, and for all nonnegative integersj less thansearchLen, the character at positionk+j ofS is the same as the character at positionj ofsearchStr; but if there is no such integerk, then return the value-1.

Thelength property of thelastIndexOf method is1.

NOTE ThelastIndexOf function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.9 String.prototype.localeCompare (that)

When thelocaleCompare method is called with one argumentthat, it returns a Number other thanNaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) withthat (converted to a String). The two Strings areS andThat. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by the system default locale, and will be negative, zero, or positive, depending on whetherS comes beforeThat in the sort order, the Strings are equal, orS comes afterThat in the sort order, respectively.

Before perform the comparisons the following steps are performed to prepare the Strings:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. LetThat beToString(that).

ThelocaleCompare method, if considered as a function of two argumentsthis andthat, is a consistent comparison function (as defined in15.4.4.11) on the set of all Strings.

The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings and to return0 when comparing Strings that are considered canonically equivalent by the Unicode standard.

If no language-sensitive comparison at all is available from the host environment, this function may perform a bitwise comparison.

NOTE 1 ThelocaleCompare method itself is not directly suitable as an argument toArray.prototype.sort because the latter requires a function of two arguments.

NOTE 2 This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment’s current locale. It is strongly recommended that this function treat Strings that are canonically equivalent according to the Unicode standard as identical (in other words, compare the Strings as if they had both been converted to Normalised Form C or D first). It is also recommended that this function not honour Unicode compatibility equivalences or decompositions.

NOTE 3 The second parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 4 ThelocaleCompare function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.10 String.prototype.match (regexp)

When thematch method is called with argumentregexp, the following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. IfType(regexp) is Object and the value of the [[Class]] internal property ofregexp is"RegExp", then letrx beregexp;
  4. Else, letrx be a new RegExp object created as if by the expressionnew RegExp(regexp) whereRegExp is the standard built-in constructor with that name.
  5. Letglobal be the result of calling the [[Get]] internal method ofrx with argument"global".
  6. Letexec be the standard built-in functionRegExp.prototype.exec (see 15.10.6.2)
  7. Ifglobal is nottrue, then
    1. Return the result of calling the [[Call]] internal method ofexec withrx as thethis value and argument list containingS.
  8. Else,global istrue
    1. Call the [[Put]] internal method ofrx with arguments"lastIndex" and 0.
    2. LetA be a new array created as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
    3. LetpreviousLastIndex be 0.
    4. Letn be 0.
    5. LetlastMatch betrue.
    6. Repeat, whilelastMatch istrue
      1. Letresult be the result of calling the [[Call]] internal method ofexec withrx as thethis value and argument list containingS.
      2. Ifresult isnull, then setlastMatch tofalse.
      3. Else,result is notnull
        1. LetthisIndex be the result of calling the [[Get]] internal method ofrx with argument"lastIndex".
        2. IfthisIndex =previousLastIndex then
          1. Call the [[Put]] internal method ofrx with arguments"lastIndex" andthisIndex+1.
          2. SetpreviousLastIndex tothisIndex+1.
        3. Else, setpreviousLastIndex tothisIndex.
        4. LetmatchStr be the result of calling the [[Get]] internal method ofresult with argument"0".
        5. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(n), theProperty Descriptor {[[Value]]:matchStr, [[Writable]]:true, [[Enumerable]]:true, [[configurable]]:true}, andfalse.
        6. Incrementn.
    7. Ifn = 0, then returnnull.
    8. ReturnA.

NOTE Thematch function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.11 String.prototype.replace (searchValue, replaceValue)

First setstring according to the following steps:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. Letstring be the result of callingToString, giving it thethis value as its argument.

IfsearchValue is a regular expression (an object whose [[Class]] internal property is"RegExp"), do the following: IfsearchValue.global isfalse, then searchstring for the first match of the regular expressionsearchValue. IfsearchValue.global istrue, then searchstring for all matches of the regular expressionsearchValue. Do the search in the same manner as inString.prototype.match, including the update ofsearchValue.lastIndex. Letm be the number of left capturing parentheses insearchValue (usingNcapturingParens as specified in15.10.2.1).

IfsearchValue is not a regular expression, letsearchString beToString(searchValue) and searchstring for the first occurrence ofsearchString. Letm be 0.

IfreplaceValue is a function, then for each matched substring, call the function with the followingm + 3 arguments. Argument 1 is the substring that matched. IfsearchValue is a regular expression, the nextm arguments are all of the captures in the MatchResult (see 15.10.2.1). Argumentm + 2 is the offset withinstring where the match occurred, and argumentm + 3 isstring. The result is a String value derived from the original input by replacing each matched substring with the corresponding return value of the function call, converted to a String if need be.

Otherwise, letnewstring denote the result of convertingreplaceValue to a String. The result is a String value derived from the original input String by replacing each matched substring with a String derived fromnewstring by replacing characters innewstring by replacement text as specified in Table 22. These$ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements. For example,"$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns"$1-$11,$1-$22". A$ innewstring that does not match any of the forms below is left as is.

Table 22 — Replacement Text Symbol Substitutions
CharactersReplacement text
$$$
$&The matched substring.
$‘The portion ofstring that precedes the matched substring.
$’The portion ofstring that follows the matched substring.
$nThenth capture, wheren is a single digit in the range1 to9 and$n is not followed by a decimal digit. Ifn≤m and thenth capture isundefined, use the empty String instead. Ifn>m, the result is implementation-defined.
$nnThennth capture, wherenn is a two-digit decimal number in the range01 to99. Ifnn≤m and thennth capture isundefined, use the empty String instead. Ifnn>m, the result is implementation-defined.

NOTE Thereplace function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.12 String.prototype.search (regexp)

When the search method is called with argumentregexp, the following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. Letstring be the result of callingToString, giving it thethis value as its argument.
  3. IfType(regexp) is Object and the value of the [[Class]] internal property ofregexp is"RegExp", then letrx beregexp;
  4. Else, letrx be a new RegExp object created as if by the expressionnew RegExp(regexp) whereRegExp is the standard built-in constructor with that name.
  5. Search the valuestring from its beginning for an occurrence of the regular expression patternrx. Letresult be a Number indicating the offset withinstring where the pattern matched, or –1 if there was no match. ThelastIndex andglobal properties ofregexp are ignored when performing the search. ThelastIndex property ofregexp is left unchanged.
  6. Returnresult.

NOTE Thesearch function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.13 String.prototype.slice (start, end)

Theslice method takes two arguments,start andend, and returns a substring of the result of converting this object to a String, starting from character positionstart and running to, but not including, character positionend (or through the end of the String ifend isundefined). Ifstart is negative, it is treated assourceLength+start wheresourceLength is the length of the String. Ifend is negative, it is treated assourceLength+end wheresourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. Letlen be the number of characters inS.
  4. LetintStart beToInteger(start).
  5. Ifend isundefined, letintEnd belen; else letintEnd beToInteger(end).
  6. IfintStart is negative, letfrom be max(len +intStart,0); else letfrom be min(intStart,len).
  7. IfintEnd is negative, letto be max(len +intEnd,0); else letto be min(intEnd,len).
  8. Letspan be max(to –from,0).
  9. Return a String containingspan consecutive characters fromS beginning with the character at positionfrom.

Thelength property of theslice method is2.

NOTE Theslice function is intentionally generic; it does not require that itsthis value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.14 String.prototype.split (separator, limit)

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences ofseparator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value ofseparator may be a String of any length or it may be a RegExp object (i.e., an object whose [[Class]] internal property is"RegExp"; see15.10).

The value ofseparator may be an empty String, an empty regular expression, or a regular expression that can match an empty String. In this case,separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, ifseparator is the empty String, the String is split up into individual characters; the length of the result array equals the length of the String, and each substring contains one character.) Ifseparator is a regular expression, only the first match at a given position of thethis String is considered, even if backtracking could yield a non-empty-substring match at that position. (For example,"ab".split(/a*?/) evaluates to the array["a","b"], while"ab".split(/a*/) evaluates to the array["","b"].)

If thethis object is (or converts to) the empty String, the result depends on whetherseparator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

Ifseparator is a regular expression that contains capturing parentheses, then each timeseparator is matched the results (including anyundefined results) of the capturing parentheses are spliced into the output array. For example,

"A<B>bold</B>and<CODE>coded</CODE>".split(/<(\/)?([^<>]+)>/)

evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined,
 "CODE", "coded", "/", "CODE", ""]

Ifseparator isundefined, then the result array contains just one String, which is thethis value (converted to a String). Iflimit is notundefined, then the output array is truncated so that it contains no more thanlimit elements.

When thesplit method is called, the following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. LetA be a new array created as if by the expressionnew Array()whereArray is the standard built-in constructor with that name.
  4. LetlengthA be 0.
  5. Iflimit isundefined, letlim = 232–1; else letlim =ToUint32(limit).
  6. Lets be the number of characters inS.
  7. Letp = 0.
  8. Ifseparator is a RegExp object (its [[Class]] is"RegExp"), letR =separator; otherwise letR =ToString(separator).
  9. Iflim = 0, returnA.
  10. Ifseparator isundefined, then
    1. Call the [[DefineOwnProperty]] internal method ofA with arguments"0",Property Descriptor {[[Value]]:S, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    2. ReturnA.
  11. Ifs = 0, then
    1. CallSplitMatch(S, 0,R) and letz be its MatchResult result.
    2. Ifz is notfailure, returnA.
    3. Call the [[DefineOwnProperty]] internal method ofA with arguments"0",Property Descriptor {[[Value]]:S, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
    4. ReturnA.
  12. Letq =p.
  13. Repeat, whileq ≠s
    1. CallSplitMatch(S, q, R) and letz be its MatchResult result.
    2. Ifz isfailure, then letq =q+1.
    3. Else,z is notfailure
      1. z must be a State. Lete bez'sendIndex and letcap bez'scaptures array.
      2. Ife =p, then letq =q+1.
      3. Else,e ≠p
        1. LetT be a String value equal to the substring ofS consisting of the characters at positionsp (inclusive) throughq (exclusive).
        2. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(lengthA),Property Descriptor {[[Value]]:T, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
        3. IncrementlengthA by 1.
        4. IflengthA =lim, returnA.
        5. Let p = e.
        6. Leti = 0.
        7. Repeat, whilei is not equal to the number of elements incap.
          1. Leti =i+1.
          2. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(lengthA),Property Descriptor {[[Value]]:cap[i], [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
          3. IncrementlengthA by 1.
          4. IflengthA =lim, returnA.
        8. Letq =p.
  14. LetT be a String value equal to the substring ofS consisting of the characters at positionsp (inclusive) throughs (exclusive).
  15. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(lengthA),Property Descriptor {[[Value]]:T, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  16. ReturnA.

The abstract operationSplitMatch takes three parameters, a StringS, an integerq, and a String or RegExpR, and performs the following in order to return a MatchResult (see 15.10.2.1):

  1. IfR is a RegExp object (its [[Class]] is"RegExp"), then
    1. Call the [[Match]] internal method ofR giving it the argumentsS andq, and return the MatchResult result.
  2. Type(R) must be String. Letr be the number of characters inR.
  3. Lets be the number of characters inS.
  4. Ifq+r >s then return the MatchResultfailure.
  5. If there exists an integeri between 0 (inclusive) andr (exclusive) such that the character at positionq+i ofS is different from the character at positioni ofR, then returnfailure.
  6. Letcap be an empty array of captures (see 15.10.2.1).
  7. Return the State (q+r,cap). (see 15.10.2.1)

Thelength property of thesplit method is2.

NOTE 1 Thesplit method ignores the value of separator.global for separators that are RegExp objects.

NOTE 2 Thesplit function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.15 String.prototype.substring (start, end)

Thesubstring method takes two arguments,start andend, and returns a substring of the result of converting this object to a String, starting from character positionstart and running to, but not including, character positionend of the String (or through the end of the String isend isundefined). The result is a String value, not a String object.

If either argument isNaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.

Ifstart is larger thanend, they are swapped.

The following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. Letlen be the number of characters inS.
  4. LetintStart beToInteger(start).
  5. Ifend isundefined, letintEnd belen; else letintEnd beToInteger(end).
  6. LetfinalStart be min(max(intStart, 0),len).
  7. LetfinalEnd be min(max(intEnd, 0),len).
  8. Letfrom be min(finalStart,finalEnd).
  9. Letto be max(finalStart,finalEnd).
  10. Return a String whose length isto -from, containing characters fromS, namely the characters with indicesfrom throughto −1, in ascending order.

Thelength property of thesubstring method is2.

NOTE Thesubstring function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.16 String.prototype.toLowerCase ( )

The following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. LetL be a String where each character ofL is either the Unicode lowercase equivalent of the corresponding character ofS or the actual corresponding character ofS if no Unicode lowercase equivalent exists.
  4. ReturnL.

For the purposes of this operation, the 16-bit code units of the Strings are treated as code points in the Unicode Basic Multilingual Plane. Surrogate code points are directly transferred fromS toL without any mapping.

The result must be derived according to the case mappings in the Unicode character database (this explicitly includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that accompanies it in Unicode 2.1.8 and later).

NOTE 1 The case mapping of some characters may produce multiple characters. In this case the result String may not be the same length as the source String. Because bothtoUpperCase andtoLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words,s.toUpperCase().toLowerCase() is not necessarily equal tos.toLowerCase().

NOTE 2 ThetoLowerCase function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.17 String.prototype.toLocaleLowerCase ( )

This function works exactly the same astoLowerCase except that its result is intended to yield the correct result for the host environment’s current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 ThetoLocaleLowerCase function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.18 String.prototype.toUpperCase ( )

This function behaves in exactly the same way asString.prototype.toLowerCase, except that characters are mapped to theiruppercase equivalents as specified in the Unicode Character Database.

NOTE ThetoUpperCase function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.19 String.prototype.toLocaleUpperCase ( )

This function works exactly the same astoUpperCase except that its result is intended to yield the correct result for the host environment’s current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

NOTE 1 The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

NOTE 2 ThetoLocaleUpperCase function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.20 String.prototype.trim ( )

The following steps are taken:

  1. CallCheckObjectCoercible passing thethis value as its argument.
  2. LetS be the result of callingToString, giving it thethis value as its argument.
  3. LetT be a String value that is a copy ofS with both leading and trailing white space removed. The definition of white space is the union ofWhiteSpace andLineTerminator.
  4. ReturnT.

NOTE Thetrim function is intentionally generic; it does not require that itsthis value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.5 Properties of String Instances

String instances inherit properties from the String prototype object and their [[Class]] internal property value is"String". String instances also have a [[PrimitiveValue]] internal property, alength property, and a set of enumerable properties with array index names.

The [[PrimitiveValue]] internal property is the String value represented by this String object. The array index named properties correspond to the individual characters of the String value. A special [[GetOwnProperty]] internal method is used to specify the number, values, and attributes of the array index named properties.

15.5.5.1 length

The number of characters in the String value represented by this String object.

Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.5.5.2 [[GetOwnProperty]] ( P )

String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (8.12.1). This special internal method provides access to named properties corresponding to the individual characters of String objects.

AssumeS is a String object andP is a String.

When the [[GetOwnProperty]] internal method ofS is called with property nameP, the following steps are taken:

  1. Letdesc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) onS with argumentP.
  2. Ifdesc is notundefined returndesc.
  3. IfToString(abs(ToInteger(P))) is not the same value asP, returnundefined.
  4. Letstr be the String value of the [[PrimitiveValue]] internal property ofS.
  5. Letindex beToInteger(P).
  6. Letlen be the number of characters instr.
  7. Iflen ≤index, returnundefined.
  8. LetresultStr be a String of length 1, containing one character fromstr, specifically the character at positionindex, where the first (leftmost) character instr is considered to be at position 0, the next one at position 1, and so on.
  9. Return aProperty Descriptor { [[Value]]:resultStr, [[Enumerable]]:true, [[Writable]]:false, [[Configurable]]:false }

15.6 Boolean Objects

15.6.1 The Boolean Constructor Called as a Function

WhenBoolean is called as a function rather than as a constructor, it performs a type conversion.

15.6.1.1 Boolean (value)

Returns a Boolean value (not a Boolean object) computed byToBoolean(value).

15.6.2 The Boolean Constructor

WhenBoolean is called as part of anew expression it is a constructor: it initialises the newly created object.

15.6.2.1 new Boolean (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Boolean prototype object, the one that is the initial value ofBoolean.prototype (15.6.3.1).

The [[Class]] internal property of the newly constructed Boolean object is set to"Boolean".

The [[PrimitiveValue]] internal property of the newly constructed Boolean object is set toToBoolean(value).

The [[Extensible]] internal property of the newly constructed object is set totrue.

15.6.3 Properties of the Boolean Constructor

The value of the [[Prototype]] internal property of the Boolean constructor is the Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is1), the Boolean constructor has the following property:

15.6.3.1 Boolean.prototype

The initial value ofBoolean.prototype is the Boolean prototype object (15.6.4).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.6.4 Properties of the Boolean Prototype Object

The Boolean prototype object is itself a Boolean object (its [[Class]] is"Boolean") whose value isfalse.

The value of the [[Prototype]] internal property of the Boolean prototype object is the standard built-in Object prototype object (15.2.4).

15.6.4.1 Boolean.prototype.constructor

The initial value ofBoolean.prototype.constructor is the built-inBoolean constructor.

15.6.4.2 Boolean.prototype.toString ( )

The following steps are taken:

  1. LetB be thethis value.
  2. IfType(B) is Boolean, then letb beB.
  3. Else ifType(B) is Object and the value of the [[Class]] internal property ofB is"Boolean", then letb be the value of the [[PrimitiveValue]] internal property ofB.
  4. Else throw aTypeError exception.
  5. Ifb istrue, then return"true"; else return"false".

15.6.4.3 Boolean.prototype.valueOf ( )

The following steps are taken:

  1. LetB be thethis value.
  2. IfType(B) is Boolean, then letb beB.
  3. Else ifType(B) is Object and the value of the [[Class]] internal property ofB is"Boolean", then letb be the value of the [[PrimitiveValue]] internal property ofB.
  4. Else throw aTypeError exception.
  5. Returnb.

15.6.5 Properties of Boolean Instances

Boolean instances inherit properties from the Boolean prototype object and their [[Class]] internal property value is"Boolean". Boolean instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Boolean value represented by this Boolean object.

15.7 Number Objects

15.7.1 The Number Constructor Called as a Function

WhenNumber is called as a function rather than as a constructor, it performs a type conversion.

15.7.1.1 Number ( [ value ] )

Returns a Number value (not a Number object) computed byToNumber(value) ifvalue was supplied, else returns+0.

15.7.2 The Number Constructor

WhenNumber is called as part of anew expression it is a constructor: it initialises the newly created object.

15.7.2.1 new Number ( [ value ] )

The [[Prototype]] internal property of the newly constructed object is set to the original Number prototype object, the one that is the initial value ofNumber.prototype (15.7.3.1).

The [[Class]] internal property of the newly constructed object is set to"Number".

The [[PrimitiveValue]] internal property of the newly constructed object is set toToNumber(value) ifvalue was supplied, else to +0.

The [[Extensible]] internal property of the newly constructed object is set totrue.

15.7.3 Properties of the Number Constructor

The value of the [[Prototype]] internal property of the Number constructor is the Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is1), the Number constructor has the following properties:

15.7.3.1 Number.prototype

The initial value ofNumber.prototype is the Number prototype object (15.7.4).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.7.3.2 Number.MAX_VALUE

The value ofNumber.MAX_VALUE is the largest positive finite value of the Number type, which is approximately1.7976931348623157 × 10308.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.7.3.3 Number.MIN_VALUE

The value ofNumber.MIN_VALUE is the smallest positive value of the Number type, which is approximately5 × 10‑324.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.7.3.4 Number.NaN

The value ofNumber.NaN isNaN.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.7.3.5 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is −∞.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.7.3.6 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.7.4 Properties of the Number Prototype Object

The Number prototype object is itself a Number object (its [[Class]] is"Number") whose value is +0.

The value of the [[Prototype]] internal property of the Number prototype object is the standard built-in Object prototype object (15.2.4).

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an Object for which the value of the [[Class]] internal property is"Number".

In the following descriptions of functions that are properties of the Number prototype object, the phrase “this Number object” refers to either the object that is thethis value for the invocation of the function or, ifType(this value) is Number, an object that is created as if by the expressionnew Number(this value) whereNumber is the standard built-in constructor with that name. Also, the phrase “this Number value” refers to either the Number value represented by this Number object, that is, the value of the [[PrimitiveValue]] internal property of this Number object or thethis value if its type is Number. ATypeError exception is thrown if thethis value is neither an object for which the value of the [[Class]] internal property is"Number" or a value whose type is Number.

15.7.4.1 Number.prototype.constructor

The initial value ofNumber.prototype.constructor is the built-inNumber constructor.

15.7.4.2 Number.prototype.toString ( [ radix ] )

The optionalradix should be an integer value in the inclusive range2 to36. Ifradix not present or isundefined the Number10 is used as the value ofradix. IfToInteger(radix) is the Number10 then this Number value is given as an argument to theToString abstract operation; the resulting String value is returned.

IfToInteger(radix) is not an integer between 2 and 36 inclusive throw aRangeError exception. IfToInteger(radix) is an integer from 2 to 36, but not 10, the result is a String representation of this Number value using the specified radix. Lettersa-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent if the radix is not 10, however the algorithm should be a generalisation of that specified in9.8.1.

ThetoString function is not generic; it throws aTypeError exception if itsthis value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.3 Number.prototype.toLocaleString()

Produces a String value that represents this Number value formatted according to the conventions of the host environment’s current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing astoString.

NOTE The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.7.4.4 Number.prototype.valueOf ( )

Returns this Number value.

