The ECMAScript standard does not include anyinput/output (I/O), such asnetworking,storage, orgraphics facilities. In practice, the web browser or other runtime system provides JavaScript APIs for I/O.
AlthoughJava and JavaScript are similar in name andsyntax, the two languages are distinct and differ greatly in design.
During these formative years of the Web,web pages could only be static, lacking the capability for dynamic behavior after the page was loaded in the browser. There was a desire in the flourishing web development scene to remove this limitation, so in 1995, Netscape decided to add aprogramming language to Navigator. They pursued two routes to achieve this: collaborating withSun Microsystems to embed theJava language, while also hiringBrendan Eich to embed theScheme language.[6]
The goal was a "language for the masses",[13] "to help nonprogrammers create dynamic, interactiveWeb sites".[14] Netscape management soon decided that the best option was for Eich to devise a new language, with syntax similar to Java and less like Scheme or other extantscripting languages.[5][6] Although the new language and itsinterpreter implementation were called LiveScript when first shipped as part of a Navigatorbeta in September 1995, the name was changed to JavaScript for the official release in December.[6][1][15][16]
The choice of theJavaScript name has caused confusion, implying that it is directly related to Java. At the time, thedot-com boom had begun and Java was a popular new language, so Eich considered the JavaScript name a marketing ploy by Netscape.[13]
Microsoft first released JScript in 1996, alongside initial support forCSS and extensions toHTML. Each of theseimplementations was noticeably different from their counterparts inNetscape Navigator.[18][19] These differences made it difficult for developers to make their websites work well in both browsers, leading to widespread use of "best viewed in Netscape" and "best viewed in Internet Explorer" logos for several years.[18][20]
The rise of JScript
Brendan Eich later said of this period: "It's still kind of asidekick language. It's considered slow or annoying. People dopop-ups or those scrolling messages in the oldstatus bar at the bottom of your oldbrowser."[13]
In November 1996,Netscape submitted JavaScript toEcma International, as the starting point for a standard specification that all browser vendors could conform to. This led to the official release of the firstECMAScript language specification in June 1997.
The standards process continued for a few years, with the release of ECMAScript 2 in June 1998 and ECMAScript 3 in December 1999. Work on ECMAScript 4 began in 2000.[17]
However, the effort to fully standardize the language was undermined by Microsoft gaining an increasingly dominant position in the browser market. By the early 2000s,Internet Explorer's market share reached 95%.[21] This meant thatJScript became the de facto standard forclient-side scripting on the Web.
Microsoft initially participated in the standards process and implemented some proposals in its JScript language, but eventually it stopped collaborating on ECMA work. Thus ECMAScript 4 was mothballed.
Growth and standardization
Logo of Technical Committee 39 ("TC39"), the committee ofEcma International that is responsible for maintaining the standard
During the period ofInternet Explorer dominance in the early 2000s, client-side scripting was stagnant. This started to change in 2004, when the successor of Netscape,Mozilla, released theFirefox browser. Firefox was well received by many, taking significant market share from Internet Explorer.[22]
In 2005, Mozilla joined ECMA International, and work started on theECMAScript for XML (E4X) standard. This led to Mozilla working jointly withMacromedia (later acquired byAdobe Systems), who were implementing E4X in their ActionScript 3 language, which was based on an ECMAScript 4 draft. The goal became standardizing ActionScript 3 as the new ECMAScript 4. To this end, Adobe Systems released theTamarin implementation as anopen source project. However, Tamarin and ActionScript 3 were too different from established client-side scripting, and without cooperation from Microsoft, ECMAScript 4 never reached fruition.
Meanwhile, very important developments were occurring in open-source communities not affiliated with ECMA work. In 2005,Jesse James Garrett released a white paper in which he coined the termAjax and described a set of technologies, of which JavaScript was the backbone, to createweb applications where data can be loaded in the background, avoiding the need for full page reloads. This sparked a renaissance period of JavaScript, spearheaded by open-source libraries and the communities that formed around them. Many new libraries were created, includingjQuery,Prototype,Dojo Toolkit, andMooTools.
