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
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. Ascripting language is a programming language that is used to manipulate, customize, 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
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 is now used to provide core scripting capabilities for a variety of
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.
4.1 Web Scripting
A web browser provides an ECMAScript
A web server provides a different
Each Web browser and server that supports ECMAScript supplies its own
4.2 Hosts and Implementations
To aid integrating ECMAScript into
Animplementation is an external source that further defines facilities enumerated in Annex
Animplementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.
Animplementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such asMath.exp
Ahost is an external source that further defines facilities listed in Annex
Ahost hook is an abstract operation that is defined in whole or in part by an external source. All
- It must return either a
normal completion or athrow completion .
Ahost-defined facility is one that defers its definition to an external source without further qualification and is listed in Annex
Ahost environment is a particular choice of definition for all
This specification follows the editorial convention of always using the most specific term. For example, if a facility is
Both
4.3 ECMAScript 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
ECMAScript defines a collection ofbuilt-in objects that round out the definition of ECMAScript entities. These built-in objects include theObject,Function,Boolean,Symbol, and variousError objects; objects that represent and manipulate numeric values includingMath,Number, andDate; the text processing objectsString andRegExp; objects that are indexed collections of values includingArray and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections includingMap andSet objects; objects supporting structured data including theJSON object,ArrayBuffer,SharedArrayBuffer, andDataView; objects supporting control abstractions including generator functions andPromise objects; and reflection objects includingProxy andReflect.
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.
Large ECMAScript programs are supported bymodules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.
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.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or vianew Date(2009, 11) creates a new Date object. Invoking aDate() produces a string representation of the current date and time rather than an object.
Every object created by a
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, while 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 anew expressions:cf1,cf2,cf3,cf4, andcf5. Each of these objects contains properties named
Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is,
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes 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
In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
an
4.4.2 implementation-defined
an
4.4.3 host-defined
same as
Editorially, see clause
4.4.4 type
set of data values as defined in clause
4.4.5 primitive value
member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or String as defined in clause
A primitive value is a datum that is represented directly at the lowest level of the language implementation.
4.4.6 object
member of the type Object
An object is a collection of properties and has a single prototype object. The prototype may be
4.4.7 constructor
The value of a
4.4.8 prototype
object that provides shared properties for other objects
When aconstructor.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.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
Any object that is not an
4.4.11 standard object
object whose semantics are defined by this specification
4.4.12 built-in object
object specified and supplied by an ECMAScript implementation
Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects.
4.4.13 undefined value
primitive value used when a variable has not been assigned a value
4.4.14 Undefined type
type whose sole value is the
4.4.15 null value
primitive value that represents the intentional absence of any object value
4.4.16 Null type
type whose sole value is the
4.4.17 Boolean value
member of the
There are only two Boolean values,
4.4.18 Boolean type
type consisting of the primitive values
4.4.19 Boolean object
member of the
A Boolean object is created by using the Booleannew expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.
4.4.20 String value
primitive value that is a
A String value is a member of the
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the
A String object is created by using the Stringnew expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary format
A Number value is a member of the
4.4.24 Number type
set of all possible Number values including
4.4.25 Number object
member of the
A Number object is created by using the Numbernew expression, supplying a Number value as an argument. The resulting object has an internal slot whose value is the Number value. A Number object can be coerced to a Number value by calling the Number
4.4.26 Infinity
Number value that is the positive infinite Number value
4.4.27 NaN
Number value that is an
4.4.28 BigInt value
primitive value corresponding to an arbitrary-precision
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the
4.4.31 Symbol value
primitive value that represents a unique, non-String Object
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the
4.4.34 function
member of the
In addition to its 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.
4.4.35 built-in function
built-in object that is a function
Examples of built-in functions includeparseInt andMath.exp. A
4.4.36 built-in constructor
built-in function that is a
Examples of built-inObject andFunction. A
4.4.37 property
part of an object that associates a key (either a String value or a Symbol value) and a value
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
4.4.38 method
function that is the value of a property
When a function is called as a method of an object, the object is passed to the function as its
4.4.39 built-in method
method that is a built-in function
Standard built-in methods are defined in this specification. A
4.4.40 attribute
internal value that defines some characteristic of a property
4.4.41 own property
property that is directly contained by its object
4.4.42 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
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause
Clauses
Clauses
Clauses
Clause