ThevalueOf function is not generic; it throws aTypeError exception if itsthis value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.5 Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation withfractionDigits digits after the decimal point. IffractionDigits isundefined, 0 is assumed. Specifically, perform the following steps:

  1. Letf beToInteger(fractionDigits). (IffractionDigits isundefined, this step produces the value0).
  2. Iff < 0 orf > 20, throw aRangeError exception.
  3. Letx be this Number value.
  4. Ifx isNaN, return the String"NaN".
  5. Lets be the empty String.
  6. Ifx < 0, then
    1. Let s be "-".
    2. Let x = –x.
  7. Ifx ≥ 1021, then
    1. Letm =ToString(x).
  8. Else,x < 1021
    1. Letn be an integer for which the exact mathematical value ofn ÷ 10f –x is as close to zero as possible. If there are two suchn, pick the largern.
    2. Ifn = 0, letm be the String"0". Otherwise, letm be the String consisting of the digits of the decimal representation ofn (in order, with no leading zeroes).
    3. Iff ≠ 0, then
      1. Letk be the number of characters inm.
      2. Ifk ≤f, then
        1. Letz be the String consisting off+1–k occurrences of the character ‘0’.
        2. Letm be the concatenation of Stringsz andm.
        3. Letk =f + 1.
      3. Leta be the firstk–f characters ofm, and letb be the remainingf characters ofm.
      4. Letm be the concatenation of the three Stringsa,".", andb.
  9. Return the concatenation of the Stringss andm.

Thelength property of thetoFixed method is1.

If thetoFixed method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour oftoFixed for values offractionDigits less than 0 or greater than 20. In this casetoFixed would not necessarily throwRangeError for such values.

NOTE The output oftoFixed may be more precise thantoString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example,

(1000000000000000128).toString() returns"1000000000000000100",
while (1000000000000000128).toFixed(0) returns"1000000000000000128".

15.7.4.6 Number.prototype.toExponential (fractionDigits)

Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point andfractionDigits digits after the significand's decimal point. IffractionDigits isundefined, include as many significand digits as necessary to uniquely specify the Number (just like inToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:

  1. Letx be this Number value.
  2. Letf beToInteger(fractionDigits).
  3. Ifx isNaN, return the String"NaN".
  4. Lets be the empty String.
  5. Ifx< 0, then
    1. Lets be"-".
    2. Letx = –x.
  6. Ifx = +∞, then
    1. Return the concatenation of the Stringss and"Infinity".
  7. IffractionDigits is notundefined and (f < 0 orf > 20), throw aRangeError exception.
  8. Ifx = 0, then
    1. Letf = 0.
    2. Letm be the String consisting off+1 occurrences of the character ‘0’.
    3. Lete = 0.
  9. Else,x ≠ 0
    1. If fractionDigits is notundefined, then
      1. Lete andn be integers such that 10f ≤n < 10f+1 and for which the exact mathematical value ofn × 10e–f –x is as close to zero as possible. If there are two such sets ofe andn, pick thee andn for whichn × 10e–f is larger.
    2. Else,fractionDigits isundefined
      1. Lete,n, andf be integers such thatf ≥ 0, 10f ≤n < 10f+1, the number value for n × 10e–f isx, andf is as small as possible. Note that the decimal representation ofn hasf+1 digits,n is not divisible by 10, and the least significant digit ofn is not necessarily uniquely determined by these criteria.
    3. Letm be the String consisting of the digits of the decimal representation ofn (in order, with no leading zeroes).
  10. Iff ≠ 0, then
    1. Leta be the first character ofm, and letb be the remainingf characters ofm.
    2. Letm be the concatenation of the three Stringsa,".", andb.
  11. Ife = 0, then
    1. Letc ="+".
    2. Letd ="0".
  12. Else
    1. Ife > 0, then letc ="+".
    2. Else,e ≤ 0
      1. Letc ="-".
      2. Lete = –e.
    3. Letd be the String consisting of the digits of the decimal representation ofe (in order, with no leading zeroes).
  13. Letm be the concatenation of the four Stringsm,"e",c, andd.
  14. Return the concatenation of the Stringss andm.

Thelength property of thetoExponential method is1.

If thetoExponential method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour oftoExponential for values offractionDigits less than 0 or greater than 20. In this casetoExponential would not necessarily throwRangeError for such values.

NOTE For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 9.b.i be used as a guideline:

  1. Lete,n, andf be integers such thatf ≥ 0, 10f ≤ n < 10f+1, the number value for n × 10e–f is x, andf is as small as possible. If there are multiple possibilities forn, choose the value ofn for whichn × 10e–f is closest in value tox. If there are two such possible values ofn, choose the one that is even.

15.7.4.7 Number.prototype.toPrecision (precision)

Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point andprecision–1 digits after the significand's decimal point or in decimal fixed notation withprecision significant digits. Ifprecision isundefined, callToString (9.8.1) instead. Specifically, perform the following steps:

  1. Letx be this Number value.
  2. Ifprecision isundefined, returnToString(x).
  3. Letp beToInteger(precision).
  4. Ifx isNaN, return the String"NaN".
  5. Lets be the empty String.
  6. Ifx < 0, then
    1. Lets be"-".
    2. Letx = –x.
  7. Ifx = +∞, then
    1. Return the concatenation of the Stringss and"Infinity".
  8. Ifp < 1 orp > 21, throw aRangeError exception.
  9. Ifx = 0, then
    1. Letm be the String consisting ofp occurrences of the character ‘0’.
    2. Lete = 0.
  10. Elsex ≠ 0,
    1. Lete andn be integers such that 10p–1 ≤n < 10p and for which the exact mathematical value ofn × 10e–p+1 –x is as close to zero as possible. If there are two such sets ofe andn, pick thee andn for whichn × 10e–p+1 is larger.
    2. Letm be the String consisting of the digits of the decimal representation ofn (in order, with no leading zeroes).
    3. Ife < –6 ore ≥p, then
      1. Leta be the first character ofm, and letb be the remainingp–1 characters ofm.
      2. Letm be the concatenation of the three Stringsa,".", andb.
      3. Ife = 0, then
        1. Letc ="+" andd ="0".
      4. Elsee ≠ 0,
        1. Ife > 0, then
          1. Letc ="+".
        2. Elsee < 0,
          1. Letc ="-".
          2. Lete = –e.
        3. Letd be the String consisting of the digits of the decimal representation ofe (in order, with no leading zeroes).
      5. Letm be the concatenation of the five Stringss,m,"e",c, andd.
  11. Ife =p–1, then return the concatenation of the Stringss andm.
  12. Ife ≥ 0, then
    1. Letm be the concatenation of the firste+1 characters ofm, the character ‘.’, and the remainingp– (e+1) characters ofm.
  13. Elsee < 0,
    1. Letm be the concatenation of the String"0.", –(e+1) occurrences of the character ‘0’, and the Stringm.
  14. Return the concatenation of the Stringss andm.

Thelength property of thetoPrecision method is1.

If thetoPrecision method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour oftoPrecision for values ofprecision less than 1 or greater than 21. In this casetoPrecision would not necessarily throwRangeError for such values.

15.7.5 Properties of Number Instances

Number instances inherit properties from the Number prototype object and their [[Class]] internal property value is"Number". Number instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Number value represented by this Number object.

15.8 The Math Object

The Math object is a single object that has some named properties, some of which are functions.

The value of the [[Prototype]] internal property of the Math object is the standard built-in Object prototype object (15.2.4). The value of the [[Class]] internal property of the Math object is"Math".

The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as a constructor with thenew operator.

The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as a function.

NOTE In this specification, the phrase “the Number value forx” has a technical meaning defined in8.5.

15.8.1 Value Properties of the Math Object

15.8.1.1 E

The Number value fore, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.8.1.2 LN10

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.8.1.3 LN2

The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.8.1.4 LOG2E

The Number value for the base-2 logarithm ofe, the base of the natural logarithms; this value is approximately1.4426950408889634.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

NOTE The value ofMath.LOG2E is approximately the reciprocal of the value ofMath.LN2.

15.8.1.5 LOG10E

The Number value for the base-10 logarithm ofe, the base of the natural logarithms; this value is approximately0.4342944819032518.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

NOTE The value ofMath.LOG10E is approximately the reciprocal of the value ofMath.LN10.

15.8.1.6 PI

The Number value forπ, the ratio of the circumference of a circle to its diameter, which is approximately3.1415926535897932.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.8.1.7 SQRT1_2

The Number value for the square root of½, which is approximately0.7071067811865476.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

NOTE The value ofMath.SQRT1_2 is approximately the reciprocal of the value ofMath.SQRT2.

15.8.1.8 SQRT2

The Number value for the square root of2, which is approximately1.4142135623730951.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.8.2 Function Properties of the Math Object

Each of the followingMath object functions applies theToNumber abstract operator to each of its arguments (in left-to-right order if there is more than one) and then performs a computation on the resulting Number value(s).

In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values described in8.5.

NOTE The behaviour of the functionsacos,asin,atan,atan2,cos,exp,log,pow,sin,sqrt, andtan is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754 arithmetic contained infdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

15.8.2.1 abs (x)

Returns the absolute value ofx; the result has the same magnitude asx but has positive sign.

  • Ifx is NaN, the result is NaN.
  • Ifx is −0, the result is +0.
  • Ifx is −∞, the result is +∞.

15.8.2.2 acos (x)

Returns an implementation-dependent approximation to the arc cosine ofx. The result is expressed in radians and ranges from+0 to+π.

  • Ifx is NaN, the result is NaN.
  • Ifx is greater than 1, the result is NaN.
  • Ifx is less than−1, the result is NaN.
  • Ifx is exactly 1, the result is +0.

15.8.2.3 asin (x)

Returns an implementation-dependent approximation to the arc sine ofx. The result is expressed in radians and ranges from−π/2 to+π/2.

  • Ifx is NaN, the result is NaN.
  • Ifx is greater than 1, the result is NaN.
  • Ifx is less than –1, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.

15.8.2.4 atan (x)

Returns an implementation-dependent approximation to the arc tangent ofx. The result is expressed in radians and ranges from−π/2 to+π/2.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞, the result is an implementation-dependent approximation to +π/2.
  • Ifx is −∞, the result is an implementation-dependent approximation to −π/2.

15.8.2.5 atan2 (y, x)

Returns an implementation-dependent approximation to the arc tangent of the quotienty/x of the argumentsy andx, where the signs ofy andx are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument namedy be first and the argument namedx be second. The result is expressed in radians and ranges from−π to+π.

  • If eitherx ory is NaN, the result is NaN.
  • Ify>0 andx is +0, the result is an implementation-dependent approximation to +π/2.
  • Ify>0 andx is −0, the result is an implementation-dependent approximation to +π/2.
  • Ify is +0 andx>0, the result is +0.
  • Ify is +0 andx is +0, the result is +0.
  • Ify is +0 andx is −0, the result is an implementation-dependent approximation to +π.
  • Ify is +0 andx<0, the result is an implementation-dependent approximation to +π.
  • Ify is −0 andx>0, the result is −0.
  • Ify is −0 andx is +0, the result is −0.
  • Ify is −0 andx is −0, the result is an implementation-dependent approximation to −π.
  • Ify is −0 andx<0, the result is an implementation-dependent approximation to −π.
  • Ify<0 andx is +0, the result is an implementation-dependent approximation to −π/2.
  • Ify<0 andx is −0, the result is an implementation-dependent approximation to −π/2.
  • Ify>0 andy is finite andx is +∞, the result is +0.
  • Ify>0 andy is finite andx is −∞, the result if an implementation-dependent approximation to +π.
  • Ify<0 andy is finite andx is +∞, the result is −0.
  • Ify<0 andy is finite andx is −∞, the result is an implementation-dependent approximation to −π.
  • Ify is +∞ andx is finite, the result is an implementation-dependent approximation to +π/2.
  • Ify is −∞ andx is finite, the result is an implementation-dependent approximation to −π/2.
  • Ify is +∞ andx is +∞, the result is an implementation-dependent approximation to +π/4.
  • Ify is +∞ andx is −∞, the result is an implementation-dependent approximation to +3π/4.
  • Ify is −∞ andx is +∞, the result is an implementation-dependent approximation to −π/4.
  • Ify is −∞ andx is −∞, the result is an implementation-dependent approximation to −3π/4.

15.8.2.6 ceil (x)

Returns the smallest (closest to−∞) Number value that is not less thanx and is equal to a mathematical integer. Ifx is already an integer, the result isx.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞, the result is +∞.
  • Ifx is −∞, the result is −∞.
  • Ifx is less than 0 but greater than -1, the result is −0.

The value ofMath.ceil(x) is the same as the value of-Math.floor(-x).

15.8.2.7 cos (x)

Returns an implementation-dependent approximation to the cosine ofx. The argument is expressed in radians.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is 1.
  • Ifx is −0, the result is 1.
  • Ifx is +∞, the result is NaN.
  • Ifx is −∞, the result is NaN.

15.8.2.8 exp (x)

Returns an implementation-dependent approximation to the exponential function ofx (e raised to the power ofx, wheree is the base of the natural logarithms).

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is 1.
  • Ifx is −0, the result is 1.
  • Ifx is +∞, the result is +∞.
  • Ifx is −∞, the result is +0.

15.8.2.9 floor (x)

Returns the greatest (closest to+∞) Number value that is not greater thanx and is equal to a mathematical integer. Ifx is already an integer, the result isx.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞, the result is +∞.
  • Ifx is −∞, the result is −∞.
  • Ifx is greater than 0 but less than 1, the result is +0.

NOTE The value ofMath.floor(x) is the same as the value of-Math.ceil(-x).

15.8.2.10 log (x)

  • Returns an implementation-dependent approximation to the natural logarithm ofx.
  • Ifx is NaN, the result is NaN.
  • Ifx is less than 0, the result is NaN.
  • Ifx is +0 or −0, the result is −∞.
  • Ifx is 1, the result is +0.
  • Ifx is +∞, the result is +∞.

15.8.2.11 max ( [ value1 [ , value2 [ , … ] ] ] )

Given zero or more arguments, callsToNumber on each of the arguments and returns the largest of the resulting values.

  • If no arguments are given, the result is −∞.
  • If any value is NaN, the result is NaN.
  • The comparison of values to determine the largest value is done as in11.8.5 except that +0 is considered to be larger than −0.

Thelength property of themax method is2.

15.8.2.12 min ( [ value1 [ , value2 [ , … ] ] ] )

Given zero or more arguments, callsToNumber on each of the arguments and returns the smallest of the resulting values.

  • If no arguments are given, the result is +∞.
  • If any value is NaN, the result is NaN.
  • The comparison of values to determine the smallest value is done as in11.8.5 except that +0 is considered to be larger than −0.

Thelength property of themin method is2.

15.8.2.13 pow (x, y)

Returns an implementation-dependent approximation to the result of raisingx to the powery.

  • Ify is NaN, the result is NaN.
  • Ify is +0, the result is 1, even ifx is NaN.
  • Ify is −0, the result is 1, even ifx is NaN.
  • Ifx is NaN andy is nonzero, the result is NaN.
  • Ifabs(x)>1 andy is +∞, the result is +∞.
  • Ifabs(x)>1 andy is −∞, the result is +0.
  • Ifabs(x)==1 andy is +∞, the result is NaN.
  • Ifabs(x)==1 andy is −∞, the result is NaN.
  • Ifabs(x)<1 andy is +∞, the result is +0.
  • Ifabs(x)<1 andy is −∞, the result is +∞.
  • Ifx is +∞ andy>0, the result is +∞.
  • Ifx is +∞ andy<0, the result is +0.
  • Ifx is −∞ andy>0 andy is an odd integer, the result is −∞.
  • Ifx is −∞ andy>0 andy is not an odd integer, the result is +∞.
  • Ifx is −∞ andy<0 andy is an odd integer, the result is −0.
  • Ifx is −∞ andy<0 andy is not an odd integer, the result is +0.
  • Ifx is +0 andy>0, the result is +0.
  • Ifx is +0 andy<0, the result is +∞.
  • Ifx is −0 andy>0 andy is an odd integer, the result is −0.
  • Ifx is −0 andy>0 andy is not an odd integer, the result is +0.
  • Ifx is −0 andy<0 andy is an odd integer, the result is −∞.
  • Ifx is −0 andy<0 andy is not an odd integer, the result is +∞.
  • Ifx<0 andx is finite andy is finite andy is not an integer, the result is NaN.

15.8.2.14 random ( )

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.

15.8.2.15 round (x)

Returns the Number value that is closest tox and is equal to a mathematical integer. If two integer Number values are equally close tox, then the result is the Number value that is closer to+∞. Ifx is already an integer, the result isx.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞, the result is +∞.
  • Ifx is −∞, the result is −∞.
  • Ifx is greater than 0 but less than 0.5, the result is +0.
  • Ifx is less than 0 but greater than or equal to -0.5, the result is −0.

NOTE 1Math.round(3.5) returns4, butMath.round(–3.5) returns–3.

NOTE 2 The value ofMath.round(x) is the same as the value ofMath.floor(x+0.5), except whenx is−0 or is less than0 but greater than or equal to-0.5; for these casesMath.round(x) returns−0, butMath.floor(x+0.5) returns+0.

15.8.2.16 sin (x)

Returns an implementation-dependent approximation to the sine ofx. The argument is expressed in radians.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞ or −∞, the result is NaN.

15.8.2.17 sqrt (x)

Returns an implementation-dependent approximation to the square root ofx.

  • Ifx is NaN, the result is NaN.
  • Ifx is less than 0, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞, the result is +∞.

15.8.2.18 tan (x)

Returns an implementation-dependent approximation to the tangent ofx. The argument is expressed in radians.

  • Ifx is NaN, the result is NaN.
  • Ifx is +0, the result is +0.
  • Ifx is −0, the result is −0.
  • Ifx is +∞ or −∞, the result is NaN.

15.9 Date Objects

15.9.1 Overview of Date Objects and Definitions of Abstract Operators

The following functions are abstract operations that operate on time values (defined in15.9.1.1). Note that, in every case, if any argument to one of these functions isNaN, the result will beNaN.

15.9.1.1 Time Values and Time Range

A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called atime value. A time value may also beNaN, indicating that the Date object does not represent a specific instant of time.

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from –9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value+0.

15.9.1.2 Day Number and Time within Day

A giventime valuet belongs to day number

Day(t) =floor(t / msPerDay)

where the number of milliseconds per day is

msPerDay = 86400000

The remainder is called the time within the day:

TimeWithinDay(t) =tmodulo msPerDay

15.9.1.3 Year Number

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and date within that year. In this system, leap years are precisely those which are (divisible by4) and ((not divisible by100) or (divisible by400)). The number of days in year numbery is therefore defined by

DaysInYear(y) = 365 if (ymodulo 4) ≠ 0
= 366 if (ymodulo 4) = 0 and (ymodulo 100) ≠ 0
= 365 if (ymodulo 100) = 0 and (ymodulo 400) ≠ 0
= 366 if (ymodulo 400) = 0

All non-leap years have365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of yeary is given by:

DayFromYear(y) = 365 × (y−1970) +floor((y−1969)/4) −floor((y−1901)/100) +floor((y−1601)/400)

Thetime value of the start of a year is:

TimeFromYear(y) =msPerDay × DayFromYear(y)

Atime value determines a year by:

YearFromTime(t) = the largest integery (closest to positive infinity) such that TimeFromYear(y) ≤t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t) = 0 if DaysInYear(YearFromTime(t)) = 365
= 1 if DaysInYear(YearFromTime(t)) = 366

15.9.1.4 Month Number

Months are identified by an integer in the range0 to11, inclusive. The mapping MonthFromTime(t) from atime valuet to a month number is defined by:

MonthFromTime(t) = 0 if 0 ≤ DayWithinYear(t) < 31
= 1 if 31 ≤ DayWithinYear (t) < 59+InLeapYear(t)
= 2 if 59+InLeapYear(t) ≤ DayWithinYear (t) < 90+InLeapYear(t)
= 3 if 90+InLeapYear(t) ≤ DayWithinYear (t) < 120+InLeapYear(t)
= 4 if 120+InLeapYear(t) ≤ DayWithinYear (t) < 151+InLeapYear(t)
= 5 if 151+InLeapYear(t) ≤ DayWithinYear (t) < 181+InLeapYear(t)
= 6 if 181+InLeapYear(t) ≤ DayWithinYear (t) < 212+InLeapYear(t)
= 7 if 212+InLeapYear(t) ≤ DayWithinYear (t) < 243+InLeapYear(t)
= 8 if 243+InLeapYear(t) ≤ DayWithinYear (t) < 273+InLeapYear(t)
= 9 if 273+InLeapYear(t) ≤ DayWithinYear (t) < 304+InLeapYear(t)
= 10 if 304+InLeapYear(t) ≤ DayWithinYear (t) < 334+InLeapYear(t)
= 11 if 334+InLeapYear(t) ≤ DayWithinYear (t) < 365+InLeapYear(t)

where

DayWithinYear(t) =Day(t)−DayFromYear(YearFromTime(t))

A month value of0 specifies January;1 specifies February;2 specifies March;3 specifies April;4 specifies May;5 specifies June;6 specifies July;7 specifies August;8 specifies September;9 specifies October;10 specifies November; and11 specifies December. Note thatMonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

15.9.1.5 Date Number

A date number is identified by an integer in the range1 through31, inclusive. The mapping DateFromTime(t) from atime valuet to a month number is defined by:

DateFromTime(t) =DayWithinYear(t)+1 ifMonthFromTime(t)=0
=DayWithinYear(t)−30 ifMonthFromTime(t)=1
=DayWithinYear(t)−58−InLeapYear(t) ifMonthFromTime(t)=2
=DayWithinYear(t)−89−InLeapYear(t) ifMonthFromTime(t)=3
=DayWithinYear(t)−119−InLeapYear(t) ifMonthFromTime(t)=4
=DayWithinYear(t)−150−InLeapYear(t) ifMonthFromTime(t)=5
=DayWithinYear(t)−180−InLeapYear(t) ifMonthFromTime(t)=6
=DayWithinYear(t)−211−InLeapYear(t) ifMonthFromTime(t)=7
=DayWithinYear(t)−242−InLeapYear(t) ifMonthFromTime(t)=8
=DayWithinYear(t)−272−InLeapYear(t) ifMonthFromTime(t)=9
=DayWithinYear(t)−303−InLeapYear(t) ifMonthFromTime(t)=10
=DayWithinYear(t)−333−InLeapYear(t) ifMonthFromTime(t)=11

15.9.1.6 Week Day

The weekday for a particulartime valuet is defined as

WeekDay(t) = (Day(t) + 4)modulo 7

A weekday value of0 specifies Sunday;1 specifies Monday;2 specifies Tuesday;3 specifies Wednesday;4 specifies Thursday;5 specifies Friday; and6 specifies Saturday. Note thatWeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

15.9.1.7 Local Time Zone Adjustment

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the localstandard time. Daylight saving time isnot reflected by LocalTZA. The value LocalTZA does not vary with time but depends only on the geographic location.