Google debuted itsChrome browser in 2008, with theV8 JavaScript engine that was faster than its competition.[23][24] The key innovation wasjust-in-time compilation (JIT),[25] so other browser vendors needed to overhaul their engines for JIT.[26]
In July 2008, these disparate parties came together for a conference inOslo. This led to the eventual agreement in early 2009 to combine all relevant work and drive the language forward. The result was the ECMAScript 5 standard, released in December 2009.
Reaching maturity
Ambitious work on the language continued for several years, culminating in an extensive collection of additions and refinements being formalized with the publication ofECMAScript 6 in 2015.[27]
The creation ofNode.js in 2009 byRyan Dahl sparked a significant increase in the usage of JavaScript outside of web browsers. Node combines theV8 engine, anevent loop, andI/OAPIs, thereby providing a stand-alone JavaScript runtime system.[28][29] As of 2018, Node had been used by millions of developers,[30] andnpm had the most modules of anypackage manager in the world.[31]
The ECMAScript draft specification is currently maintained openly onGitHub,[32] and editions are produced via regular annual snapshots.[32] Potential revisions to the language are vetted through a comprehensive proposal process.[33][34] Now, instead of edition numbers, developers check the status of upcoming features individually.[32]
The current JavaScript ecosystem has manylibraries andframeworks, established programming practices, and substantial usage of JavaScript outside of web browsers.[16] Plus, with the rise ofsingle-page applications and other JavaScript-heavy websites, severaltranspilers have been created to aid the development process.[35]
A letter was circulated in September 2024, spearheaded byRyan Dahl, calling on Oracle to free the JavaScript trademark.[40]Brendan Eich, the original creator of JavaScript, was among the over 14,000 signatories who supported the initiative.
Website client-side usage
JavaScript is the dominantclient-sidescripting language of the Web, with 99% of allwebsites using it for this purpose.[10] Scripts are embedded in or included fromHTML documents and interact with theDOM.
Loading newweb page content without reloading the page, viaAjax or aWebSocket. For example,users ofsocial media can send and receive messages without leaving the current page.
Web page animations, such as fading objects in and out, resizing, and moving them.
However, the term "Vanilla JS" was coined for websites not using any libraries or frameworks at all, instead relying entirely on standard JavaScript functionality.[43]
Oracle used to provide Nashorn, a JavaScript interpreter, as part of theirJava Development Kit (JDK) API library along withjjs a command line interpreter as of JDK version 8. It was removed in JDK 15. As a replacement Oracle offered GraalJS which can also be used with theOpenJDK which allows one to create and reference Java objects in JavaScript code and add runtime scripting in JavaScript to applications written in Java.[49][50][51][52]
The firstengines for JavaScript were mereinterpreters of thesource code, but all relevant modern engines usejust-in-time compilation for improved performance.[56] JavaScript engines are typically developed byweb browser vendors, and every major browser has one. In a browser, the JavaScript engine runs in concert with therendering engine via theDocument Object Model andWeb IDL bindings.[57] However, the use of JavaScript engines is not limited to browsers; for example, theV8 engine is a core component of theNode.jsruntime system.[58] They are also calledECMAScript engines, after the official name of the specification. With the advent ofWebAssembly, some engines can also execute this code in the samesandbox as regular JavaScript code.[59][58]
Runtime system
A JavaScript engine must be embedded within aruntime system (such as aweb browser or a standalone system) to enable scripts to interact with the broader environment. The runtime system includes the necessary APIs forinput/output operations, such asnetworking,storage, andgraphics, and provides the ability to import scripts.
JavaScript is a single-threaded language. The runtime processesmessages from aqueue one at a time, and it calls afunction associated with each new message, creating acall stack frame with the function'sarguments andlocal variables. The call stack shrinks and grows based on the function's needs. When the call stack is empty upon function completion, JavaScript proceeds to the next message in the queue. This is called theevent loop, described as "run to completion" because each message is fully processed before the next message is considered. However, the language'sconcurrency model describes the event loop asnon-blocking: program I/O is performed usingevents andcallback functions. This means, for example, that JavaScript can process a mouse click while waiting for a database query to return information.[60]
The notable standalone runtimes areNode.js,Deno, andBun.