15.9.1.8 Daylight Saving Time Adjustment

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend only on four things:

(1) the time since the beginning of the year

t –TimeFromYear(YearFromTime(t))

(2) whethert is in a leap year

InLeapYear(t)

(3) the week day of the beginning of the year

WeekDay(TimeFromYear(YearFromTime(t)))

and (4) the geographic location.

The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had been used at the time. This avoids complications such as taking into account the years that the locale observed daylight saving time year round.

If the host environment provides functionality for determining daylight saving time, the implementation of ECMAScript is free to map the year in question to an equivalent year (same leap-year-ness and same starting week day for the year) for which the host environment provides daylight saving time information. The only restriction is that all equivalent years should produce the same result.

15.9.1.9 Local Time

Conversion from UTC to local time is defined by

LocalTime(t) =t +LocalTZA +DaylightSavingTA(t)

Conversion from local time to UTC is defined by

UTC(t) =t –LocalTZA –DaylightSavingTA(t –LocalTZA)

Note thatUTC(LocalTime(t)) is not necessarily always equal tot.

15.9.1.10 Hours, Minutes, Second, and Milliseconds

The following functions are useful in decomposing time values:

HourFromTime(t) =floor(t / msPerHour)modulo HoursPerDay
MinFromTime(t) =floor(t / msPerMinute)modulo MinutesPerHour
SecFromTime(t) =floor(t / msPerSecond)modulo SecondsPerMinute
msFromTime(t) = tmodulo msPerSecond

where

HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000
msPerMinute = 60000 = msPerSecond × SecondsPerMinute
msPerHour = 3600000 = msPerMinute × MinutesPerHour

15.9.1.11 MakeTime (hour, min, sec, ms)

The operator MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:

  1. Ifhour is not finite ormin is not finite orsec is not finite orms is not finite, returnNaN.
  2. Leth beToInteger(hour).
  3. Letm beToInteger(min).
  4. Lets beToInteger(sec).
  5. Letmilli beToInteger(ms).
  6. Lett beh*msPerHour+m*msPerMinute+s*msPerSecond+milli, performing the arithmetic according to IEEE 754 rules (that is, as if using the ECMAScript operators* and+).
  7. Returnt.

15.9.1.12 MakeDay (year, month, date)

The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:

  1. Ifyear is not finite ormonth is not finite ordate is not finite, returnNaN.
  2. Lety beToInteger(year).
  3. Letm beToInteger(month).
  4. Letdt beToInteger(date).
  5. Letym bey +floor(m /12).
  6. Letmn bemmodulo 12.
  7. Find a valuet such thatYearFromTime(t)==ym andMonthFromTime(t)==mn andDateFromTime(t)== 1; but if this is not possible (because some argument is out of range), returnNaN.
  8. ReturnDay(t) +dt − 1.

15.9.1.13 MakeDate (day, time)

The operator MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:

  1. Ifday is not finite ortime is not finite, returnNaN.
  2. Returnday ×msPerDay +time.

15.9.1.14 TimeClip (time)

The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:

  1. Iftime is not finite, returnNaN.
  2. Ifabs(time) > 8.64 x 1015, returnNaN.
  3. Return an implementation-dependent choice of eitherToInteger(time) orToInteger(time) + (+0). (Adding a positive zero converts−0 to+0.)

NOTE The point of step 3 is that an implementation is permitted a choice of internal representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal representation may or may not distinguish−0 and+0.

15.9.1.15 Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows:YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

YYYY is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.

- “-” (hyphen) appears literally twice in the string.

MM is the month of the year from 01 (January) to 12 (December).

DD is the day of the month from 01 to 31.

T “T” appears literally in the string, to indicate the beginning of the time element.

HH is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.

: “:” (colon) appears literally twice in the string.

mm is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.

ss is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.

. “.” (dot) appears literally in the string.

sss is the number of complete milliseconds since the start of the second as three decimal digits.

Z is the time zone offset specified as “Z” (for UTC) or either “+” or “-” followed by a time expressionHH:mm

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional time zone offset appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss

All numbers must be base10. If theMM orDD fields are absent “01” is used as the value. If theHH,mm, orss fields are absent “00” is used as the value and the value of an absentsss field is “000”. The value of an absent time zone offset is “Z”.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.

NOTE 1 As every day both starts and ends with midnight, the two notations00:00 and24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time:1995-02-04T24:00 and1995-02-05T00:00

NOTE 2 There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies numeric representations of date and time.

15.9.1.15.1 Extended years

ECMAScript requires the ability to specify6 digit years (extended years); approximately285,426 years, either forward or backward, from 01 January, 1970 UTC. To represent years before0 or after9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year representation shall have2 extra year digits and is always prefixed with a + or – sign. The year0 is considered positive and hence prefixed with a + sign.

NOTE Examples of extended years:

-283457-03-21T15:00:59.008Z   283458 B.C.
-000001-01-01T00:00:00Z          2 B.C.
+000000-01-01T00:00:00Z         1 B.C.
+000001-01-01T00:00:00Z         1 A.D.
+001970-01-01T00:00:00Z         1970 A.D.
+002009-12-15T00:00:00Z         2009 A.D.
+287396-10-12T08:59:00.992Z 287396 A.D.

15.9.2 The Date Constructor Called as a Function

WhenDate is called as a function rather than as a constructor, it returns a String representing the current time (UTC).

NOTE The function callDate(…) is not equivalent to the object creation expressionnew Date(…) with the same arguments.

15.9.2.1 Date ( [ year [, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A String is created and returned as if by the expression(new Date()).toString() whereDate is the standard built-in constructor with that name andtoString is the standard built-in methodDate.prototype.toString.

15.9.3 The Date Constructor

WhenDate is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.9.3.1 new Date (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )

WhenDate is called with two to seven arguments, it computes the date fromyear,month, and (optionally)date,hours,minutes,seconds andms.

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value ofDate.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to"Date".

The [[Extensible]] internal property of the newly constructed object is set totrue.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

  1. Lety beToNumber(year).
  2. Letm beToNumber(month).
  3. Ifdate is supplied then letdt beToNumber(date); else letdt be1.
  4. Ifhours is supplied then leth beToNumber(hours); else leth be0.
  5. Ifminutes is supplied then letmin beToNumber(minutes); else letmin be0.
  6. Ifseconds is supplied then lets beToNumber(seconds); else lets be0.
  7. Ifms is supplied then letmilli beToNumber(ms); else letmilli be0.
  8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then letyr be 1900+ToInteger(y); otherwise, letyr bey.
  9. LetfinalDate beMakeDate(MakeDay(yr,m,dt),MakeTime(h,min,s,milli)).
  10. Set the [[PrimitiveValue]] internal property of the newly constructed object toTimeClip(UTC(finalDate)).

15.9.3.2 new Date (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value ofDate.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to"Date".

The [[Extensible]] internal property of the newly constructed object is set totrue.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

  1. Letv beToPrimitive(value).
  2. IfType(v) is String, then
    1. Parsev as a date, in exactly the same manner as for theparse method (15.9.4.2); letV be thetime value for this date.
  3. Else, letV beToNumber(v).
  4. Set the [[PrimitiveValue]] internal property of the newly constructed object toTimeClip(V) and return.

15.9.3.3 new Date ( )

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value ofDate.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to"Date".

The [[Extensible]] internal property of the newly constructed object is set totrue.

The [[PrimitiveValue]] internal property of the newly constructed object is set to thetime value (UTC) identifying the current time.

15.9.4 Properties of the Date Constructor

The value of the [[Prototype]] internal property of the Date constructor is the Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is7), the Date constructor has the following properties:

15.9.4.1 Date.prototype

The initial value ofDate.prototype is the built-in Date prototype object (15.9.5).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.9.4.2 Date.parse (string)

Theparse function applies theToString operator to its argument and interprets the resulting String as a date and time; it returns a Number, the UTCtime value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the format of the String according to the rules called out in Date Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognisable Strings or dates containing illegal element values in the format String shall causeDate.parse to returnNaN.

Ifx is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:

x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced byDate.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (15.9.1.15) and that could not be produced in that implementation by thetoString ortoUTCString method.

15.9.4.3 Date.UTC (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )

When theUTC function is called with fewer than two arguments, the behaviour is implementation-dependent. When theUTC function is called with two to seven arguments, it computes the date fromyear,month and (optionally)date,hours,minutes,seconds andms. The following steps are taken:

  1. Lety beToNumber(year).
  2. Letm beToNumber(month).
  3. Ifdate is supplied then letdt beToNumber(date); else letdt be1.
  4. Ifhours is supplied then leth beToNumber(hours); else leth be0.
  5. Ifminutes is supplied then letmin beToNumber(minutes); else letmin be0.
  6. Ifseconds is supplied then lets beToNumber(seconds); else lets be0.
  7. Ifms is supplied then letmilli beToNumber(ms); else letmilli be0.
  8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then letyr be 1900+ToInteger(y); otherwise, letyr bey.
  9. ReturnTimeClip(MakeDate(MakeDay(yr,m,dt),MakeTime(h,min,s,milli))).

Thelength property of theUTC function is7.

NOTE TheUTC function differs from theDate constructor in two ways: it returns atime value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.

15.9.4.4 Date.now ( )

Thenow function return a Number value that is thetime value designating the UTC date and time of the occurrence of the call tonow.

15.9.5 Properties of the Date Prototype Object

The Date prototype object is itself a Date object (its [[Class]] is"Date") whose [[PrimitiveValue]] isNaN.

The value of the [[Prototype]] internal property of the Date prototype object is the standard built-in Object prototype object (15.2.4).

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is thethis value for the invocation of the function. Unless explicitly noted otherwise, none of these functions are generic; aTypeError exception is thrown if thethis value is not an object for which the value of the [[Class]] internal property is"Date". Also, the phrase “thistime value” refers to the Number value for the time represented by this Date object, that is, the value of the [[PrimitiveValue]] internal property of this Date object.

15.9.5.1 Date.prototype.constructor

The initial value ofDate.prototype.constructor is the built-inDate constructor.

15.9.5.2 Date.prototype.toString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form.

NOTE For any Date valued whose milliseconds amount is zero, the result ofDate.parse(d.toString()) is equal tod.valueOf(). See15.9.4.2.

15.9.5.3 Date.prototype.toDateString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form.

15.9.5.4 Date.prototype.toTimeString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form.

15.9.5.5 Date.prototype.toLocaleString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment’s current locale.

NOTE The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.6 Date.prototype.toLocaleDateString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment’s current locale.

NOTE The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.7 Date.prototype.toLocaleTimeString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment’s current locale.

NOTE The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.8 Date.prototype.valueOf ( )

ThevalueOf function returns a Number, which isthis time value.

15.9.5.9 Date.prototype.getTime ( )

  1. Returnthis time value.

15.9.5.10 Date.prototype.getFullYear ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnYearFromTime(LocalTime(t)).

15.9.5.11 Date.prototype.getUTCFullYear ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnYearFromTime(t).

15.9.5.12 Date.prototype.getMonth ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnMonthFromTime(LocalTime(t)).

15.9.5.13 Date.prototype.getUTCMonth ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnMonthFromTime(t).

15.9.5.14 Date.prototype.getDate ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnDateFromTime(LocalTime(t)).

15.9.5.15 Date.prototype.getUTCDate ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnDateFromTime(t).

15.9.5.16 Date.prototype.getDay ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnWeekDay(LocalTime(t)).

15.9.5.17 Date.prototype.getUTCDay ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnWeekDay(t).

15.9.5.18 Date.prototype.getHours ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnHourFromTime(LocalTime(t)).

15.9.5.19 Date.prototype.getUTCHours ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnHourFromTime(t).

15.9.5.20 Date.prototype.getMinutes ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnMinFromTime(LocalTime(t)).

15.9.5.21 Date.prototype.getUTCMinutes ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnMinFromTime(t).

15.9.5.22 Date.prototype.getSeconds ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnSecFromTime(LocalTime(t)).

15.9.5.23 Date.prototype.getUTCSeconds ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnSecFromTime(t).

15.9.5.24 Date.prototype.getMilliseconds ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnmsFromTime(LocalTime(t)).

15.9.5.25 Date.prototype.getUTCMilliseconds ( )

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnmsFromTime(t).

15.9.5.26 Date.prototype.getTimezoneOffset ( )

Returns the difference between local time and UTC time in minutes.

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. Return (t −LocalTime(t)) /msPerMinute.

15.9.5.27 Date.prototype.setTime (time)

  1. Letv beTimeClip(ToNumber(time)).
  2. Set the [[PrimitiveValue]] internal property of this Date object tov.
  3. Returnv.

15.9.5.28 Date.prototype.setMilliseconds (ms)

  1. Lett be the result ofLocalTime(this time value).
  2. Lettime beMakeTime(HourFromTime(t),MinFromTime(t),SecFromTime(t),ToNumber(ms)).
  3. Letu beTimeClip(UTC(MakeDate(Day(t),time))).
  4. Set the [[PrimitiveValue]] internal property of this Date object tou.
  5. Returnu.

15.9.5.29 Date.prototype.setUTCMilliseconds (ms)

  1. Lett bethis time value.
  2. Lettime beMakeTime(HourFromTime(t),MinFromTime(t),SecFromTime(t),ToNumber(ms)).
  3. Letv beTimeClip(MakeDate(Day(t),time)).
  4. Set the [[PrimitiveValue]] internal property of this Date object tov.
  5. Returnv.

15.9.5.30 Date.prototype.setSeconds (sec [, ms ] )

Ifms is not specified, this behaves as ifms were specified with the valuegetMilliseconds().

  1. Lett be the result ofLocalTime(this time value).
  2. Lets beToNumber(sec).
  3. Ifms is not specified, then letmilli bemsFromTime(t); otherwise, letmilli beToNumber(ms).
  4. Letdate beMakeDate(Day(t),MakeTime(HourFromTime(t),MinFromTime(t),s,milli)).
  5. Letu beTimeClip(UTC(date)).
  6. Set the [[PrimitiveValue]] internal property of this Date object tou.
  7. Returnu.

Thelength property of thesetSeconds method is2.

15.9.5.31 Date.prototype.setUTCSeconds (sec [, ms ] )

Ifms is not specified, this behaves as ifms were specified with the valuegetUTCMilliseconds().

  1. Lett bethis time value.
  2. Lets beToNumber(sec).
  3. Ifms is not specified, then letmilli bemsFromTime(t); otherwise, letmilli beToNumber(ms).
  4. Letdate beMakeDate(Day(t),MakeTime(HourFromTime(t),MinFromTime(t),s,milli)).
  5. Letv beTimeClip(date).
  6. Set the [[PrimitiveValue]] internal property of this Date object tov.
  7. Returnv.

Thelength property of thesetUTCSeconds method is2.

15.9.5.32 Date.prototype.setMinutes (min [, sec [, ms ] ] )

Ifsec is not specified, this behaves as ifsec were specified with the valuegetSeconds().

Ifms is not specified, this behaves as ifms were specified with the valuegetMilliseconds().

  1. Lett be the result ofLocalTime(this time value).
  2. Letm beToNumber(min).
  3. Ifsec is not specified, then lets beSecFromTime(t); otherwise, lets beToNumber(sec).
  4. Ifms is not specified, then letmilli bemsFromTime(t); otherwise, letmilli beToNumber(ms).
  5. Letdate beMakeDate(Day(t),MakeTime(HourFromTime(t),m,s,milli)).
  6. Letu beTimeClip(UTC(date)).
  7. Set the [[PrimitiveValue]] internal property of this Date object tou.
  8. Returnu.

Thelength property of thesetMinutes method is3.

15.9.5.33 Date.prototype.setUTCMinutes (min [, sec [, ms ] ] )

Ifsec is not specified, this behaves as ifsec were specified with the valuegetUTCSeconds().

Ifms is not specified, this function behaves as ifms were specified with the value return bygetUTCMilliseconds().

  1. Lett bethis time value.
  2. Letm beToNumber(min).
  3. Ifsec is not specified, then lets beSecFromTime(t); otherwise, lets beToNumber(sec).
  4. Ifms is not specified, then letmilli bemsFromTime(t); otherwise, letmilli beToNumber(ms).
  5. Letdate beMakeDate(Day(t),MakeTime(HourFromTime(t),m,s,milli)).
  6. Letv beTimeClip(date).
  7. Set the [[PrimitiveValue]] internal property of this Date object tov.
  8. Returnv.

Thelength property of thesetUTCMinutes method is3.

15.9.5.34 Date.prototype.setHours (hour [, min [, sec [, ms ] ] ] )

Ifmin is not specified, this behaves as ifmin were specified with the valuegetMinutes().

Ifsec is not specified, this behaves as ifsec were specified with the valuegetSeconds().

Ifms is not specified, this behaves as ifms were specified with the valuegetMilliseconds().

  1. Lett be the result ofLocalTime(this time value).
  2. Leth beToNumber(hour).
  3. Ifmin is not specified, then letm beMinFromTime(t); otherwise, letm beToNumber(min).
  4. If Ifsec is not specified, then lets beSecFromTime(t); otherwise, lets beToNumber(sec).
  5. Ifms is not specified, then letmilli bemsFromTime(t); otherwise, letmilli beToNumber(ms).
  6. Letdate beMakeDate(Day(t),MakeTime(h,m,s,milli)).
  7. Letu beTimeClip(UTC(date)).
  8. Set the [[PrimitiveValue]] internal property of this Date object tou.
  9. Returnu.

Thelength property of thesetHours method is4.

15.9.5.35 Date.prototype.setUTCHours (hour [, min [, sec [, ms ] ] ] )

Ifmin is not specified, this behaves as ifmin were specified with the valuegetUTCMinutes().

Ifsec is not specified, this behaves as ifsec were specified with the valuegetUTCSeconds().

Ifms is not specified, this behaves as ifms were specified with the valuegetUTCMilliseconds().

  1. Lett bethis time value.
  2. Leth beToNumber(hour).
  3. Ifmin is not specified, then letm beMinFromTime(t); otherwise, letm beToNumber(min).
  4. Ifsec is not specified, then lets beSecFromTime(t); otherwise, lets beToNumber(sec).
  5. Ifms is not specified, then letmilli bemsFromTime(t); otherwise, letmilli beToNumber(ms).
  6. LetnewDate beMakeDate(Day(t),MakeTime(h,m,s,milli)).
  7. Letv beTimeClip(newDate).
  8. Set the [[PrimitiveValue]] internal property of this Date object tov.
  9. Returnv.

Thelength property of thesetUTCHours method is4.

15.9.5.36 Date.prototype.setDate (date)

  1. Lett be the result ofLocalTime(this time value).
  2. Letdt beToNumber(date).
  3. LetnewDate beMakeDate(MakeDay(YearFromTime(t),MonthFromTime(t),dt),TimeWithinDay(t)).
  4. Letu beTimeClip(UTC(newDate)).
  5. Set the [[PrimitiveValue]] internal property of this Date object tou.
  6. Returnu.

15.9.5.37 Date.prototype.setUTCDate (date)

  1. Lett bethis time value.
  2. Letdt beToNumber(date).
  3. LetnewDate beMakeDate(MakeDay(YearFromTime(t),MonthFromTime(t),dt),TimeWithinDay(t)).
  4. Letv beTimeClip(newDate).
  5. Set the [[PrimitiveValue]] internal property of this Date object tov.
  6. Returnv.

15.9.5.38 Date.prototype.setMonth (month [, date ] )

Ifdate is not specified, this behaves as ifdate were specified with the valuegetDate().

  1. Lett be the result ofLocalTime(this time value).
  2. Letm beToNumber(month).
  3. Ifdate is not specified, then letdt beDateFromTime(t); otherwise, letdt beToNumber(date).
  4. LetnewDate beMakeDate(MakeDay(YearFromTime(t),m,dt),TimeWithinDay(t)).
  5. Letu beTimeClip(UTC(newDate)).
  6. Set the [[PrimitiveValue]] internal property of this Date object tou.
  7. Returnu.

Thelength property of thesetMonth method is2.

15.9.5.39 Date.prototype.setUTCMonth (month [, date ] )

Ifdate is not specified, this behaves as ifdate were specified with the valuegetUTCDate().

  1. Lett bethis time value.
  2. Letm beToNumber(month).
  3. Ifdate is not specified, then letdt beDateFromTime(t); otherwise, letdt beToNumber(date).
  4. LetnewDate beMakeDate(MakeDay(YearFromTime(t),m,dt),TimeWithinDay(t)).
  5. Letv beTimeClip(newDate).
  6. Set the [[PrimitiveValue]] internal property of this Date object tov.
  7. Returnv.

Thelength property of thesetUTCMonth method is2.

15.9.5.40 Date.prototype.setFullYear (year [, month [, date ] ] )

Ifmonth is not specified, this behaves as ifmonth were specified with the valuegetMonth().

Ifdate is not specified, this behaves as ifdate were specified with the valuegetDate().

  1. Lett be the result ofLocalTime(this time value); but ifthis time value isNaN, lett be+0.
  2. Lety beToNumber(year).
  3. Ifmonth is not specified, then letm beMonthFromTime(t); otherwise, letm beToNumber(month).
  4. Ifdate is not specified, then letdt beDateFromTime(t); otherwise, letdt beToNumber(date).
  5. LetnewDate beMakeDate(MakeDay(y,m,dt),TimeWithinDay(t)).
  6. Letu beTimeClip(UTC(newDate)).
  7. Set the [[PrimitiveValue]] internal property of this Date object tou.
  8. Returnu.

Thelength property of thesetFullYear method is3.

15.9.5.41 Date.prototype.setUTCFullYear (year [, month [, date ] ] )

Ifmonth is not specified, this behaves as ifmonth were specified with the valuegetUTCMonth().

Ifdate is not specified, this behaves as ifdate were specified with the valuegetUTCDate().

  1. Lett bethis time value; but ifthis time value isNaN, lett be+0.
  2. Lety beToNumber(year).
  3. Ifmonth is not specified, then letm beMonthFromTime(t); otherwise, letm beToNumber(month).
  4. Ifdate is not specified, then letdt beDateFromTime(t); otherwise, letdt beToNumber(date).
  5. LetnewDate beMakeDate(MakeDay(y,m,dt),TimeWithinDay(t)).
  6. Letv beTimeClip(newDate).
  7. Set the [[PrimitiveValue]] internal property of this Date object tov.
  8. Returnv.

Thelength property of thesetUTCFullYear method is3.

15.9.5.42 Date.prototype.toUTCString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in a convenient, human-readable form in UTC.

NOTE The intent is to produce a String representation of a date that is more readable than the format specified in15.9.1.15. It is not essential that the chosen format be unambiguous or easily machine parsable. If an implementation does not have a preferred human-readable format it is recommended to use the format defined in15.9.1.15 but with a space rather than a “T” used to separate the date and time elements.

15.9.5.43 Date.prototype.toISOString ( )

This function returns a String value represent the instance in time represented by this Date object. The format of the String is the Date Time string format defined in15.9.1.15. All fields are present in the String. The time zone is always UTC, denoted by the suffix Z. If thetime value of this object is not a finite Number aRangeError exception is thrown.

15.9.5.44 Date.prototype.toJSON ( key )

This function provides a String representation of a Date object for use byJSON.stringify (15.12.3).

When thetoJSON method is called with argumentkey, the following steps are taken:

  1. LetO be the result of callingToObject, giving it thethis value as its argument.
  2. Lettv beToPrimitive(O, hint Number).
  3. Iftv is a Number and is not finite, returnnull.
  4. LettoISO be the result of calling the [[Get]] internal method ofO with argument "toISOString".
  5. IfIsCallable(toISO) isfalse, throw aTypeError exception.
  6. Return the result of calling the [[Call]] internal method oftoISO withO as thethis value and an empty argument list.

NOTE 1 The argument is ignored.

NOTE 2 ThetoJSON function is intentionally generic; it does not require that itsthis value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have atoISOString method. An object is free to use the argumentkey to filter its stringification.

15.9.6 Properties of Date Instances

Date instances inherit properties from the Date prototype object and their [[Class]] internal property value is"Date". Date instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property istime value represented by this Date object.

15.10 RegExp (Regular Expression) Objects

A RegExp object contains a regular expression and the associated flags.

NOTE The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.

15.10.1 Patterns

TheRegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion ofPattern.

Syntax

Pattern::
Disjunction
Disjunction::
Alternative
Alternative|Disjunction
Alternative::
[empty]
AlternativeTerm
Term::
Assertion
Atom
AtomQuantifier
Assertion::
^
$
\b
\B
(?=Disjunction)
(?!Disjunction)
Quantifier::
QuantifierPrefix
QuantifierPrefix?
QuantifierPrefix::
*
+
?
{DecimalDigits}
{DecimalDigits,}
{DecimalDigits,DecimalDigits}
Atom::
PatternCharacter
.
\AtomEscape
CharacterClass
(Disjunction)
(?:Disjunction)
PatternCharacter::
SourceCharacterbut not one of
^$\.*+?()[]{}|
AtomEscape::
DecimalEscape
CharacterEscape
CharacterClassEscape
CharacterEscape::
ControlEscape
cControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscape
ControlEscape::one of
fnrtv
ControlLetter::one of
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
IdentityEscape::
SourceCharacterbut notIdentifierPart
<ZWJ>
<ZWNJ>
DecimalEscape::
DecimalIntegerLiteral[lookahead ∉DecimalDigit]
CharacterClassEscape::one of
dDsSwW
CharacterClass::
[[lookahead ∉ {^}]ClassRanges]
[^ClassRanges]
ClassRanges::
[empty]
NonemptyClassRanges
NonemptyClassRanges::
ClassAtom
ClassAtomNonemptyClassRangesNoDash
ClassAtom-ClassAtomClassRanges
NonemptyClassRangesNoDash::
ClassAtom
ClassAtomNoDashNonemptyClassRangesNoDash
ClassAtomNoDash-ClassAtomClassRanges
ClassAtom::
-
ClassAtomNoDash
ClassAtomNoDash::
SourceCharacterbut not one of\or]or-
\ClassEscape
ClassEscape::
DecimalEscape
b
CharacterEscape
CharacterClassEscape

15.10.2 Pattern Semantics

A regular expression pattern is converted into an internal procedure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The internal procedure is used as the value of a RegExp object’s [[Match]] internal property.

15.10.2.1 Notation

The descriptions below use the following variables:

  • Input is the String being matched by the regular expression pattern. The notationinput[n] means thenth character ofinput, wheren can range between 0 (inclusive) andInputLength (exclusive).

  • InputLength is the number of characters in theInput String.

  • NcapturingParens is the total number of left capturing parentheses (i.e. the total number of times theAtom::(Disjunction) production is expanded) in the pattern. A left capturing parenthesis is any( pattern character that is matched by the( terminal of theAtom::(Disjunction) production.

  • IgnoreCase is the setting of the RegExp object'signoreCase property.

  • Multiline is the setting of the RegExp object’smultiline property.

Furthermore, the descriptions below use the following internal data structures:

  • ACharSet is a mathematical set of characters.

  • AState is an ordered pair(endIndex,captures) whereendIndex is an integer andcaptures is an internal array ofNcapturingParens values.States are used to represent partial match states in the regular expression matching algorithms. TheendIndex is one plus the index of the last input character matched so far by the pattern, whilecaptures holds the results of capturing parentheses. Thenth element ofcaptures is either a String that represents the value obtained by thenth set of capturing parentheses orundefined if thenth set of capturing parentheses hasn’t been reached yet. Due to backtracking, manyStates may be in use at any time during the matching process.

  • AMatchResult is either aState or the special tokenfailure that indicates that the match failed.

  • AContinuation procedure is an internal closure (i.e. an internal procedure with some arguments already bound to values) that takes oneState argument and returns aMatchResult result. If an internal closure references variables bound in the function that creates the closure, the closure uses the values that these variables had at the time the closure was created. TheContinuation attempts to match the remaining portion (specified by the closure's already-bound arguments) of the pattern against the input String, starting at the intermediate state given by itsState argument. If the match succeeds, theContinuation returns the finalState that it reached; if the match fails, theContinuation returnsfailure.

  • AMatcher procedure is an internal closure that takes two arguments -- aState and aContinuation -- and returns aMatchResult result. AMatcher attempts to match a middle subpattern (specified by the closure's already-bound arguments) of the pattern against the input String, starting at the intermediate state given by itsState argument. TheContinuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a newState, theMatcher then callsContinuation on that newState to test if the rest of the pattern can match as well. If it can, theMatcher returns theState returned byContinuation; if not, theMatcher may try different choices at its choice points, repeatedly callingContinuation until it either succeeds or all possibilities have been exhausted.

  • AnAssertionTester procedure is an internal closure that takes aState argument and returns a Boolean result. The assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current place in the input String and returnstrue if the condition matched orfalse if not.

  • AnEscapeValue is either a character or an integer. AnEscapeValue is used to denote the interpretation of aDecimalEscape escape sequence: a characterch means that the escape sequence is interpreted as the characterch, while an integern means that the escape sequence is interpreted as a backreference to thenth set of capturing parentheses.

15.10.2.2 Pattern

The productionPattern::Disjunction evaluates as follows:

  1. EvaluateDisjunction to obtain a Matcherm.
  2. Return an internal closure that takes two arguments, a Stringstr and an integerindex, and performs the following:
    1. LetInput be the given Stringstr. This variable will be used throughout the algorithms in15.10.2.
    2. LetInputLength be the length ofInput. This variable will be used throughout the algorithms in15.10.2.
    3. Letc be a Continuation that always returns its State argument as a successful MatchResult.
    4. Letcap be an internal array ofNcapturingParensundefined values, indexed 1 throughNcapturingParens.
    5. Letx be the State (index,cap).
    6. Callm(x,c) and return its result.

NOTE A Pattern evaluates ("compiles") to an internal procedure value.RegExp.prototype.exec can then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in15.10.2 are designed so that compiling a pattern may throw aSyntaxError exception; on the other hand, once the pattern is successfully compiled, applying its result internal procedure to find a match in a String cannot throw an exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).

15.10.2.3 Disjunction

The productionDisjunction::Alternative evaluates by evaluatingAlternative to obtain aMatcher and returning thatMatcher.

The productionDisjunction::Alternative|Disjunction evaluates as follows:

  1. EvaluateAlternative to obtain a Matcherm1.
  2. EvaluateDisjunction to obtain a Matcherm2.
  3. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following:
    1. Callm1(x,c) and letr be its result.
    2. Ifr isn'tfailure, returnr.
    3. Callm2(x,c) and return its result.

NOTE The| regular expression operator separates two alternatives. The pattern first tries to match the leftAlternative (followed by the sequel of the regular expression); if it fails, it tries to match the rightDisjunction (followed by the sequel of the regular expression). If the leftAlternative, the rightDisjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the leftAlternative. If choices in the leftAlternative are exhausted, the rightDisjunction is tried instead of the leftAlternative. Any capturing parentheses inside a portion of the pattern skipped by| produceundefined values instead of Strings. Thus, for example,

/a|ab/.exec("abc")

returns the result"a" and not"ab". Moreover,

/((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

["abc", "ab", undefined, "ab", "c", "c", undefined]

15.10.2.4 Alternative

The productionAlternative::[empty] evaluates by returning a Matcher that takes two arguments, a Statex and a Continuationc, and returns the result of callingc(x).

The productionAlternative::AlternativeTerm evaluates as follows:

  1. EvaluateAlternative to obtain a Matcherm1.
  2. EvaluateTerm to obtain a Matcherm2.
  3. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following:
    1. Create a Continuationd that takes a State argumenty and returns the result of callingm2(y,c).
    2. Callm1(x,d) and return its result.

NOTE ConsecutiveTerms try to simultaneously match consecutive portions of the input String. If the leftAlternative, the rightTerm, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the rightTerm, and all choices in the rightTerm are tried before moving on to the next choice in the leftAlternative.

15.10.2.5 Term

The productionTerm::Assertion evaluates by returning an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following:

  1. EvaluateAssertion to obtain an AssertionTestert.
  2. Callt(x) and letr be the resulting Boolean value.
  3. Ifr isfalse, returnfailure.
  4. Callc(x) and return its result.

The productionTerm::Atom evaluates by evaluatingAtom to obtain a Matcher and returning that Matcher.

The productionTerm::AtomQuantifier evaluates as follows:

  1. EvaluateAtom to obtain a Matcherm.
  2. EvaluateQuantifier to obtain the three results: an integermin, an integer (or ∞)max, and Booleangreedy.
  3. Ifmax is finite and less thanmin, then throw aSyntaxError exception.
  4. LetparenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion'sTerm. This is the total number of times theAtom::(Disjunction) production is expanded prior to this production'sTerm plus the total number ofAtom::(Disjunction) productions enclosing thisTerm.
  5. LetparenCount be the number of left capturing parentheses in the expansion of this production'sAtom. This is the total number ofAtom::(Disjunction) productions enclosed by this production'sAtom.
  6. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following:
    1. Call RepeatMatcher(m,min,max,greedy,x,c,parenIndex,parenCount) and return its result.

The abstract operationRepeatMatcher takes eight parameters, a Matcherm, an integermin, an integer (or ∞)max, a Booleangreedy, a Statex, a Continuationc, an integerparenIndex, and an integerparenCount, and performs the following:

  1. Ifmax is zero, then callc(x) and return its result.
  2. Create an internal Continuation closured that takes one State argumenty and performs the following:
    1. Ifmin is zero andy'sendIndex is equal tox'sendIndex, then returnfailure.
    2. Ifmin is zero then letmin2 be zero; otherwise letmin2 bemin–1.
    3. Ifmax is ∞, then letmax2 be ∞; otherwise letmax2 bemax–1.
    4. Call RepeatMatcher(m,min2,max2,greedy,y,c,parenIndex,parenCount) and return its result.
  3. Letcap be a fresh copy ofx'scaptures internal array.
  4. For every integerk that satisfiesparenIndex <k andk ≤parenIndex+parenCount, setcap[k] toundefined.
  5. Lete bex'sendIndex.
  6. Letxr be the State (e,cap).
  7. Ifmin is not zero, then callm(xr,d) and return its result.
  8. Ifgreedy isfalse, then
    1. Callc(x) and letz be its result.
    2. Ifz is notfailure, returnz.
    3. Callm(xr,d) and return its result.
  9. Callm(xr,d) and letz be its result.
  10. Ifz is notfailure, returnz.
  11. Callc(x) and return its result.

NOTE 1 AnAtom followed by aQuantifier is repeated the number of times specified by theQuantifier. AQuantifier can be non-greedy, in which case theAtom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case theAtom pattern is repeated as many times as possible while still matching the sequel. TheAtom pattern is repeated rather than the input String that it matches, so different repetitions of theAtom can match different input substrings.

NOTE 2 If theAtom and the sequel of the regular expression all have choice points, theAtom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition ofAtom. All choices in the last (nth) repetition ofAtom are tried before moving on to the next choice in the next-to-last (n–1)st repetition ofAtom; at which point it may turn out that more or fewer repetitions ofAtom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n-1)st repetition ofAtom and so on.

Compare

/a[a-z]{2,4}/.exec("abcdefghi")

which returns"abcde" with

/a[a-z]{2,4}?/.exec("abcdefghi")

which returns"abc".

Consider also

/(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

["aaba", "ba"]

and not any of:

["aabaac", "aabaac"]
["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:

"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/,"$1")

which returns the gcd in unary notation"aaaaa".

NOTE 3 Step 4 of the RepeatMatcher clearsAtom's captures each timeAtom is repeated. We can see its behaviour in the regular expression

/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")

which returns the array

["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost* clears all captured Strings contained in the quantifiedAtom, which in this case includes capture Strings numbered 2, 3, 4, and 5.

NOTE 4 Step 1 of the RepeatMatcher'sd closure states that, once the minimum number of repetitions has been satisfied, any more expansions ofAtom that match the empty String are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:

/(a*)*/.exec("b")

or the slightly more complicated:

/(a*)b\1+/.exec("baaaac")

which returns the array

["b", ""]

15.10.2.6 Assertion

The productionAssertion::^ evaluates by returning an internal AssertionTester closure that takes a State argumentx and performs the following:

  1. Lete bex'sendIndex.
  2. Ife is zero, returntrue.
  3. IfMultiline isfalse, returnfalse.
  4. If the characterInput[e–1] is one ofLineTerminator, returntrue.
  5. Returnfalse.

The productionAssertion::$ evaluates by returning an internal AssertionTester closure that takes a State argumentx and performs the following:

  1. Lete bex'sendIndex.
  2. Ife is equal toInputLength, returntrue.
  3. Ifmultiline isfalse, returnfalse.
  4. If the characterInput[e] is one ofLineTerminator, returntrue.
  5. Returnfalse.

The productionAssertion::\b evaluates by returning an internal AssertionTester closure that takes a State argumentx and performs the following:

  1. Lete bex'sendIndex.
  2. CallIsWordChar(e–1) and leta be the Boolean result.
  3. CallIsWordChar(e) and letb be the Boolean result.
  4. Ifa istrue andb isfalse, returntrue.
  5. Ifa isfalse andb istrue, returntrue.
  6. Returnfalse.

The productionAssertion::\B evaluates by returning an internal AssertionTester closure that takes a State argumentx and performs the following:

  1. Lete bex'sendIndex.
  2. CallIsWordChar(e–1) and leta be the Boolean result.
  3. CallIsWordChar(e) and let b be the Boolean result.
  4. If a istrue and b isfalse, returnfalse.
  5. If a isfalse and b istrue, returnfalse.
  6. Returntrue.

The productionAssertion::(?=Disjunction) evaluates as follows:

  1. EvaluateDisjunction to obtain a Matcherm.
  2. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following steps:
    1. Letd be a Continuation that always returns its State argument as a successful MatchResult.
    2. Callm(x,d) and letr be its result.
    3. Ifr isfailure, returnfailure.
    4. Lety ber's State.
    5. Letcap bey'scaptures internal array.
    6. Letxe bex'sendIndex.
    7. Letz be the State (xe,cap).
    8. Callc(z) and return its result.

The productionAssertion::(?!Disjunction) evaluates as follows:

  1. EvaluateDisjunction to obtain a Matcherm.
  2. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following steps:
    1. Letd be a Continuation that always returns its State argument as a successful MatchResult.
    2. Callm(x,d) and letr be its result.
    3. Ifr isn'tfailure, returnfailure.
    4. Callc(x) and return its result.

The abstract operationIsWordChar takes an integer parametere and performs the following:

  1. Ife == –1 ore ==InputLength, returnfalse.
  2. Letc be the characterInput[e].
  3. Ifc is one of the sixty-three characters below, returntrue.
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789_
  1. Returnfalse.

15.10.2.7 Quantifier

The productionQuantifier::QuantifierPrefix evaluates as follows:

  1. EvaluateQuantifierPrefix to obtain the two results: an integermin and an integer (or ∞)max.
  2. Return the three resultsmin,max, andtrue.

The productionQuantifier::QuantifierPrefix? evaluates as follows:

  1. EvaluateQuantifierPrefix to obtain the two results: an integermin and an integer (or ∞)max.
  2. Return the three resultsmin,max, andfalse.

The productionQuantifierPrefix::* evaluates by returning the two results 0 and ∞.

The productionQuantifierPrefix::+ evaluates by returning the two results 1 and ∞.

The productionQuantifierPrefix::? evaluates by returning the two results 0 and 1.

The productionQuantifierPrefix::{DecimalDigits} evaluates as follows:

  1. Leti be the MV ofDecimalDigits (see 7.8.3).
  2. Return the two resultsi andi.

The productionQuantifierPrefix::{DecimalDigits,} evaluates as follows:

  1. Leti be the MV ofDecimalDigits.
  2. Return the two resultsi and ∞.

The productionQuantifierPrefix::{DecimalDigits,DecimalDigits} evaluates as follows:

  1. Leti be the MV of the firstDecimalDigits.
  2. Letj be the MV of the secondDecimalDigits.
  3. Return the two resultsi andj.

15.10.2.8 Atom

The productionAtom::PatternCharacter evaluates as follows:

  1. Letch be the character represented byPatternCharacter.
  2. LetA be a one-element CharSet containing the characterch.
  3. CallCharacterSetMatcher(A,false) and return its Matcher result.

The productionAtom::. evaluates as follows:

  1. LetA be the set of all characters exceptLineTerminator.
  2. CallCharacterSetMatcher(A,false) and return its Matcher result.

The productionAtom::\AtomEscape evaluates by evaluatingAtomEscape to obtain a Matcher and returning that Matcher.

The productionAtom::CharacterClass evaluates as follows:

  1. EvaluateCharacterClass to obtain a CharSetA and a Booleaninvert.
  2. CallCharacterSetMatcher(A,invert) and return its Matcher result.

The productionAtom::(Disjunction) evaluates as follows:

  1. EvaluateDisjunction to obtain a Matcherm.
  2. LetparenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's initial left parenthesis. This is the total number of times theAtom::(Disjunction) production is expanded prior to this production'sAtom plus the total number ofAtom::(Disjunction) productions enclosing thisAtom.
  3. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following steps:
    1. Create an internal Continuation closured that takes one State argumenty and performs the following steps:
      1. Letcap be a fresh copy ofy'scaptures internal array.
      2. Letxe bex'sendIndex.
      3. Letye bey'sendIndex.
      4. Lets be a fresh String whose characters are the characters ofInput at positionsxe (inclusive) throughye (exclusive).
      5. Setcap[parenIndex+1] tos.
      6. Letz be the State (ye,cap).
      7. Callc(z) and return its result.
    2. Callm(x,d) and return its result.

The productionAtom::(?:Disjunction) evaluates by evaluatingDisjunction to obtain a Matcher and returning that Matcher.

The abstract operationCharacterSetMatcher takes two arguments, a CharSetA and a Boolean flaginvert, and performs the following:

  1. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following steps:
    1. Lete bex'sendIndex.
    2. Ife ==InputLength, returnfailure.
    3. Letch be the characterInput[e].
    4. Letcc be the result ofCanonicalize(ch).
    5. Ifinvert isfalse, then
      1. If there does not exist a membera of setA such thatCanonicalize(a) ==cc, returnfailure.
    6. Elseinvert istrue,
      1. If there exists a membera of setA such thatCanonicalize(a) ==cc, returnfailure.
    7. Letcap bex'scaptures internal array.
    8. Lety be the State (e+1,cap).
    9. Callc(y) and return its result.

The abstract operationCanonicalize takes a character parameterch and performs the following steps:

  1. IfIgnoreCase isfalse, returnch.
  2. Letu bech converted to upper case as if by calling the standard built-in methodString.prototype.toUpperCase on the one-character Stringch.
  3. Ifu does not consist of a single character, returnch.
  4. Letcu beu's character.
  5. Ifch's code unit value is greater than or equal to decimal 128 andcu's code unit value is less than decimal 128, then returnch.
  6. Returncu.

NOTE 1 Parentheses of the form(Disjunction) serve both to group the components of theDisjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching internal procedure. To inhibit the capturing behaviour of parentheses, use the form(?:Disjunction) instead.

NOTE 2 The form(?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern insideDisjunction must match at the current position, but the current position is not advanced before matching the sequel. IfDisjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a(?= form (this unusual behaviour is inherited from Perl). This only matters when theDisjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.