Features
The following features are common to all conforming ECMAScript implementations unless explicitly specified otherwise. The number of cited reserved words including keywords is 50-60 and varies depending on the implementation.
JavaScript supports much of thestructured programming syntax fromC (e.g.,if statements,while loops,switch statements,do while loops, etc.). One partial exception isscoping: originally JavaScript only hadfunction scoping withvar;block scoping was added in ECMAScript 2015 with the keywordslet andconst. Like C, JavaScript makes a distinction betweenexpressions andstatements. One syntactic difference from C isautomatic semicolon insertion, which allow semicolons (which terminate statements) to be omitted.[61]
JavaScript isweakly typed, which means certain types are implicitly cast depending on the operation used.[62]
The binary+ operator casts both operands to a string unless both operands are numbers. This is because the addition operator doubles as a concatenation operator
The binary- operator always casts both operands to a number
Both unary operators (+,-) always cast the operand to a number. However,+ always casts toNumber (binary64) while- preservesBigInt (integer)[63]
Values are cast to strings like the following:[62]
Strings are left as-is
Numbers are converted to their string representation
Arrays have their elements cast to strings after which they are joined by commas (,)
Other objects are converted to the string[object Object] whereObject is the name of the constructor of the object
Values are cast to numbers by casting to strings and then casting the strings to numbers. These processes can be modified by definingtoString andvalueOf functions on theprototype for string and number casting respectively.
JavaScript has received criticism for the way it implements these conversions as the complexity of the rules can be mistaken for inconsistency.[64][62] For example, when adding a number to a string, the number will be cast to a string before performing concatenation, but when subtracting a number from a string, the string is cast to a number before performing subtraction.
Often also mentioned is{} + [] resulting in0 (number). This is misleading: the{} is interpreted as an empty code block instead of an empty object, and the empty array is cast to a number by the remaining unary+ operator. If the expression is wrapped in parentheses -({} + []) – the curly brackets are interpreted as an empty object and the result of the expression is"[object Object]" as expected.[62]
JavaScript includes aneval function that can execute statements provided as strings at run-time.
Object-orientation (prototype-based)
Prototypal inheritance in JavaScript is described byDouglas Crockford as:
You make prototype objects, and then ... make new instances. Objects are mutable in JavaScript, so we can augment the new instances, giving them new fields and methods. These can then act as prototypes for even newer objects. We don't need classes to make lots of similar objects... Objects inherit from objects. What could be more object oriented than that?[67]
In JavaScript, anobject is anassociative array, augmented with a prototype (see below); each key provides the name for an objectproperty, and there are two syntactical ways to specify such a name: dot notation (obj.x = 10) and bracket notation (obj["x"] = 10). A property may be added, rebound, or deleted at run-time. Mostproperties of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using afor...in loop.
JavaScript usesprototypes where many other object-oriented languages useclasses forinheritance,[68] but it's still possible to simulate most class-based features with the prototype system.[69] Additionally,ECMAScript version 6 (released June 2015) introduced the keywordsclass,extends andsuper, which serve as syntactic sugar to abstract the underlying prototypal inheritance system with a more conventional interface. Constructors are declared by specifying a method namedconstructor, and all classes are automatically subclasses of the base class Object, similarly to Java.
classPerson{constructor(name){this.name=name;}}classStudentextendsPerson{constructor(name,id){super(name);this.id=id;}}constbob=newStudent("Robert",12345);console.log(bob.name);// Robert
Though the underlying object mechanism is still based on prototypes, the newer syntax is similar to other object oriented languages. Private variables are declared by prefixing the field name with anumber sign (#), andpolymorphism is not directly supported, although it can be emulated by manually calling different functions depending on the number and type of arguments provided.[70]
Functions as object constructors
Functions double as object constructors, along with their typical role. Prefixing a function call withnew will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from theObject prototype).[71] ECMAScript 5 offers theObject.create method, allowing explicit creation of an instance without automatically inheriting from theObject prototype (older environments can assign the prototype tonull).[72] The constructor'sprototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the function used as a constructor.