For example,

/(?=(a+))/.exec("baaabac")

matches the empty String immediately after the firstb and therefore returns the array:

["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

/(?=(a+))a*b\1/.exec("baaabac")

This expression returns

["aba", "a"]

and not:

["aaaba", "a"]

NOTE 3 The form(?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern insideDisjunction must fail to match at the current position. The current position is not advanced before matching the sequel.Disjunction can contain capturing parentheses, but backreferences to them only make sense from withinDisjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always returnundefined because the negative lookahead must fail for the pattern to succeed. For example,

/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")

looks for ana not immediately followed by some positive number n ofa's, ab, another na's (specified by the first\2) and ac. The second\2 is outside the negative lookahead, so it matches againstundefined and therefore always succeeds. The whole expression returns the array:

["baaabaac", "ba", undefined, "abaac"]

In case-insignificant matches all characters are implicitly converted to upper case immediately before they are compared. However, if converting a character to upper case would expand that character into more than one character (such as converting"ß" (\u00DF) into"SS"), then the character is left as-is instead. The character is also left as-is if it is not an ASCII character but converting it to upper case would make it into an ASCII character. This prevents Unicode characters such as\u0131 and\u017F from matching regular expressions such as/[a‑z]/i, which are only intended to match ASCII letters. Furthermore, if these conversions were allowed, then/[^\W]/i would match each ofa,b, …,h, but noti ors.

15.10.2.9 AtomEscape

The productionAtomEscape::DecimalEscape evaluates as follows:

  1. EvaluateDecimalEscape to obtain an EscapeValueE.
  2. IfE is a character, then
    1. Letch beE's character.
    2. LetA be a one-element CharSet containing the characterch.
    3. CallCharacterSetMatcher(A,false) and return its Matcher result.
  3. E must be an integer. Letn be that integer.
  4. Ifn=0 orn>NCapturingParens then throw aSyntaxError exception.
  5. Return an internal Matcher closure that takes two arguments, a Statex and a Continuationc, and performs the following:
    1. Letcap bex'scaptures internal array.
    2. Lets becap[n].
    3. Ifs isundefined, then callc(x) and return its result.
    4. Lete bex'sendIndex.
    5. Letlen bes's length.
    6. Letf bee+len.
    7. Iff>InputLength, returnfailure.
    8. If there exists an integeri between 0 (inclusive) andlen (exclusive) such thatCanonicalize(s[i]) is not the same character asCanonicalize(Input [e+i]), then returnfailure.
    9. Lety be the State (f,cap).
    10. Callc(y) and return its result.

The productionAtomEscape::CharacterEscape evaluates as follows:

  1. EvaluateCharacterEscape to obtain a characterch.
  2. LetA be a one-element CharSet containing the characterch.
  3. CallCharacterSetMatcher(A,false) and return its Matcher result.

The productionAtomEscape::CharacterClassEscape evaluates as follows:

  1. EvaluateCharacterClassEscape to obtain a CharSetA.
  2. CallCharacterSetMatcher(A,false) and return its Matcher result.

NOTE An escape sequence of the form\ followed by a nonzero decimal numbern matches the result of thenth set of capturing parentheses (see 15.10.2.11). It is an error if the regular expression has fewer thann capturing parentheses. If the regular expression hasn or more capturing parentheses but thenth one isundefined because it has not captured anything, then the backreference always succeeds.

15.10.2.10 CharacterEscape

The productionCharacterEscape::ControlEscape evaluates by returning the character according to Table 23.

Table 23 — ControlEscape Character Values
ControlEscapeCode UnitNameSymbol
t\u0009horizontal tab<HT>
n\u000Aline feed (new line)<LF>
v\u000Bvertical tab<VT>
f\u000Cform feed<FF>
r\u000Dcarriage return<CR>

The productionCharacterEscape::cControlLetter evaluates as follows:

  1. Letch be the character represented byControlLetter.
  2. Leti bech's code unit value.
  3. Letj be the remainder of dividingi by 32.
  4. Return the character whose code unit value isj.

The productionCharacterEscape::HexEscapeSequence evaluates by evaluating the CV of theHexEscapeSequence (see 7.8.4) and returning its character result.

The productionCharacterEscape::UnicodeEscapeSequence evaluates by evaluating the CV of theUnicodeEscapeSequence (see 7.8.4) and returning its character result.

The productionCharacterEscape::IdentityEscape evaluates by returning the character represented byIdentityEscape.

15.10.2.11 DecimalEscape

The productionDecimalEscape::DecimalIntegerLiteral[lookahead ∉DecimalDigit] evaluates as follows:

  1. Leti be the MV ofDecimalIntegerLiteral.
  2. Ifi is zero, return the EscapeValue consisting of a <NUL> character (Unicode value 0000).
  3. Return the EscapeValue consisting of the integeri.

The definition of “the MV ofDecimalIntegerLiteral” is in7.8.3.

NOTE If\ is followed by a decimal numbern whose first digit is not0, then the escape sequence is considered to be a backreference. It is an error ifn is greater than the total number of left capturing parentheses in the entire regular expression.\0 represents the <NUL> character and cannot be followed by a decimal digit.

15.10.2.12 CharacterClassEscape

The productionCharacterClassEscape::d evaluates by returning the ten-element set of characters containing the characters0 through9 inclusive.

The productionCharacterClassEscape::D evaluates by returning the set of all characters not included in the set returned byCharacterClassEscape::d .

The productionCharacterClassEscape::s evaluates by returning the set of characters containing the characters that are on the right-hand side of theWhiteSpace (7.2) orLineTerminator (7.3) productions.

The productionCharacterClassEscape::S evaluates by returning the set of all characters not included in the set returned byCharacterClassEscape::s .

The productionCharacterClassEscape::w evaluates by returning the set of characters containing the sixty-three characters:

abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789_

The productionCharacterClassEscape::W evaluates by returning the set of all characters not included in the set returned byCharacterClassEscape::w .

15.10.2.13 CharacterClass

The productionCharacterClass::[[lookahead ∉ {^}]ClassRanges] evaluates by evaluatingClassRanges to obtain a CharSet and returning that CharSet and the Booleanfalse.

The productionCharacterClass::[^ClassRanges] evaluates by evaluatingClassRanges to obtain a CharSet and returning that CharSet and the Booleantrue.

15.10.2.14 ClassRanges

The productionClassRanges::[empty] evaluates by returning the empty CharSet.

The productionClassRanges::NonemptyClassRanges evaluates by evaluatingNonemptyClassRanges to obtain a CharSet and returning that CharSet.

15.10.2.15 NonemptyClassRanges

The productionNonemptyClassRanges::ClassAtom evaluates by evaluatingClassAtom to obtain a CharSet and returning that CharSet.

The productionNonemptyClassRanges::ClassAtomNonemptyClassRangesNoDash evaluates as follows:

  1. EvaluateClassAtom to obtain a CharSetA.
  2. EvaluateNonemptyClassRangesNoDash to obtain a CharSetB.
  3. Return the union of CharSetsA andB.

The productionNonemptyClassRanges::ClassAtom-ClassAtomClassRanges evaluates as follows:

  1. Evaluate the firstClassAtom to obtain a CharSetA.
  2. Evaluate the secondClassAtom to obtain a CharSetB.
  3. EvaluateClassRanges to obtain a CharSetC.
  4. CallCharacterRange(A,B) and letD be the resulting CharSet.
  5. Return the union of CharSetsD andC.

The abstract operationCharacterRange takes two CharSet parametersA andB and performs the following:

  1. IfA does not contain exactly one character orB does not contain exactly one character then throw aSyntaxError exception.
  2. Leta be the one character in CharSetA.
  3. Letb be the one character in CharSetB.
  4. Leti be the code unit value of charactera.
  5. Letj be the code unit value of characterb.
  6. Ifi >j then throw aSyntaxError exception.
  7. Return the set containing all characters numberedi throughj, inclusive.

15.10.2.16 NonemptyClassRangesNoDash

The productionNonemptyClassRangesNoDash::ClassAtom evaluates by evaluatingClassAtom to obtain a CharSet and returning that CharSet.

The productionNonemptyClassRangesNoDash::ClassAtomNoDashNonemptyClassRangesNoDash evaluates as follows:

  1. EvaluateClassAtomNoDash to obtain a CharSetA.
  2. EvaluateNonemptyClassRangesNoDash to obtain a CharSetB.
  3. Return the union of CharSetsA andB.

The productionNonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRanges evaluates as follows:

  1. EvaluateClassAtomNoDash to obtain a CharSetA.
  2. EvaluateClassAtom to obtain a CharSetB.
  3. EvaluateClassRanges to obtain a CharSetC.
  4. CallCharacterRange(A,B) and letD be the resulting CharSet.
  5. Return the union of CharSetsD andC.

NOTE 1ClassRanges can expand into singleClassAtoms and/or ranges of twoClassAtoms separated by dashes. In the latter case theClassRanges includes all characters between the firstClassAtom and the secondClassAtom, inclusive; an error occurs if eitherClassAtom does not represent a single character (for example, if one is\w) or if the firstClassAtom's code unit value is greater than the secondClassAtom's code unit value.

NOTE 2 Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern/[E-F]/i matches only the lettersE,F,e, andf, while the pattern/[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols[,\,],^,_, and`.

NOTE 3 A- character can be treated literally or it can denote a range. It is treated literally if it is the first or last character ofClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

15.10.2.17 ClassAtom

The productionClassAtom::- evaluates by returning the CharSet containing the one character-.

The productionClassAtom::ClassAtomNoDash evaluates by evaluatingClassAtomNoDash to obtain a CharSet and returning that CharSet.

15.10.2.18 ClassAtomNoDash

The productionClassAtomNoDash::SourceCharacterbut not one of\or]or- evaluates by returning a one-element CharSet containing the character represented bySourceCharacter.

The productionClassAtomNoDash::\ClassEscape evaluates by evaluatingClassEscape to obtain a CharSet and returning that CharSet.

15.10.2.19 ClassEscape

The productionClassEscape::DecimalEscape evaluates as follows:

  1. EvaluateDecimalEscape to obtain an EscapeValueE.
  2. IfE is not a character then throw aSyntaxError exception.
  3. Letch beE's character.
  4. Return the one-element CharSet containing the characterch.

The productionClassEscape::b evaluates by returning the CharSet containing the one character <BS> (Unicode value 0008).

The productionClassEscape::CharacterEscape evaluates by evaluatingCharacterEscape to obtain a character and returning a one-element CharSet containing that character.

The productionClassEscape::CharacterClassEscape evaluates by evaluatingCharacterClassEscape to obtain a CharSet and returning that CharSet.

NOTE AClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for\b,\B, and backreferences. Inside aCharacterClass,\b means the backspace character, while\B and backreferences raise errors. Using a backreference inside aClassAtom causes an error.

15.10.3 The RegExp Constructor Called as a Function

15.10.3.1 RegExp(pattern, flags)

Ifpattern is an objectR whose [[Class]] internal property is"RegExp" andflags isundefined, then returnR unchanged. Otherwise call the standard built-inRegExp constructor (15.10.4.1) as if by the expressionnew RegExp(pattern,flags) and return the object constructed by that constructor.

15.10.4 The RegExp Constructor

WhenRegExp is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.10.4.1 new RegExp(pattern, flags)

Ifpattern is an objectR whose [[Class]] internal property is"RegExp" andflags isundefined, then letP be thepattern used to constructR and letF be the flags used to constructR. Ifpattern is an objectR whose [[Class]] internal property is"RegExp" andflags is notundefined, then throw aTypeError exception. Otherwise, letP be the empty String ifpattern isundefined andToString(pattern) otherwise, and letF be the empty String ifflags isundefined andToString(flags) otherwise.

If the characters ofP do not have the syntactic formPattern, then throw aSyntaxError exception. Otherwise let the newly constructed object have a [[Match]] internal property obtained by evaluating ("compiling") the characters ofP as aPattern as described in15.10.2.

IfF contains any character other than"g","i", or"m", or if it contains the same character more than once, then throw aSyntaxError exception.

If aSyntaxError exception is not thrown, then:

LetS be a String in the form of aPattern equivalent toP, in which certain characters are escaped as described below.S may or may not be identical toP orpattern; however, the internal procedure that would result from evaluatingS as aPattern must behave identically to the internal procedure given by the constructed object's [[Match]] internal property.

The characters/ occurring in the pattern shall be escaped inS as necessary to ensure that the String value formed by concatenating the Strings"/",S,"/", andF can be parsed (in an appropriate lexical context) as aRegularExpressionLiteral that behaves identically to the constructed regular expression. For example, ifP is"/", thenS could be"\/" or"\u002F", among other possibilities, but not"/", because/// followed byF would be parsed as aSingleLineComment rather than aRegularExpressionLiteral. IfP is the empty String, this specification can be met by lettingS be"(?:)".

The following properties of the newly constructed object are data properties with the attributes that are specified in15.10.7. The [[Value]] of each property is set as follows:

Thesource property of the newly constructed object is set toS.

Theglobal property of the newly constructed object is set to a Boolean value that istrue ifF contains the character"g" andfalse otherwise.

TheignoreCase property of the newly constructed object is set to a Boolean value that istrue ifF contains the character"i" andfalse otherwise.

Themultiline property of the newly constructed object is set to a Boolean value that istrue ifF contains the character"m" andfalse otherwise.

ThelastIndex property of the newly constructed object is set to0.

The [[Prototype]] internal property of the newly constructed object is set to the standard built-in RegExp prototype object as specified in15.10.6.

The [[Class]] internal property of the newly constructed object is set to"RegExp".

NOTE If pattern is aStringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be recognised by RegExp, any backslash\ characters must be escaped within theStringLiteral to prevent them being removed when the contents of theStringLiteral are formed.

15.10.5 Properties of the RegExp Constructor

The value of the [[Prototype]] internal property of the RegExp constructor is the standard built-in Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is2), the RegExp constructor has the following properties:

15.10.5.1 RegExp.prototype

The initial value ofRegExp.prototype is the RegExp prototype object (15.10.6).

This property shall have the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.10.6 Properties of the RegExp Prototype Object

The value of the [[Prototype]] internal property of the RegExp prototype object is the standard built-in Object prototype object (15.2.4). The RegExp prototype object is itself a regular expression object; its [[Class]] is"RegExp". The initial values of the RegExp prototype object’s data properties (15.10.7) are set as if the object was created by the expressionnew RegExp() whereRegExp is that standard built-in constructor with that name.

The RegExp prototype object does not have avalueOf property of its own; however, it inherits thevalueOf property from the Object prototype object.

In the following descriptions of functions that are properties of the RegExp prototype object, the phrase “this RegExp object” refers to the object that is thethis value for the invocation of the function; aTypeError exception is thrown if thethis value is not an object or an object for which the value of the [[Class]] internal property is not"RegExp".

15.10.6.1 RegExp.prototype.constructor

The initial value ofRegExp.prototype.constructor is the standard built-inRegExp constructor.

15.10.6.2 RegExp.prototype.exec(string)

Performs a regular expression match ofstring against the regular expression and returns an Array object containing the results of the match, ornull ifstring did not match.

The StringToString(string) is searched for an occurrence of the regular expression pattern as follows:

  1. LetR be this RegExp object.
  2. LetS be the value ofToString(string).
  3. Letlength be the length ofS.
  4. LetlastIndex be the result of calling the [[Get]] internal method ofR with argument "lastIndex".
  5. Leti be the value ofToInteger(lastIndex).
  6. Letglobal be the result of calling the [[Get]] internal method ofR with argument "global".
  7. Ifglobal isfalse, then leti = 0.
  8. LetmatchSucceeded befalse.
  9. Repeat, whilematchSucceeded isfalse
    1. Ifi < 0 ori >length, then
      1. Call the [[Put]] internal method ofR with arguments "lastIndex", 0, andtrue.
      2. Returnnull.
    2. Call the [[Match]] internal method ofR with argumentsS andi.
    3. If [[Match]] returnedfailure, then
      1. Let i = i+1.
    4. else
      1. Letr be the State result of the call to [[Match]].
      2. SetmatchSucceeded totrue.
  10. Lete ber'sendIndex value.
  11. Ifglobal istrue,
    1. Call the [[Put]] internal method ofR with arguments "lastIndex",e, andtrue.
  12. Letn be the length ofr'scaptures array. (This is the same value as 15.10.2.1'sNCapturingParens.)
  13. LetA be a new array created as if by the expressionnew Array() whereArray is the standard built-in constructor with that name.
  14. LetmatchIndex bei.
  15. Call the [[DefineOwnProperty]] internal method ofA with arguments "index",Property Descriptor {[[Value]]:matchIndex, [[Writable]:true, [[Enumerable]]:true, [[Configurable]]:true}, andtrue.
  16. Call the [[DefineOwnProperty]] internal method ofA with arguments "input",Property Descriptor {[[Value]]:S, [[Writable]:true, [[Enumerable]]:true, [[Configurable]]:true}, andtrue.
  17. Call the [[DefineOwnProperty]] internal method ofA with arguments "length",Property Descriptor {[[Value]]:n + 1}, andtrue.
  18. LetmatchedSubstr be the matched substring (i.e. the portion ofS between offseti inclusive and offsete exclusive).
  19. Call the [[DefineOwnProperty]] internal method ofA with arguments "0",Property Descriptor {[[Value]]:matchedSubstr, [[Writable]:true, [[Enumerable]]:true, [[Configurable]]:true}, andtrue.
  20. For each integeri such thati > 0 andi ≤n
    1. LetcaptureI beith element ofr'scaptures array.
    2. Call the [[DefineOwnProperty]] internal method ofA with argumentsToString(i),Property Descriptor {[[Value]]:captureI, [[Writable]:true, [[Enumerable]]:true, [[Configurable]]:true}, andtrue.
  21. ReturnA.

15.10.6.3 RegExp.prototype.test(string)

The following steps are taken:

  1. Letmatch be the result of evaluating theRegExp.prototype.exec (15.10.6.2) algorithm upon this RegExp object usingstring as the argument.
  2. Ifmatch is not null, then returntrue; else returnfalse.

15.10.6.4 RegExp.prototype.toString()

Return the String value formed by concatenating the Strings"/", the String value of thesource property of this RegExp object, and"/"; plus"g" if theglobal property istrue,"i" if theignoreCase property istrue, and"m" if themultiline property istrue.

NOTE The returned String has the form of aRegularExpressionLiteral that evaluates to another RegExp object with the same behaviour as this object.

15.10.7 Properties of RegExp Instances

RegExp instances inherit properties from the RegExp prototype object and their [[Class]] internal property value is"RegExp". RegExp instances also have a [[Match]] internal property and alength property.

The value of the [[Match]] internal property is an implementation dependent representation of thePattern of the RegExp object.

RegExp instances also have the following properties.

15.10.7.1 source

The value of thesource property is a String in the form of aPattern representing the current regular expression. This property shall have the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.10.7.2 global

The value of theglobal property is a Boolean value indicating whether the flags contained the character“g”. This property shall have the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.10.7.3 ignoreCase

The value of theignoreCase property is a Boolean value indicating whether the flags contained the character“i”. This property shall have the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.10.7.4 multiline

The value of themultiline property is a Boolean value indicating whether the flags contained the character“m”. This property shall have the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.10.7.5 lastIndex

The value of thelastIndex property specifies the String position at which to start the next match. It is coerced to an integer when used (see 15.10.6.2). This property shall have the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false }.

NOTE Unlike the other standard built-in properties of RegExp instances,lastIndex is writable.

15.11 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

15.11.1 The Error Constructor Called as a Function

WhenError is called as a function rather than as a constructor, it creates and initialises a new Error object. Thus the function callError(…) is equivalent to the object creation expressionnew Error(…) with the same arguments.

15.11.1.1 Error (message)

The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object, the one that is the initial value ofError.prototype (15.11.3.1).

The [[Class]] internal property of the newly constructed object is set to"Error".

The [[Extensible]] internal property of the newly constructed object is set totrue.

If the argumentmessage is notundefined, themessage own property of the newly constructed object is set toToString(message).

15.11.2 The Error Constructor

WhenError is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.11.2.1 new Error (message)

The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object, the one that is the initial value ofError.prototype (15.11.3.1).

The [[Class]] internal property of the newly constructed Error object is set to"Error".

The [[Extensible]] internal property of the newly constructed object is set totrue.

If the argumentmessage is notundefined, themessage own property of the newly constructed object is set toToString(message).

15.11.3 Properties of the Error Constructor

The value of the [[Prototype]] internal property of the Error constructor is the Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is1), the Error constructor has the following property:

15.11.3.1 Error.prototype

The initial value ofError.prototype is the Error prototype object (15.11.4).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.11.4 Properties of the Error Prototype Object

The Error prototype object is itself an Error object (its [[Class]] is"Error").

The value of the [[Prototype]] internal property of the Error prototype object is the standard built-in Object prototype object (15.2.4).

15.11.4.1 Error.prototype.constructor

The initial value ofError.prototype.constructor is the built-inError constructor.

15.11.4.2 Error.prototype.name

The initial value ofError.prototype.name is"Error".

15.11.4.3 Error.prototype.message

The initial value ofError.prototype.message is the empty String.

15.11.4.4 Error.prototype.toString ( )

The following steps are taken:

  1. LetO be thethis value.
  2. IfType(O) is not Object, throw aTypeError exception.
  3. Letname be the result of calling the [[Get]] internal method ofO with argument"name".
  4. Ifname isundefined, then letname be"Error"; else letname beToString(name).
  5. Letmsg be the result of calling the [[Get]] internal method ofO with argument"message".
  6. Ifmsg is undefined, then letmsg be the empty String; else letmsg beToString(msg).
  7. Ifmsg is undefined, then letmsg be the empty String; else letmsg beToString(msg).
  8. Ifname is the empty String, returnmsg.
  9. Ifmsg is the empty String, returnname.
  10. Return the result of concatenatingname,":", a single space character, andmsg.

15.11.5 Properties of Error Instances

Error instances inherit properties from the Error prototype object and their [[Class]] internal property value is"Error". Error instances have no special properties.

15.11.6 Native Error Types Used in This Standard

One of theNativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in15.11.7.

15.11.6.1 EvalError

This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.

15.11.6.2 RangeError

Indicates a numeric value has exceeded the allowable range. See15.4.2.2,15.4.5.1,15.7.4.2,15.7.4.5,15.7.4.6,15.7.4.7, and15.9.5.43.