// This code is completely equivalent to the previous snippetfunctionPerson(name){this.name=name;}functionStudent(name,id){Person.call(this,name);this.id=id;}varbob=newStudent("Robert",12345);console.log(bob.name);// Robert
JavaScript's built-in classes, such asArray andObject, also have prototypes that can be modified. However, it's generally considered bad practice tomodify built-in objects, because third-party code may use or inherit methods and properties from these objects, and may not expect the prototype to be modified.[73]
Unlike in many object-oriented languages, in JavaScript there is no distinction between a function definition and amethod definition. Rather, the distinction occurs during function calling. When a function is called as a method of an object, the function's localthis keyword is bound to that object for that invocation.
JavaScriptfunctions arefirst-class; a function is considered to be an object.[74] As such, a function may have properties and methods, such as.call() and.bind().[75]
Anested function is a function defined within another function. It is created each time the outer function is invoked.
In addition, each nested function forms alexical closure: thelexical scope of the outer function (including any constant, local variable, or argument value) becomes part of the internal state of each inner function object, even after execution of the outer function concludes.[76]
JavaScript natively supports various function-based implementations ofRole[77] patterns likeTraits[78][79] andMixins.[80] Such a function defines additional behavior by at least one method bound to thethis keyword within itsfunction body. A Role then has to be delegated explicitly viacall orapply to objects that need to feature additional behavior that is not shared via the prototype chain.
Object composition and inheritance
Whereas explicit function-based delegation does covercomposition in JavaScript, implicit delegation already happens every time the prototype chain is walked in order to, e.g., find a method that might be related to but is not directly owned by an object. Once the method is found it gets called within this object's context. Thusinheritance in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.
An indefinite number of parameters can be passed to a function. The function can access them throughformal parameters and also through the localarguments object.Variadic functions can also be created by using thebind method.
Like in many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, theseliterals form the basis of theJSON data format.
A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. JavaScript supplies combinator methods, which allow developers to combine multiple JavaScript promises and do operations based on different scenarios. The methods introduced are: Promise.race, Promise.all, Promise.allSettled and Promise.any.
Async/await allows an asynchronous, non-blocking function to be structured in a way similar to an ordinary synchronous function. Asynchronous, non-blocking code can be written, with minimal overhead, structured similarly to traditional synchronous, blocking code.
Vendor-specific extensions
Historically, someJavaScript engines supported these non-standard features:
Variables in JavaScript can be defined using either thevar,[82]let[83] orconst[84] keywords. Variables defined without keywords will be defined at the global scope.
Arrow functions were first introduced in6th Edition – ECMAScript 2015. They shorten the syntax for writing functions in JavaScript. Arrow functions are anonymous, so a variable is needed to refer to them in order to invoke them after their creation, unless surrounded by parenthesis and executed immediately.
Here is an example of JavaScript syntax.
// Declares a function-scoped variable named `x`, and implicitly assigns the// special value `undefined` to it. Variables without value are automatically// set to undefined.// var is generally considered bad practice and let and const are usually preferred.varx;// Variables can be manually set to `undefined` like soletx2=undefined;// Declares a block-scoped variable named `y`, and implicitly sets it to// `undefined`. The `let` keyword was introduced in ECMAScript 2015.lety;// Declares a block-scoped, un-reassignable variable named `z`, and sets it to// a string literal. The `const` keyword was also introduced in ECMAScript 2015,// and must be explicitly assigned to.// The keyword `const` means constant, hence the variable cannot be reassigned// as the value is `constant`.constz="this value cannot be reassigned!";// Declares a global-scoped variable and assigns 3. This is generally considered// bad practice, and will not work if strict mode is on.t=3;// Declares a variable named `myNumber`, and assigns a number literal (the value// `2`) to it.letmyNumber=2;// Reassigns `myNumber`, setting it to a string literal (the value `"foo"`).// JavaScript is a dynamically-typed language, so this is legal.myNumber="foo";
JavaScript and theDOM provide the potential for malicious authors to deliver scripts to run on a client computer via the Web. Browser authors minimize this risk using two restrictions. First, scripts run in asandbox in which they can only perform Web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by thesame-origin policy: scripts from one website do not have access to information such as usernames, passwords, or cookies sent to another site. Most JavaScript-related security bugs are breaches of either the same origin policy or the sandbox.