15.11.6.3 ReferenceError

Indicate that an invalid reference value has been detected. See8.7.1,8.7.2,10.2.1,10.2.1.1.4,10.2.1.2.4, and11.13.1.

15.11.6.4 SyntaxError

Indicates that a parsing error has occurred. See11.1.5,11.3.1,11.3.2,11.4.1,11.4.4,11.4.5,11.13.1,11.13.2,12.2.1,12.10.1,12.14.1,13.1,15.1.2.1,15.3.2.1,15.10.2.2,15.10.2.5,15.10.2.9,15.10.2.15,15.10.2.19,15.10.4.1, and15.12.2.

15.11.6.5 TypeError

Indicates the actual type of an operand is different than the expected type. See8.6.2,8.7.2,8.10.5,8.12.5,8.12.7,8.12.8,8.12.9,9.9,9.10,10.2.1,10.2.1.1.3,10.6,11.2.2,11.2.3,11.4.1,11.8.6,11.8.7,11.3.1,13.2,13.2.3, 15,15.2.3.2,15.2.3.3,15.2.3.4,15.2.3.5,15.2.3.6,15.2.3.7,15.2.3.8,15.2.3.9,15.2.3.10,15.2.3.11,15.2.3.12,15.2.3.13,15.2.3.14,15.2.4.3,15.3.4.2,15.3.4.3,15.3.4.4,15.3.4.5,15.3.4.5.2,15.3.4.5.3,15.3.5,15.3.5.3,15.3.5.4,15.4.4.3,15.4.4.11,15.4.4.16,15.4.4.17,15.4.4.18,15.4.4.19,15.4.4.20,15.4.4.21,15.4.4.22,15.4.5.1,15.5.4.2,15.5.4.3,15.6.4.2,15.6.4.3,15.7.4,15.7.4.2,15.7.4.4,15.9.5,15.9.5.44,15.10.4.1,15.10.6,15.11.4.4 and15.12.3.

15.11.6.6 URIError

Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition. See15.1.3.

15.11.7NativeError Object Structure

When an ECMAScript implementation detects a runtime error, it throws an instance of one of theNativeError objects defined in15.11.6. Each of these objects has the structure described below, differing only in the name used as the constructor name instead ofNativeError, in thename property of the prototype object, and in the implementation-definedmessage property of the prototype object.

For each error object, references toNativeError in the definition should be replaced with the appropriate error object name from15.11.6.

15.11.7.1NativeError Constructors Called as Functions

When aNativeError constructor is called as a function rather than as a constructor, it creates and initialises a new object. A call of the object as a function is equivalent to calling it as a constructor with the same arguments.

15.11.7.2NativeError (message)

The [[Prototype]] internal property of the newly constructed object is set to the prototype object for this error constructor. The [[Class]] internal property of the newly constructed object is set to"Error". The [[Extensible]] internal property of the newly constructed object is set totrue.

If the argumentmessage is notundefined, themessage own property of the newly constructed object is set toToString(message).

15.11.7.3 TheNativeError Constructors

When aNativeError constructor is called as part of anew expression, it is a constructor: it initialises the newly created object.

15.11.7.4 newNativeError (message)

The [[Prototype]] internal property of the newly constructed object is set to the prototype object for thisNativeError constructor. The [[Class]] internal property of the newly constructed object is set to"Error". The [[Extensible]] internal property of the newly constructed object is set totrue.

If the argumentmessage is notundefined, themessage own property of the newly constructed object is set toToString(message).

15.11.7.5 Properties of theNativeError Constructors

The value of the [[Prototype]] internal property of aNativeError constructor is the Function prototype object (15.3.4).

Besides the internal properties and thelength property (whose value is1), eachNativeError constructor has the following property:

15.11.7.6NativeError.prototype

The initial value ofNativeError.prototype is aNativeError prototype object (15.11.7.7). EachNativeError constructor has a separate prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false }.

15.11.7.7 Properties of theNativeError Prototype Objects

EachNativeError prototype object is an Error object (its [[Class]] is"Error").

The value of the [[Prototype]] internal property of eachNativeError prototype object is the standard built-in Error prototype object (15.11.4).

15.11.7.8NativeError.prototype.constructor

The initial value of theconstructor property of the prototype for a givenNativeError constructor is theNativeError constructor function itself (15.11.7).

15.11.7.9NativeError.prototype.name

The initial value of thename property of the prototype for a givenNativeError constructor is the name of the constructor (the name used instead ofNativeError).

15.11.7.10NativeError.prototype.message

The initial value of themessage property of the prototype for a givenNativeError constructor is the empty String.

NOTE The prototypes for the NativeError constructors do not themselves provide atoString function, but instances of errors will inherit it from the Error prototype object.

15.11.7.11 Properties ofNativeError Instances

NativeError instances inherit properties from theirNativeError prototype object and their [[Class]] internal property value is"Error".NativeError instances have no special properties.

15.12 The JSON Object

TheJSON object is a single object that contains two functions,parse andstringify, that are used to parse and construct JSON texts. The JSON Data Interchange Format is described in RFC 4627 <http://www.ietf.org/rfc/rfc4627.txt>. The JSON interchange format used in this specification is exactly that described by RFC 4627 with two exceptions:

The value of the [[Prototype]] internal property of the JSON object is the standard built-in Object prototype object (15.2.4). The value of the [[Class]] internal property of the JSON object is"JSON". The value of the [[Extensible]] internal property of the JSON object is set totrue.

The JSON object does not have a [[Construct]] internal property; it is not possible to use the JSON object as a constructor with thenew operator.

The JSON object does not have a [[Call]] internal property; it is not possible to invoke the JSON object as a function.

15.12.1 The JSON Grammar

JSON.stringify produces a String that conforms to the following JSON grammar. JSON.parse accepts a String that conforms to the JSON grammar.

15.12.1.1 The JSON Lexical Grammar

JSON is similar to ECMAScript source text in that it consists of a sequence of characters conforming to the rules ofSourceCharacter. The JSON Lexical Grammar defines the tokens that make up a JSON text similar to the manner that the ECMAScript lexical grammar defines the tokens of an ECMAScript source text. The JSON Lexical grammar only recognises the white space character specified by the productionJSONWhiteSpace. The JSON lexical grammar shares some productions with the ECMAScript lexical grammar. All nonterminal symbols of the grammar that do not begin with the characters “JSON” are defined by productions of the ECMAScript lexical grammar.

Syntax

JSONWhiteSpace::

<TAB>
<CR>
<LF>
<SP>
JSONString::
"JSONStringCharactersopt"
JSONStringCharacters::
JSONStringCharacterJSONStringCharactersopt

JSONStringCharacter::

SourceCharacterbut not one of"or\or U+0000through U+001F
\ JSONEscapeSequence
JSONEscapeSequence::
JSONEscapeCharacter
UnicodeEscapeSequence
JSONEscapeCharacter::one of
"/\bfnrt
JSONNumber::
-optDecimalIntegerLiteralJSONFractionoptExponentPartopt
JSONFraction::
.DecimalDigits
JSONNullLiteral::
NullLiteral
JSONBooleanLiteral::
BooleanLiteral

15.12.1.2 The JSON Syntactic Grammar

The JSON Syntactic Grammar defines a valid JSON text in terms of tokens defined by the JSON lexical grammar. The goal symbol of the grammar isJSONText.

Syntax

JSONText:
JSONValue
JSONValue:
JSONNullLiteral
JSONBooleanLiteral
JSONObject
JSONArray
JSONString
JSONNumber
JSONObject:
{}
{JSONMemberList}
JSONMember:
JSONString:JSONValue
JSONMemberList:
JSONMember
JSONMemberList,JSONMember
JSONArray:
[]
[JSONElementList]
JSONElementList:
JSONValue
JSONElementList,JSONValue

15.12.2 parse ( text [ , reviver ] )

Theparse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format is a restricted form of ECMAScript literal. JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript arrays. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, andnull. JSON uses a more limited set of white space characters thanWhiteSpace and allows Unicode code points U+2028 and U+2029 to directly appear inJSONString literals without using an escape sequence. The process of parsing is similar to11.1.4 and11.1.5 as constrained by the JSON grammar.

The optionalreviver parameter is a function that takes two parameters, (key andvalue). It can filter and transform the results. It is called with each of thekey/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returnsundefined then the property is deleted from the result.

  1. LetJText beToString(text).
  2. ParseJText using the grammars in15.12.1. Throw aSyntaxError exception ifJText did not conform to the JSON grammar for the goal symbolJSONText.
  3. Letunfiltered be the result of parsing and evaluatingJText as if it was the source text of an ECMAScriptProgram but usingJSONString in place ofStringLiteral. Note that sinceJText conforms to the JSON grammar this result will be either a primitive value or an object that is defined by either anArrayLiteral or anObjectLiteral.
  4. IfIsCallable(reviver) istrue, then
    1. Letroot be a new object created as if by the expressionnew Object(), whereObject is the standard built-in constructor with that name.
    2. Call the [[DefineOwnProperty]] internal method ofroot with the empty String, the PropertyDescriptor {[[Value]]:unfiltered, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse as arguments.
    3. Return the result of calling the abstract operation Walk, passingroot and the empty String. The abstract operation Walk is described below.
  5. Else
    1. Returnunfiltered.

The abstract operation Walk is a recursive abstract operation that takes two parameters: aholder object and the Stringname of a property in that object. Walk uses the value ofreviver that was originally passed to the above parse function.

  1. Letval be the result of calling the [[Get]] internal method ofholder with argumentname.
  2. Ifval is an object, then
    1. If the [[Class]] internal property ofval is"Array"
      1. SetI to 0.
      2. Letlen be the result of calling the [[Get]] internal method ofval with argument"length".
      3. Repeat whileI <len,
        1. LetnewElement be the result of calling the abstract operation Walk, passingval andToString(I).
        2. IfnewElement isundefined, then
          1. Call the [[Delete]] internal method ofval withToString(I) andfalse as arguments.
        3. Else
          1. Call the [[DefineOwnProperty]] internal method ofval with argumentsToString(I), theProperty Descriptor {[[Value]]:newElement, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, andfalse.
        4. Add 1 toI.
    2. Else
      1. Letkeys be an internalList of String values consisting of the names of all the own properties ofval whose [[Enumerable]] attribute istrue. The ordering of the Strings should be the same as that used by theObject.keys standard built-in function.
      2. For each StringP inkeys do,
        1. LetnewElement be the result of calling the abstract operation Walk, passingval andP.
        2. IfnewElement isundefined, then
          1. Call the [[Delete]] internal method ofval withP andfalse as arguments.
        3. Else
          1. Call the [[DefineOwnProperty]] internal method ofval with argumentsP, theProperty Descriptor {[[Value]]:newElement, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  3. Return the result of calling the [[Call]] internal method ofreviver passingholder as thethis value and with an argument list consisting ofname andval.

It is not permitted for a conforming implementation ofJSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.

NOTE In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.

15.12.3 stringify ( value [ , replacer [ , space ] ] )

Thestringify function returns a String in JSON format representing an ECMAScript value. It can take three parameters. The first parameter is required. Thevalue parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number ornull. The optionalreplacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as a white list for selecting the object properties that will be stringified. The optionalspace parameter is a String or Number that allows the result to have white space injected into it to improve human readability.

These are the steps in stringifying an object:

  1. Letstack be an emptyList.
  2. Letindent be the empty String.
  3. LetPropertyList andReplacerFunction beundefined.
  4. IfType(replacer) is Object, then
    1. IfIsCallable(replacer) istrue, then
      1. LetReplacerFunction bereplacer.
    2. Else if the [[Class]] internal property ofreplacer is"Array", then
      1. LetPropertyList be an empty internalList
      2. For each valuev of a property ofreplacer that has an array index property name. The properties are enumerated in the ascending array index order of their names.
        1. Letitem beundefined.
        2. IfType(v) is String then letitem bev.
        3. Else ifType(v) is Number then letitem beToString(v).
        4. Else ifType(v) is Object then,
          1. If the [[Class]] internal property ofv is"String" or"Number" then letitem beToString(v).
        5. Ifitem is not undefined anditem is not currently an element ofPropertyList then,
          1. Appenditem to the end ofPropertyList.
  5. IfType(space) is Object then,
    1. If the [[Class]] internal property ofspace is"Number" then,
      1. Letspace beToNumber(space).
    2. Else if the [[Class]] internal property ofspace is"String" then,
      1. Letspace beToString(space).
  6. IfType(space) is Number
    1. Letspace be min(10,ToInteger(space)).
    2. Setgap to a String containingspace space characters. This will be the empty String ifspace is less than 1.
  7. Else ifType(space) is String
    1. If the number of characters inspace is 10 or less, setgap tospace otherwise setgap to a String consisting of the first 10 characters ofspace.
  8. Else
    1. Setgap to the empty String.
  9. Letwrapper be a new object created as if by the expressionnew Object(), whereObject is the standard built-in constructor with that name.
  10. Call the [[DefineOwnProperty]] internal method ofwrapper with arguments the empty String, theProperty Descriptor {[[Value]]:value, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}, andfalse.
  11. Return the result of calling the abstract operationStr with the empty String andwrapper.

The abstract operationStr(key,holder) has access toReplacerFunction from the invocation of thestringify method. Its algorithm is as follows:

  1. Letvalue be the result of calling the [[Get]] internal method ofholder with argumentkey.
  2. IfType(value) is Object, then
    1. LettoJSON be the result of calling the [[Get]] internal method ofvalue with argument"toJSON".
    2. IfIsCallable(toJSON) istrue
      1. Letvalue be the result of calling the [[Call]] internal method oftoJSON passingvalue as thethis value and with an argument list consisting ofkey.
  3. IfReplacerFunction is notundefined, then
    1. Letvalue be the result of calling the [[Call]] internal method ofReplacerFunction passingholder as thethis value and with an argument list consisting ofkey andvalue.
  4. IfType(value) is Object then,
    1. If the [[Class]] internal property ofvalue is"Number" then,
      1. Letvalue beToNumber(value).
    2. Else if the [[Class]] internal property ofvalue is"String" then,
      1. Letvalue beToString(value).
    3. Else if the [[Class]] internal property ofvalue is"Boolean" then,
      1. Letvalue be the value of the [[PrimitiveValue]] internal property ofvalue.
  5. Ifvalue isnull then return"null".
  6. Ifvalue istrue then return"true".
  7. Ifvalue isfalse then return"false".
  8. IfType(value) is String, then return the result of calling the abstract operationQuote with argumentvalue.
  9. IfType(value) is Number
    1. Ifvalue is finite then returnToString(value).
    2. Else, return"null".
  10. IfType(value) is Object, andIsCallable(value) isfalse
    1. If the [[Class]] internal property ofvalue is"Array" then
      1. Return the result of calling the abstract operationJA with argumentvalue.
    2. Else, return the result of calling the abstract operationJO with argumentvalue.
  11. Returnundefined.

The abstract operationQuote(value) wraps a String value in double quotes and escapes characters within it.

  1. Letproduct be the double quote character.
  2. For each characterC invalue
    1. IfC is the double quote character or the backslash character
      1. Letproduct be the concatenation ofproduct and the backslash character.
      2. Letproduct be the concatenation ofproduct andC.
    2. Else ifC is backspace, formfeed, newline, carriage return, or tab
      1. Letproduct be the concatenation ofproduct and the backslash character.
      2. Letabbrev be the character corresponding to the value ofC as follows:
        backspace"b"
        formfeed"f"
        newline"n"
        carriage return"r"
        tab"t"
      3. Letproduct be the concatenation ofproduct andabbrev.
    3. Else ifC is a control character having a code unit value less than the space character
      1. Letproduct be the concatenation ofproduct and the backslash character.
      2. Letproduct be the concatenation ofproduct and"u".
      3. Lethex be the result of converting the numeric code unit value ofC to a String of four hexadecimal digits.
      4. Letproduct be the concatenation ofproduct andhex.
    4. Else
      1. Letproduct be the concatenation ofproduct andC.
  3. Letproduct be the concatenation ofproduct and the double quote character.
  4. Returnproduct.

The abstract operationJO(value) serializes an object. It has access to thestack,indent,gap,PropertyList,ReplacerFunction, andspace of the invocation of the stringify method.

  1. Ifstack containsvalue then throw aTypeError exception because the structure is cyclical.
  2. Appendvalue tostack.
  3. Letstepback beindent.
  4. Letindent be the concatenation ofindent andgap.
  5. IfPropertyList is notundefined, then
    1. LetK bePropertyList.
  6. Else
    1. LetK be an internalList of Strings consisting of the names of all the own properties ofvalue whose [[Enumerable]] attribute istrue. The ordering of the Strings should be the same as that used by theObject.keys standard built-in function.
  7. Letpartial be an emptyList.
  8. For each elementP ofK.
    1. LetstrP be the result of calling the abstract operationStr with argumentsP andvalue.
    2. IfstrP is notundefined
      1. Letmember be the result of calling the abstract operationQuote with argumentP.
      2. Letmember be the concatenation ofmember and the colon character.
      3. Ifgap is not the empty String
        1. Letmember be the concatenation ofmember and thespace character.
      4. Letmember be the concatenation ofmember andstrP.
      5. Appendmember topartial.
  9. Ifpartial is empty, then
    1. Letfinal be"{}".
  10. Else
    1. Ifgap is the empty String
      1. Letproperties be a String formed by concatenating all the element Strings ofpartial with each adjacent pair of Strings separated with the comma character. A comma is not inserted either before the first String or after the last String.
      2. Letfinal be the result of concatenating"{",properties, and"}".
    2. Elsegap is not the empty String
      1. Letseparator be the result of concatenating the comma character, the line feed character, andindent.
      2. Letproperties be a String formed by concatenating all the element Strings ofpartial with each adjacent pair of Strings separated withseparator. Theseparator String is not inserted either before the first String or after the last String.
      3. Letfinal be the result of concatenating"{", the line feed character,indent,properties, the line feed character,stepback, and"}".
  11. Remove the last element ofstack.
  12. Letindent bestepback.
  13. Returnfinal.

The abstract operationJA(value) serializes an array. It has access to thestack,indent,gap, andspace of the invocation of the stringify method. The representation of arrays includes only the elements between zero andarray.length– 1 inclusive. Named properties are excluded from the stringification. An array is stringified as an open left bracket, elements separated by comma, and a closing right bracket.

  1. Ifstack containsvalue then throw aTypeError exception because the structure is cyclical.
  2. Appendvalue tostack.
  3. Letstepback beindent.
  4. Letindent be the concatenation ofindent andgap.
  5. Letpartial be an emptyList.
  6. Letlen be the result of calling the [[Get]] internal method of value with argument"length".
  7. Letindex be 0.
  8. Repeat whileindex <len
    1. LetstrP be the result of calling the abstract operationStr with argumentsToString(index) andvalue.
    2. IfstrP isundefined
      1. Append"null" topartial.
    3. Else
      1. AppendstrP topartial.
    4. Incrementindex by 1.
  9. Ifpartial is empty ,then
    1. Letfinal be"[]".
  10. Else
    1. Ifgap is the empty String
      1. Letproperties be a String formed by concatenating all the element Strings ofpartial with each adjacent pair of Strings separated with the comma character. A comma is not inserted either before the first String or after the last String.
      2. Letfinal be the result of concatenating"[",properties, and"]".
    2. Else
      1. Letseparator be the result of concatenating the comma character, the line feed character, andindent.
      2. Letproperties be a String formed by concatenating all the element Strings ofpartial with each adjacent pair of Strings separated withseparator. Theseparator String is not inserted either before the first String or after the last String.
      3. Letfinal be the result of concatenating"[", the line feed character,indent,properties, the line feed character,stepback, and"]".
  11. Remove the last element ofstack.
  12. Letindent bestepback.
  13. Returnfinal.

NOTE 1 JSON structures are allowed to be nested to any depth, but they must be acyclic. Ifvalue is or contains a cyclic structure, then the stringify function must throw aTypeError exception. This is an example of a value that cannot be stringified:

a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw an TypeError.

NOTE 2 Symbolic primitive values are rendered as follows:

  • Thenull value is rendered in JSON text as the Stringnull.
  • Theundefined value is not rendered.
  • Thetrue value is rendered in JSON text as the Stringtrue.
  • Thefalse value is rendered in JSON text as the Stringfalse.

NOTE 3 String values are wrapped in double quotes. The characters" and\ are escaped with\ prefixes. Control characters are replaced with escape sequences\uHHHH, or with the shorter forms,\b (backspace),\f (formfeed),\n (newline),\r (carriage return),\t (tab).

NOTE 4 Finite numbers are stringified as if by callingToString(number).NaN and Infinity regardless of sign are represented as the Stringnull.

NOTE 5 Values that do not have a JSON representation (such asundefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the Stringnull. In objects an unrepresentable value causes the property to be excluded from stringification.

NOTE 6 An object is rendered as an opening left brace followed by zero or more properties, separated with commas, closed with a right brace. A property is a quoted String representing the key or property name, a colon, and then the stringified property value. An array is rendered as an opening left bracket followed by zero or more values, separated with commas, closed with a right bracket.

16 Errors

An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. Anearly error is an error that can be detected and reported prior to the evaluation of any construct in theProgram containing the error. An implementation must report early errors in aProgram prior to the first evaluation of thatProgram. Early errors ineval code are reported at the timeeval is called but prior to evaluation of any construct within theeval code. All errors that are not early errors are runtime errors.

An implementation must treat any instance of the following kinds of errors as an early error:

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.

An implementation shall report all errors as specified, except for the following:

Annex A(informative) Grammar Summary

A.1 Lexical Grammar

See clause 6
SourceCharacter::
any Unicode code unit
See clause 7
InputElementDiv::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
See clause 7
InputElementRegExp::
WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral
See 7.2
WhiteSpace::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<BOM>
<USP>
See 7.3
LineTerminator::
<LF>
<CR>
<LS>
<PS>
See 7.3
LineTerminatorSequence::
<LF>
<CR>[lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>
See 7.4
Comment::
MultiLineComment
SingleLineComment
See 7.4
MultiLineComment::
/*MultiLineCommentCharsopt*/
See 7.4
MultiLineCommentChars::
MultiLineNotAsteriskCharMultiLineCommentCharsopt
*PostAsteriskCommentCharsopt
See 7.4
PostAsteriskCommentChars::
MultiLineNotForwardSlashOrAsteriskCharMultiLineCommentCharsopt
*PostAsteriskCommentCharsopt
See 7.4
MultiLineNotAsteriskChar::
SourceCharacterbut not*
See 7.4
MultiLineNotForwardSlashOrAsteriskChar::
SourceCharacterbut not one of/or*
See 7.4
SingleLineComment::
//SingleLineCommentCharsopt
See 7.4
SingleLineCommentChars::
SingleLineCommentCharSingleLineCommentCharsopt
See 7.4
SingleLineCommentChar::
SourceCharacterbut notLineTerminator
See 7.5
Token::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
See 7.6
Identifier::
IdentifierNamebut notReservedWord
See 7.6
IdentifierName::
IdentifierStart
IdentifierNameIdentifierPart
See 7.6
IdentifierStart::
UnicodeLetter
$
_
\UnicodeEscapeSequence
See 7.6
IdentifierPart::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
<ZWNJ>
<ZWJ>
See 7.6
UnicodeLetter::
any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.
See 7.6
UnicodeCombiningMark::
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”
See 7.6
UnicodeDigit::
any character in the Unicode category “Decimal number (Nd)”
See 7.6
UnicodeConnectorPunctuation::
any character in the Unicode category “Connector punctuation (Pc)”
See 7.6.1
ReservedWord::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
See 7.6.1.1
Keyword::one of
breakdoinstanceoftypeof
caseelsenewvar
catchfinallyreturnvoid
continueforswitchwhile
debuggerfunctionthiswith
defaultifthrow
deleteintry
See 7.6.1.2
FutureReservedWord::one of
classenumextendssuper
constexportimport

The following tokens are also considered to beFutureReservedWords when parsingstrict mode code (see 10.1.1).

implementsletprivatepublic
interfacepackageprotectedstatic
yield
See 7.7
Punctuator::one of
{}()[]
.;,<><=
>===!====!==
+-*%++--
<<>>>>>&|^
!~&&||?:
=+=-=*=%=<<=
>>=>>>=&=|=^=
See 7.7
DivPunctuator::one of
//=
See 7.8
Literal::
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral
See 7.8.1
NullLiteral::
null
See 7.8.2
BooleanLiteral::
true
false
See 7.8.3
NumericLiteral::
DecimalLiteral
HexIntegerLiteral
See 7.8.3
DecimalLiteral::
DecimalIntegerLiteral.DecimalDigitsoptExponentPartopt
.DecimalDigitsExponentPartopt
DecimalIntegerLiteralExponentPartopt
See 7.8.3
DecimalIntegerLiteral::
0
NonZeroDigitDecimalDigitsopt
See 7.8.3
DecimalDigits::
DecimalDigit
DecimalDigitsDecimalDigit
See 7.8.3
DecimalDigit::one of
0123456789
See 7.8.3
NonZeroDigit::one of
123456789
See 7.8.3
ExponentPart::
ExponentIndicatorSignedInteger
See 7.8.3
ExponentIndicator::one of
eE
See 7.8.3
SignedInteger::
DecimalDigits
+DecimalDigits
-DecimalDigits
See 7.8.3
HexIntegerLiteral::
0xHexDigit
0XHexDigit
HexIntegerLiteralHexDigit
See 7.8.3
HexDigit::one of
0123456789abcdefABCDEF
See 7.8.4
StringLiteral::
"DoubleStringCharactersopt"
'SingleStringCharactersopt'
See 7.8.4
DoubleStringCharacters::
DoubleStringCharacterDoubleStringCharactersopt
See 7.8.4
SingleStringCharacters::
SingleStringCharacterSingleStringCharactersopt
See 7.8.4
DoubleStringCharacter::
SourceCharacterbut not one of"or\orLineTerminator
\EscapeSequence
LineContinuation
See 7.8.4
SingleStringCharacter::
SourceCharacterbut not one of'or\orLineTerminator
\EscapeSequence
LineContinuation
See 7.8.4
LineContinuation::
\LineTerminatorSequence
See 7.8.4
EscapeSequence::
CharacterEscapeSequence
0[lookahead ∉DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence
See 7.8.4
CharacterEscapeSequence::
SingleEscapeCharacter
NonEscapeCharacter
See 7.8.4
SingleEscapeCharacter::one of
'"\bfnrtv
See 7.8.4
NonEscapeCharacter::
SourceCharacterbut not one ofEscapeCharacterorLineTerminator
See 7.8.4
EscapeCharacter::
SingleEscapeCharacter
DecimalDigit
x
u
See 7.8.4
HexEscapeSequence::
xHexDigitHexDigit
See 7.8.4
UnicodeEscapeSequence::
uHexDigitHexDigitHexDigitHexDigit
See 7.8.5
RegularExpressionLiteral::
/RegularExpressionBody/RegularExpressionFlags
See 7.8.5
RegularExpressionBody::
RegularExpressionFirstCharRegularExpressionChars
See 7.8.5
RegularExpressionChars::
[empty]
RegularExpressionCharsRegularExpressionChar
See 7.8.5
RegularExpressionFirstChar::
RegularExpressionNonTerminatorbut not one of*or\or/or[
RegularExpressionBackslashSequence
RegularExpressionClass
See 7.8.5
RegularExpressionChar::
RegularExpressionNonTerminatorbut not\or/or[
RegularExpressionBackslashSequence
RegularExpressionClass
See 7.8.5
RegularExpressionBackslashSequence::
\RegularExpressionNonTerminator
See 7.8.5
RegularExpressionNonTerminator::
SourceCharacterbut notLineTerminator
See 7.8.5
RegularExpressionClass::
[RegularExpressionClassChars]
See 7.8.5
RegularExpressionClassChars::
[empty]
RegularExpressionClassCharsRegularExpressionClassChar
See 7.8.5
RegularExpressionClassChar::
RegularExpressionNonTerminatorbut not]or\
RegularExpressionBackslashSequence
See 7.8.5
RegularExpressionFlags::
[empty]
RegularExpressionFlagsIdentifierPart

A.2 Number Conversions

See 9.3.1
StringNumericLiteral:::
StrWhiteSpaceopt
StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt
See 9.3.1
StrWhiteSpace:::
StrWhiteSpaceCharStrWhiteSpaceopt
See 9.3.1
StrWhiteSpaceChar:::
WhiteSpace
LineTerminator
See 9.3.1
StrNumericLiteral:::
StrDecimalLiteral
HexIntegerLiteral
See 9.3.1
StrDecimalLiteral:::
StrUnsignedDecimalLiteral
+StrUnsignedDecimalLiteral
-StrUnsignedDecimalLiteral
See 9.3.1
StrUnsignedDecimalLiteral:::
Infinity
DecimalDigits.DecimalDigitsoptExponentPartopt
.DecimalDigitsExponentPartopt
DecimalDigitsExponentPartopt
See 9.3.1
DecimalDigits:::
DecimalDigit
DecimalDigitsDecimalDigit
See 9.3.1
DecimalDigit:::one of
0123456789
See 9.3.1
ExponentPart:::
ExponentIndicatorSignedInteger
See 9.3.1
ExponentIndicator:::one of
eE
See 9.3.1
SignedInteger:::
DecimalDigits
+DecimalDigits
-DecimalDigits
See 9.3.1
HexIntegerLiteral:::
0xHexDigit
0XHexDigit
HexIntegerLiteralHexDigit
See 9.3.1
HexDigit:::one of
0123456789abcdefABCDEF

A.3 Expressions

See 11.1
PrimaryExpression:
this
Identifier
Literal
ArrayLiteral
ObjectLiteral
(Expression)
See 11.1.4
ArrayLiteral:
[Elisionopt]
[ElementList]
[ElementList,Elisionopt]
See 11.1.4
ElementList:
ElisionoptAssignmentExpression
ElementList,ElisionoptAssignmentExpression
See 11.1.4
Elision:
,
Elision,
See 11.1.5
ObjectLiteral:
{}
{PropertyNameAndValueList}
{PropertyNameAndValueList,}
See 11.1.5
PropertyNameAndValueList:
PropertyAssignment
PropertyNameAndValueList,PropertyAssignment
See 11.1.5
PropertyAssignment:
PropertyName:AssignmentExpression
getPropertyName(){FunctionBody}
setPropertyName(PropertySetParameterList){FunctionBody}
See 11.1.5
PropertyName:
IdentifierName
StringLiteral
NumericLiteral
See 11.1.5
PropertySetParameterList:
Identifier
See 11.2
MemberExpression:
PrimaryExpression
FunctionExpression
MemberExpression[Expression]
MemberExpression.IdentifierName
newMemberExpressionArguments
See 11.2
NewExpression:
MemberExpression
newNewExpression
See 11.2
CallExpression:
MemberExpressionArguments
CallExpressionArguments
CallExpression[Expression]
CallExpression.IdentifierName
See 11.2
Arguments:
()
(ArgumentList)
See 11.2
ArgumentList:
AssignmentExpression
ArgumentList,AssignmentExpression
See 11.2
LeftHandSideExpression:
NewExpression
CallExpression
See 11.3
PostfixExpression:
LeftHandSideExpression
LeftHandSideExpression[noLineTerminator here]++
LeftHandSideExpression[noLineTerminator here]--
See 11.4
UnaryExpression:
PostfixExpression
deleteUnaryExpression
voidUnaryExpression
typeofUnaryExpression
++UnaryExpression
--UnaryExpression
+UnaryExpression
-UnaryExpression
~UnaryExpression
!UnaryExpression
See 11.5
MultiplicativeExpression:
UnaryExpression
MultiplicativeExpression*UnaryExpression
MultiplicativeExpression/UnaryExpression
MultiplicativeExpression%UnaryExpression
See 11.6
AdditiveExpression:
MultiplicativeExpression
AdditiveExpression+MultiplicativeExpression
AdditiveExpression-MultiplicativeExpression
See 11.7
ShiftExpression:
AdditiveExpression
ShiftExpression<<AdditiveExpression
ShiftExpression>>AdditiveExpression
ShiftExpression>>>AdditiveExpression
See 11.8
RelationalExpression:
ShiftExpression
RelationalExpression<ShiftExpression
RelationalExpression>ShiftExpression
RelationalExpression<=ShiftExpression
RelationalExpression>=ShiftExpression
RelationalExpressioninstanceofShiftExpression
RelationalExpressioninShiftExpression
See 11.8
RelationalExpressionNoIn:
ShiftExpression
RelationalExpressionNoIn<ShiftExpression
RelationalExpressionNoIn>ShiftExpression
RelationalExpressionNoIn<=ShiftExpression
RelationalExpressionNoIn>=ShiftExpression
RelationalExpressionNoIninstanceofShiftExpression
See 11.9
EqualityExpression:
RelationalExpression
EqualityExpression==RelationalExpression
EqualityExpression!=RelationalExpression
EqualityExpression===RelationalExpression
EqualityExpression!==RelationalExpression
See 11.9
EqualityExpressionNoIn:
RelationalExpressionNoIn
EqualityExpressionNoIn==RelationalExpressionNoIn
EqualityExpressionNoIn!=RelationalExpressionNoIn
EqualityExpressionNoIn===RelationalExpressionNoIn
EqualityExpressionNoIn!==RelationalExpressionNoIn
See 11.10
BitwiseANDExpression:
EqualityExpression
BitwiseANDExpression&EqualityExpression
See 11.10
BitwiseANDExpressionNoIn:
EqualityExpressionNoIn
BitwiseANDExpressionNoIn&EqualityExpressionNoIn
See 11.10
BitwiseXORExpression:
BitwiseANDExpression
BitwiseXORExpression^BitwiseANDExpression
See 11.10
BitwiseXORExpressionNoIn:
BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn^BitwiseANDExpressionNoIn
See 11.10
BitwiseORExpression:
BitwiseXORExpression
BitwiseORExpression|BitwiseXORExpression
See 11.10
BitwiseORExpressionNoIn:
BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn|BitwiseXORExpressionNoIn
See 11.11
LogicalANDExpression:
BitwiseORExpression
LogicalANDExpression&&BitwiseORExpression
See 11.11
LogicalANDExpressionNoIn:
BitwiseORExpressionNoIn
LogicalANDExpressionNoIn&&BitwiseORExpressionNoIn
See 11.11
LogicalORExpression:
LogicalANDExpression
LogicalORExpression||LogicalANDExpression
See 11.11
LogicalORExpressionNoIn:
LogicalANDExpressionNoIn
LogicalORExpressionNoIn||LogicalANDExpressionNoIn
See 11.12
ConditionalExpression:
LogicalORExpression
LogicalORExpression?AssignmentExpression:AssignmentExpression
See 11.12
ConditionalExpressionNoIn:
LogicalORExpressionNoIn
LogicalORExpressionNoIn?AssignmentExpression:AssignmentExpressionNoIn
See 11.13
AssignmentExpression:
ConditionalExpression
LeftHandSideExpression=AssignmentExpression
LeftHandSideExpressionAssignmentOperatorAssignmentExpression
See 11.13
AssignmentExpressionNoIn:
ConditionalExpressionNoIn
LeftHandSideExpression=AssignmentExpressionNoIn
LeftHandSideExpressionAssignmentOperatorAssignmentExpressionNoIn
See 11.13
AssignmentOperator:one of
*=/=%=+=-=<<=>>=>>>=&=^=|=
See 11.14
Expression:
AssignmentExpression
Expression,AssignmentExpression
See 11.14
ExpressionNoIn:
AssignmentExpressionNoIn
ExpressionNoIn,AssignmentExpressionNoIn

A.4 Statements

See clause 12
Statement:
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement
See 12.1
Block:
{StatementListopt}
See 12.1
StatementList:
Statement
StatementListStatement
See 12.2
VariableStatement:
varVariableDeclarationList;
See 12.2
VariableDeclarationList:
VariableDeclaration
VariableDeclarationList,VariableDeclaration
See 12.2
VariableDeclarationListNoIn:
VariableDeclarationNoIn
VariableDeclarationListNoIn,VariableDeclarationNoIn
See 12.2
VariableDeclaration:
IdentifierInitialiseropt
See 12.2
VariableDeclarationNoIn:
IdentifierInitialiserNoInopt
See 12.2
Initialiser:
=AssignmentExpression
See 12.2
InitialiserNoIn:
=AssignmentExpressionNoIn
See 12.3
EmptyStatement:
;
See 12.4
ExpressionStatement:
[lookahead ∉ {{,function}]Expression;
See 12.5
IfStatement:
if(Expression)StatementelseStatement
if(Expression)Statement
See 12.6
IterationStatement:
doStatementwhile(Expression);
while(Expression)Statement
for(ExpressionNoInopt;Expressionopt;Expressionopt)Statement
for(varVariableDeclarationListNoIn;Expressionopt;Expressionopt)Statement
for(LeftHandSideExpressioninExpression)Statement
for(varVariableDeclarationNoIninExpression)Statement
See 12.7
ContinueStatement:
continue;
continue[noLineTerminator here]Identifier;
See 12.8
BreakStatement:
break;
break[noLineTerminator here]Identifier;
See 12.9
ReturnStatement:
return;
return[noLineTerminator here]Expression;
See 12.10
WithStatement:
with(Expression)Statement
See 12.11
SwitchStatement:
switch(Expression)CaseBlock
See 12.11
CaseBlock:
{CaseClausesopt}
{CaseClausesoptDefaultClauseCaseClausesopt}
See 12.11
CaseClauses:
CaseClause
CaseClausesCaseClause
See 12.11
CaseClause:
caseExpression:StatementListopt
See 12.11
DefaultClause:
default:StatementListopt
See 12.12
LabelledStatement:
Identifier:Statement
See 12.13
ThrowStatement:
throw[noLineTerminator here]Expression;
See 12.14
TryStatement:
tryBlockCatch
tryBlockFinally
tryBlockCatchFinally
See 12.14
Catch:
catch(Identifier)Block
See 12.14
Finally:
finallyBlock
See 12.15
DebuggerStatement:
debugger;

A.5 Functions and Programs

See clause 13
FunctionDeclaration:
functionIdentifier(FormalParameterListopt){FunctionBody}
See clause 13
FunctionExpression:
functionIdentifieropt(FormalParameterListopt){FunctionBody}
See clause 13
FormalParameterList:
Identifier
FormalParameterList,Identifier
See clause 13
FunctionBody:
SourceElementsopt
See clause 14
Program:
SourceElementsopt
See clause 14
SourceElements:
SourceElement
SourceElementsSourceElement
See clause 14
SourceElement:
Statement
FunctionDeclaration

A.6 Universal Resource Identifier Character Classes

See 15.1.3
uri:::
uriCharactersopt
See 15.1.3
uriCharacters:::
uriCharacteruriCharactersopt
See 15.1.3
uriCharacter:::
uriReserved
uriUnescaped
uriEscaped
See 15.1.3
uriReserved:::one of
;/?:@&=+$,
See 15.1.3
uriUnescaped:::
uriAlpha
DecimalDigit
uriMark
See 15.1.3
uriEscaped:::
%HexDigitHexDigit
See 15.1.3
uriAlpha:::one of
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
See 15.1.3
uriMark:::one of
-_.!~*'()

A.7 Regular Expressions

See 15.10.1
Pattern::
Disjunction
See 15.10.1
Disjunction::
Alternative
Alternative|Disjunction
See 15.10.1
Alternative::
[empty]
AlternativeTerm
See 15.10.1
Term::
Assertion
Atom
AtomQuantifier
See 15.10.1
Assertion::
^
$
\b
\B
(?=Disjunction)
(?!Disjunction)
See 15.10.1
Quantifier::
QuantifierPrefix
QuantifierPrefix?
See 15.10.1
QuantifierPrefix::
*
+
?
{DecimalDigits}
{DecimalDigits,}
{DecimalDigits,DecimalDigits}
See 15.10.1
Atom::
PatternCharacter
.
\AtomEscape
CharacterClass
(Disjunction)
(?:Disjunction)
See 15.10.1
PatternCharacter::
SourceCharacterbut not one of-
^$\.*+?()[]{}|
See 15.10.1
AtomEscape::
DecimalEscape
CharacterEscape
CharacterClassEscape
See 15.10.1
CharacterEscape::
ControlEscape
cControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscape
See 15.10.1
ControlEscape::one of
fnrtv
See 15.10.1
ControlLetter::one of
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
See 15.10.1
IdentityEscape::
SourceCharacterbut notIdentifierPart
<ZWJ>
<ZWNJ>
See 15.10.1
DecimalEscape::
DecimalIntegerLiteral[lookahead ∉DecimalDigit]
See 15.10.1
CharacterClassEscape::one of
dDsSwW
See 15.10.1
CharacterClass::
[[lookahead ∉ {^}]ClassRanges]
[^ClassRanges]
See 15.10.1
ClassRanges::
[empty]
NonemptyClassRanges
See 15.10.1
NonemptyClassRanges::
ClassAtom
ClassAtomNonemptyClassRangesNoDash
ClassAtom–ClassAtomClassRanges
See 15.10.1
NonemptyClassRangesNoDash::
ClassAtom
ClassAtomNoDashNonemptyClassRangesNoDash
ClassAtomNoDash–ClassAtomClassRanges
See 15.10.1
ClassAtom::
-
ClassAtomNoDash
See 15.10.1
ClassAtomNoDash::
SourceCharacterbut not one of\or]or-
\ClassEscape
See 15.10.1
ClassEscape::
DecimalEscape
b
CharacterEscape
CharacterClassEscape

A.8 JSON

A.8.1 JSON Lexical Grammar

See 15.12.1.1
JSONWhiteSpace::
<TAB>
<CR>
<LF>
<SP>
See 15.12.1.1
JSONString::
"JSONStringCharactersopt"
See 15.12.1.1
JSONStringCharacters::
JSONStringCharacterJSONStringCharactersopt
See 15.12.1.1
JSONStringCharacter::
SourceCharacterbut not one of"or\orU+0000 through U+001F
\JSONEscapeSequence
See 15.12.1.1
JSONEscapeSequence::
JSONEscapeCharacter
UnicodeEscapeSequence
See 15.12.1.1
JSONEscapeCharacter::one of
"/\bfnrt
See 15.12.1.1
JSONNumber::
-optDecimalIntegerLiteralJSONFractionoptExponentPartopt
See 15.12.1.1
JSONFraction::
.DecimalDigits
See 15.12.1.1
JSONNullLiteral::
NullLiteral
See 15.12.1.1
JSONBooleanLiteral::
BooleanLiteral

A.8.2 JSON Syntactic Grammar

See 15.12.1.2
JSONText:
JSONValue
See 15.12.1.2
JSONValue:
JSONNullLiteral
JSONBooleanLiteral
JSONObject
JSONArray
JSONString
JSONNumber
See 15.12.1.2
JSONObject:
{}
{JSONMemberList}
See 15.12.1.2
JSONMember:
JSONString:JSONValue
See 15.12.1.2
JSONMemberList:
JSONMember
JSONMemberList,JSONMember
See 15.12.1.2
JSONArray:
[]
[JSONElementList]
See 15.12.1.2
JSONElementList:
JSONValue
JSONElementList,JSONValue

Annex B(informative) Compatibility

B.1 Additional Syntax

Past editions of ECMAScript have included additional syntax and semantics for specifying octal literals and octal escape sequences. These have been removed from this edition of ECMAScript. This non-normative annex presents uniform syntax and semantics for octal literals and octal escape sequences for compatibility with some older ECMAScript programs.

B.1.1 Numeric Literals

The syntax and semantics of7.8.3 can be extended as follows except that this extension is not allowed forstrict mode code:

Syntax

NumericLiteral::
DecimalLiteral
HexIntegerLiteral
OctalIntegerLiteral
OctalIntegerLiteral::
0OctalDigit
OctalIntegerLiteralOctalDigit
OctalDigit::one of
01234567

Semantics

  • The MV ofNumericLiteral::OctalIntegerLiteral is the MV ofOctalIntegerLiteral.
  • The MV ofOctalDigit::0 is 0.
  • The MV ofOctalDigit::1 is 1.
  • The MV ofOctalDigit::2 is 2.
  • The MV ofOctalDigit::3 is 3.
  • The MV ofOctalDigit::4 is 4.
  • The MV ofOctalDigit::5 is 5.
  • The MV ofOctalDigit::6 is 6.
  • The MV ofOctalDigit::7 is 7.
  • The MV ofOctalIntegerLiteral::0OctalDigit is the MV ofOctalDigit.
  • The MV ofOctalIntegerLiteral::OctalIntegerLiteralOctalDigit is (the MV ofOctalIntegerLiteral times 8) plus the MV ofOctalDigit.

B.1.2 String Literals

The syntax and semantics of7.8.4 can be extended as follows except that this extension is not allowed forstrict mode code:

Syntax

EscapeSequence::
CharacterEscapeSequence
OctalEscapeSequence
HexEscapeSequence
UnicodeEscapeSequence
OctalEscapeSequence::
OctalDigit[lookahead ∉DecimalDigit]
ZeroToThreeOctalDigit[lookahead ∉DecimalDigit]
FourToSevenOctalDigit
ZeroToThreeOctalDigitOctalDigit
ZeroToThree::one of
0123
FourToSeven::one of
4567

Semantics

  • The CV ofEscapeSequence::OctalEscapeSequence is the CV of theOctalEscapeSequence.

  • The CV ofOctalEscapeSequence::OctalDigit [lookahead ∉DecimalDigit] is the character whose code unit value is the MV of theOctalDigit.

  • The CV ofOctalEscapeSequence::ZeroToThreeOctalDigit [lookahead ∉DecimalDigit] is the character whose code unit value is (8 times the MV of theZeroToThree) plus the MV of theOctalDigit.

  • The CV ofOctalEscapeSequence::FourToSevenOctalDigit is the character whose code unit value is (8 times the MV of theFourToSeven) plus the MV of theOctalDigit.

  • The CV ofOctalEscapeSequence::ZeroToThreeOctalDigitOctalDigit is the character whose code unit value is (64 (that is, 82) times the MV of theZeroToThree) plus (8 times the MV of the firstOctalDigit) plus the MV of the secondOctalDigit.

  • The MV ofZeroToThree::0 is 0.

  • The MV ofZeroToThree::1 is 1.

  • The MV ofZeroToThree::2 is 2.

  • The MV ofZeroToThree::3 is 3.

  • The MV ofFourToSeven::4 is 4.

  • The MV ofFourToSeven::5 is 5.

  • The MV ofFourToSeven::6 is 6.

  • The MV ofFourToSeven::7 is 7.

B.2 Additional Properties

Some implementations of ECMAScript have included additional properties for some of the standard native objects. This non-normative annex suggests uniform semantics for such properties without making the properties or their semantics part of this standard.

B.2.1 escape (string)

Theescape function is a property of the global object. It computes a new version of a String value in which certain characters have been replaced by a hexadecimal escape sequence.

For those characters being replaced whose code unit value is0xFF or less, a two-digit escape sequence of the form%xx is used. For those characters being replaced whose code unit value is greater than0xFF, a four-digit escape sequence of the form%uxxxx is used.

When theescape function is called with one argumentstring, the following steps are taken:

  1. CallToString(string).
  2. Compute the number of characters in Result(1).
  3. LetR be the empty string.
  4. Letk be 0.
  5. Ifk equals Result(2), returnR.
  6. Get the character (represented as a 16-bit unsigned integer) at positionk within Result(1).
  7. If Result(6) is one of the 69 nonblank characters
    “ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./”
    then go to step 13.
  8. If Result(6), is less than 256, go to step 11.
  9. LetS be a String containing six characters“%uwxyz” wherewxyz are four hexadecimal digits encoding the value of Result(6).
  10. Go to step 14.
  11. LetS be a String containing three characters“%xy” wherexy are two hexadecimal digits encoding the value of Result(6).
  12. Go to step 14.
  13. LetS be a String containing the single character Result(6).
  14. LetR be a new String value computed by concatenating the previous value ofR andS.
  15. Increasek by 1.
  16. Go to step 5.

NOTE The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.

B.2.2 unescape (string)

Theunescape function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by theescape function is replaced with the character that it represents.

When theunescape function is called with one argumentstring, the following steps are taken:

  1. CallToString(string).
  2. Compute the number of characters in Result(1).
  3. LetR be the empty String.
  4. Letk be 0.
  5. Ifk equals Result(2), returnR.
  6. Letc be the character at positionk within Result(1).
  7. Ifc is not%, go to step 18.
  8. Ifk is greater than Result(2)−6, go to step 14.
  9. If the character at positionk+1 within Result(1) is notu, go to step 14.
  10. If the four characters at positionsk+2,k+3,k+4, andk+5 within Result(1) are not all hexadecimal digits, go to step 14.
  11. Letc be the character whose code unit value is the integer represented by the four hexadecimal digits at positionsk+2,k+3,k+4, andk+5 within Result(1).
  12. Increase k by 5.
  13. Go to step 18.
  14. Ifk is greater than Result(2)−3, go to step 18.
  15. If the two characters at positionsk+1 andk+2 within Result(1) are not both hexadecimal digits, go to step 18.
  16. Letc be the character whose code unit value is the integer represented by two zeroes plus the two hexadecimal digits at positionsk+1 andk+2 within Result(1).
  17. Increasek by 2.
  18. LetR be a new String value computed by concatenating the previous value ofR andc.
  19. Increasek by 1.
  20. Go to step 5.

B.2.3 String.prototype.substr (start, length)

Thesubstr method takes two arguments,start andlength, and returns a substring of the result of converting the this object to a String, starting from character positionstart and running forlength characters (or through the end of the String iflength isundefined). Ifstart is negative, it is treated as(sourceLength+start) wheresourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

  1. CallToString, giving it thethis value as its argument.
  2. CallToInteger(start).
  3. Iflength isundefined, use+∞; otherwise callToInteger(length).
  4. Compute the number of characters in Result(1).
  5. If Result(2) is positive or zero, use Result(2); else use max(Result(4)+Result(2),0).
  6. Compute min(max(Result(3),0), Result(4)–Result(5)).
  7. If Result(6) ≤ 0, return the empty String “”.
  8. Return a String containing Result(6) consecutive characters from Result(1) beginning with the character at position Result(5).

Thelength property of thesubstr method is2.

NOTE Thesubstr function is intentionally generic; it does not require that itsthis value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

B.2.4 Date.prototype.getYear ( )

NOTE ThegetFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When thegetYear method is called with no arguments, the following steps are taken:

  1. Lett bethis time value.
  2. Ift isNaN, returnNaN.
  3. ReturnYearFromTime(LocalTime(t)) − 1900.

B.2.5 Date.prototype.setYear (year)

NOTE ThesetFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When thesetYear method is called with one argumentyear, the following steps are taken:

  1. Lett be the result ofLocalTime(this time value); but ifthis time value isNaN, lett be+0.
  2. CallToNumber(year).
  3. If Result(2) isNaN, set the [[PrimitiveValue]] internal property of thethis value toNaN and returnNaN.
  4. If Result(2) is notNaN and 0 ≤ToInteger(Result(2)) ≤ 99 then Result(4) isToInteger(Result(2)) + 1900. Otherwise, Result(4) is Result(2).
  5. ComputeMakeDay(Result(4),MonthFromTime(t),DateFromTime(t)).
  6. ComputeUTC(MakeDate(Result(5),TimeWithinDay(t))).
  7. Set the [[PrimitiveValue]] internal property of thethis value toTimeClip(Result(6)).
  8. Return the value of the [[PrimitiveValue]] internal property of thethis value.

B.2.6 Date.prototype.toGMTString ( )

NOTE The propertytoUTCString is preferred. ThetoGMTString property is provided principally for compatibility with old code. It is recommended that thetoUTCString property be used in new ECMAScript code.

The Function object that is the initial value ofDate.prototype.toGMTString is the same Function object that is the initial value ofDate.prototype.toUTCString.

Annex C(informative) The Strict Mode of ECMAScript

The strict mode restriction and exceptions

Annex D(informative) Corrections and Clarifications in the 5th Edition with Possible 3rd Edition Compatibility Impact

Throughout: In the Edition 3 specification the meaning of phrases such as “as if by the expressionnew Array()” are subject to misinterpretation. In the Edition 5 specification text for all internal references and invocations of standard built-in objects and methods has been clarified by making it explicit that the intent is that the actual built-in object is to be used rather than the current dynamic value of the correspondingly named property.

11.8.2,11.8.3,11.8.5: ECMAScript generally uses a left to right evaluation order, however the Edition 3 specification language for the > and <= operators resulted in a partial right to left order. The specification has been corrected for these operators such that it now specifies a full left to right evaluation order. However, this change of order is potentially observable if side-effects occur during the evaluation process.

11.1.4: Edition 5 clarifies the fact that a trailing comma at the end of anArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.

11.2.3: Edition 5 reverses the order of steps 2 and 3 of the algorithm. The original order as specified in Editions 1 through 3 was incorrectly specified such that side-effects of evaluatingArguments could affect the result of evaluatingMemberExpression.

12.4: In Edition 3, an object is created, as if bynew Object()to serve as the scope for resolving the name of the exception parameter passed to acatch clause of atry statement. If the actual exception object is a function and it is called from within thecatch clause, the scope object will be passed as thethis value of the call. The body of the function can then define new properties on itsthis value and those property names become visible identifiers bindings within the scope of thecatch clause after the function returns. In Edition 5, when an exception parameter is called as a function,undefined is passed as thethis value.

13: In Edition 3, the algorithm for the productionFunctionExpression with anIdentifier adds an object created as if bynew Object() to the scope chain to serve as a scope for looking up the name of the function. The identifier resolution rules (10.1.4 in Edition 3) when applied to such an object will, if necessary, follow the object’s prototype chain when attempting to resolve an identifier. This means all the properties of Object.prototype are visible as identifiers within that scope. In practice most implementations of Edition 3 have not implemented this semantics. Edition 5 changes the specified semantics by using aDeclarative Environment Record to bind the name of the function.

14: In Edition 3, the algorithm for the productionSourceElements:SourceElementsSourceElement did not correctly propagate statement result values in the same manner asBlock. This could result in theeval function producing an incorrect result when evaluating aProgram text. In practice most implementations of Edition 3 have implemented the correct propagation rather than what was specified in Edition 5.

15.10.6: RegExp.prototype is now a RegExp object rather than an instance of Object. The value of its [[Class]] internal property which is observable using Object.prototype.toString is now “RegExp” rather than “Object”.

Annex E(informative) Additions and Changes in the 5th Edition that Introduce Incompatibilities with the 3rd Edition

7.1: Unicode format control characters are no longer stripped from ECMAScript source text before processing. In Edition 5, if such a character appears in aStringLiteral orRegularExpressionLiteral the character will be incorporated into the literal where in Edition 3 the character would not be incorporated into the literal.

7.2: Unicode character <BOM> is now treated as whitespace and its presence in the middle of what appears to be an identifier could result in a syntax error which would not have occurred in Edition 3

7.3: Line terminator characters that are preceded by an escape sequence are now allowed within a string literal token. In Edition 3 a syntax error would have been produced.

7.8.5: Regular expression literals now return a unique object each time the literal is evaluated. This change is detectable by any programs that test the object identity of such literal values or that are sensitive to the shared side effects.

7.8.5: Edition 5 requires early reporting of any possible RegExp constructor errors that would be produced when converting aRegularExpressionLiteral to a RegExp object. Prior to Edition 5 implementations were permitted to defer the reporting of such errors until the actual execution time creation of the object.

7.8.5: In Edition 5 unescaped “/” characters may appear as aCharacterClass in a regular expression literal. In Edition 3 such a character would have been interpreted as the final character of the literal.

10.4.2: In Edition 5, indirect calls to theeval function usethe global environment as both the variable environment andlexical environment for the eval code. In Edition 3, the variable and lexical environments of the caller of an indirecteval was used as the environments for the eval code.

15.4.4: In Edition 5 all methods ofArray.prototype are intentionally generic. In Edition 3toString andtoLocaleString were not generic and would throw aTypeError exception if applied to objects that were not instances of Array.

10.6: In Edition 5 the array indexed properties of argument objects that correspond to actual formal parameters are enumerable. In Edition 3, such properties were not enumerable.

10.6: In Edition 5 the value of the [[Class]] internal property of an arguments object is"Arguments". In Edition 3, it was"Object". This is observable iftoString is called as a method of an arguments object.

12.6.4: for-in statements no longer throw aTypeError if thein expression evaluates tonull orundefined. Instead, the statement behaves as if the value of the expression was an object with no enumerable properties.

15: In Edition 5, the following new properties are defined on built-in objects that exist in Edition 3:Object.getPrototypeOf,Object.getOwnPropertyDescriptor,Object.getOwnPropertyNames,Object.create,Object.defineProperty,Object.defineProperties,Object.seal,Object.freeze,Object.preventExtensions,Object.isSealed,Object.isFrozen,Object.isExtensible,Object.keys,Function.prototype.bind,Array.prototype.indexOf,Array.prototype.lastIndexOf,Array.prototype.every,Array.prototype.some,Array.prototype.forEach,Array.prototype.map,Array.prototype.filter,Array.prototype.reduce,Array.prototype.reduceRight,String.prototype.trim,Date.now,Date.prototype.toISOString, Date.prototype.toJSON.

15: Implementations are now required to ignore extra arguments to standard built-in methods unless otherwise explicitly specified. In Edition 3 the handling of extra arguments was unspecified and implementations were explicitly allowed to throw aTypeError exception.

15.1.1: The value propertiesNaN,Infinity, andundefined of the Global Object have been changed to be read-only properties.

15.1.2.1. Implementations are no longer permitted to restrict the use of eval in ways that are not a direct call. In addition, any invocation of eval that is not a direct call usesthe global environment as its variable environment rather than the caller’s variable environment.

15.1.2.2: The specification of the functionparseInt no longer allows implementations to treat Strings beginning with a0 character as octal values.

15.3.4.3: In Edition 3, aTypeError is thrown if the second argument passed toFunction.prototype.apply is neither an array object nor an arguments object. In Edition 5, the second argument may be any kind of generic array-like object that has a validlength property.

15.3.4.3, 15.3.4.4: In Edition 3 passingundefined ornull as the first argument to eitherFunction.prototype.apply orFunction.prototype.call causes the global object to be passed to the indirectly invoked target function as thethis value. If the first argument is a primitive value the result of callingToObject on the primitive value is passed as thethis value. In Edition 5, these transformations are not performed and the actual first argument value is passed as thethis value. This difference will normally be unobservable to existing ECMAScript Edition 3 code because a corresponding transformation takes place upon activation of the target function. However, depending upon the implementation, this difference may be observable by host object functions called usingapply orcall. In addition, invoking a standard built-in function in this manner withnull orundefined passed as the this value will in many cases cause behaviour in Edition 5 implementations that differ from Edition 3 behaviour. In particular, in Edition 5 built-in functions that are specified to actually use the passedthis value as an object typically throw aTypeError exception if passednull orundefined as thethis value.

15.3.5.2: In Edition 5, theprototype property of Function instances is not enumerable. In Edition 3, this property was enumerable.

15.5.5.2: In Edition 5, the individual characters of a String object’s [[PrimitiveValue] may be accessed as array indexed properties of the String object. These properties are non-writable and non-configurable and shadow any inherited properties with the same names. In Edition 3, these properties did not exist and ECMAScript code could dynamically add and remove writable properties with such names and could access inherited properties with such names.

15.9.4.2:Date.parse is now required to first attempt to parse its argument as an ISO format string. Programs that use this format but depended upon implementation specific behaviour (including failure) may behave differently.

15.10.2.12: In Edition 5,\s now additionally matches <BOM>.

15.10.4.1: In Edition 3, the exact form of the String value of thesource property of an object created by theRegExp constructor is implementation defined. In Edition 5, the String must conform to certain specified requirements and hence may be different from that produced by an Edition 3 implementation.

15.10.6.4: In Edition 3, the result ofRegExp.prototype.toString need not be derived from the value of the RegExp object’ssource property. In Edition 5 the result must be derived from thesource property in a specified manner and hence may be different from the result produced by an Edition 3 implementation.

15.11.2.1,15.11.4.3: In Edition 5, if an initial value for themessage property of an Error object is not specified via theError constructor the initial value of the property is the empty String. In Edition 3, such an initial value is implementation defined.

15.11.4.4: In Edition 3, the result ofError.prototype.toString is implementation defined. In Edition 5, the result is fully specified and hence may differ from some Edition 3 implementations.

15.12: In Edition 5, the nameJSON is defined inthe global environment. In Edition 3, testing for the presence of that name will show it to be undefined unless it is defined by the program or implementation.

Annex F(informative) Technically Significant Corrections and Clarifications in the 5.1 Edition

7.8.4: CV definitions added forDoubleStringCharacter::LineContinuation andSingleStringCharacter::LineContinuation .

10.2.1.1.3: The argument S is not ignored. It controls whether an exception is thrown when attempting to set an immutable binding.

10.2.1.2.2: In algorithm step 5,true is passed as the last argument to [[DefineOwnProperty]].

10.5: Former algorithm step 5.e is now 5.f and a new step 5.e was added to restore compatibility with 3rd Edition when redefining global functions.

11.5.3: In the final bullet item, use of IEEE 754 round-to-nearest mode is specified.

12.6.3: MissingToBoolean restored in step 3.a.ii of both algorithms.

12.6.4: Additional final sentences in each of the last two paragraphs clarify certain property enumeration requirements.

12.7,12.8,12.9: BNF modified to clarify that acontinue orbreak statement without anIdentifier or areturn statement without anExpression may have aLineTerminator before the semi-colon.

12.14: Step 3 of algorithm 1 and step 2.a of algorithm 3 are corrected such that thevalue field ofB is passed as a parameter rather thanB itself.

15.1.2.2: In step 2 of algorithm, clarify thatS may be the empty string.

15.1.2.3: In step 2 of algorithm clarify thattrimmedString may be the empty string.

15.1.3: Added notes clarifying that ECMAScript’s URI syntax is based upon RFC 2396 and not the newer RFC 3986. In the algorithm for Decode, a step was removed that immediately preceded the current step 4.d.vii.10.a because it tested for a condition that cannot occur.

15.2.3.7: Corrected use of variableP in steps 5 and 6 of algorithm.

15.2.4.2: Edition 5 handling ofundefined andnull asthis value caused existing code to fail. Specification modified to maintain compatibility with such code. New steps 1 and 2 added to the algorithm.

15.3.4.3: Steps 5 and 7 of Edition 5 algorithm have been deleted because they imposed requirements upon theargArray argument that are inconsistent with other uses of generic array-like objects.

15.4.4.12: In step 9.a, incorrect reference torelativeStart was replaced with a reference toactualStart.

15.4.4.15: Clarified that the default value forfromIndex is the length minus 1 of the array.

15.4.4.18: In step 9 of the algorithm,undefined is now the specified return value.

15.4.4.22: In step 9.c.ii the first argument to the [[Call]] internal method has been changed toundefined for consistency with the definition ofArray.prototype.reduce.

15.4.5.1: In Algorithm steps 3.l.ii and 3.l.iii the variable name was inverted resulting in an incorrectly inverted test.

15.5.4.9: Normative requirement concerning canonically equivalent strings deleted from paragraph following algorithm because it is listed as a recommendation in NOTE 2.

15.5.4.14: Insplit algorithm step 11.a and 13.a, the positional order of the arguments toSplitMatch was corrected to match the actual parameter signature ofSplitMatch. In step 13.a.iii.7.d,lengthA replacesA.length.

15.5.5.2: In first paragraph, removed the implication that the individual character property access had “array index” semantics. Modified algorithm steps 3 and 5 such that they do not enforce “array index” requirement.

15.9.1.15: Specified legal value ranges for fields that lacked them. Eliminated “time-only” formats. Specified default values for all optional fields.

15.10.2.2: The step numbers of the algorithm for the internal closure produced by step 2 were incorrectly numbered in a manner that implied that they were steps of the outer algorithm.

15.10.2.6: In the abstract operationIsWordChar the first character in the list in step 3 is “a” rather than “A”.

15.10.2.8: In the algorithm for the closure returned by the abstract operationCharacterSetMatcher, the variable defined by step 3 and passed as an argument in step 4 was renamed toch in order to avoid a name conflict with a formal parameter of the closure.

15.10.6.2: Step 9.e was deleted because It performed an extra increment ofi.

15.11.1.1: Removed requirement that themessage own property is set to the empty String when themessage argument isundefined.

15.11.1.2: Removed requirement that themessage own property is set to the empty String when themessage argument isundefined.

15.11.4.4: Steps 6-10 modified/added to correctly deal with missing or emptymessage property value.

15.11.1.2: Removed requirement that themessage own property is set to the empty String when themessage argument isundefined.

15.12.3: In step 10.b.iii of theJA internal operation, the last element of the concatenation is “]”.

B.2.1: Added to NOTE that the encoding is based upon RFC 1738 rather than the newer RFC 3986.

Annex C: An item was added corresponding to 7.6.12 regardingFutureReservedWords in strict mode.

Bibliography

  1. IEEE Std 754-2008: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2008)
  2. The Unicode Consortium. The Unicode Standard, Version 3.0, defined by: The Unicode Standard, Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5)
  3. Unicode Inc. (2010), Unicode Technical Report #15: Unicode Normalization Forms
  4. ISO 8601:2004(E)Data elements and interchange formats – Informationinterchange --Representation of dates and times
  5. RFC 1738 "Uniform Resource Locators (URL)", available at <http://tools.ietf.org/html/rfc1738>
  6. RFC 2396 "Uniform Resource Identifiers (URI): Generic Syntax", available at <http://tools.ietf.org/html/rfc2396>
  7. RFC 3629 "UTF-8, a transformation format of ISO 10646", available at <http://tools.ietf.org/html/rfc3629>
  8. RFC 4627 "The application/json Media Type for JavaScript Object Notation (JSON)" , available at <http://tools.ietf.org/html/rfc4627>
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