There are subsets of general JavaScript—ADsafe, Secure ECMAScript (SES)—that provide greater levels of security, especially on code created by third parties (such as advertisements).[85][86] Closure Toolkit is another project for safe embedding and isolation of third-party JavaScript and HTML.[87]
Content Security Policy is the main intended method of ensuring that only trusted code is executed on a Web page.
A common JavaScript-related security problem iscross-site scripting (XSS), a violation of thesame-origin policy. XSS vulnerabilities occur when an attacker can cause a target Website, such as an online banking website, to include a malicious script in the webpage presented to a victim. The script in this example can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim's authorization. One important solution to XSS vulnerabilities isHTML sanitization.
Some browsers include partial protection againstreflected XSS attacks, in which the attacker provides a URL including malicious script. However, even users of those browsers are vulnerable to other XSS attacks, such as those where the malicious code is stored in a database. Only correct design of Web applications on the server-side can fully prevent XSS.
XSS vulnerabilities can also occur because of implementation mistakes by browser authors.[88]
Another cross-site vulnerability iscross-site request forgery (CSRF). In CSRF, code on an attacker's site tricks the victim's browser into taking actions the user did not intend at a target site (like transferring money at a bank). When target sites rely solely on cookies for request authentication, requests originating from code on the attacker's site can carry the same valid login credentials of the initiating user. In general, the solution to CSRF is to require an authentication value in a hidden form field, and not only in the cookies, to authenticate any request that might have lasting effects. Checking the HTTP Referrer header can also help.
"JavaScript hijacking" is a type of CSRF attack in which a<script> tag on an attacker's site exploits a page on the victim's site that returns private information such asJSON or JavaScript. Possible solutions include:
requiring an authentication token in thePOST andGET parameters for any response that returns private information.
Misplaced trust in the client
Developers of client-server applications must recognize that untrusted clients may be under the control of attackers. The author of an application should not assume that their JavaScript code will run as intended (or at all) because any secret embedded in the code could be extracted by a determined adversary. Some implications are:
Website authors cannot perfectly conceal how their JavaScript operates because the raw source code must be sent to the client. The code can beobfuscated, but obfuscation can be reverse-engineered.
JavaScript form validation only provides convenience for users, not security. If a site verifies that the user agreed to its terms of service, or filters invalid characters out of fields that should only contain numbers, it must do so on the server, not only the client.
Scripts can be selectively disabled, so JavaScript cannot be relied on to prevent operations such as right-clicking on an image to save it.[89]
It is considered very bad practice to embed sensitive information such as passwords in JavaScript because it can be extracted by an attacker.[90]
Prototype pollution is a runtime vulnerability in which attackers can overwrite arbitrary properties in an object's prototype.
Misplaced trust in developers
Package management systems such asnpm and Bower are popular with JavaScript developers. Such systems allow a developer to easily manage their program's dependencies upon other developers' program libraries. Developers trust that the maintainers of the libraries will keep them secure and up to date, but that is not always the case. A vulnerability has emerged because of this blind trust. Relied-upon libraries can have new releases that cause bugs or vulnerabilities to appear in all programs that rely upon the libraries. Inversely, a library can go unpatched with known vulnerabilities out in the wild. In a study done looking over a sample of 133,000 websites, researchers found 37% of the websites included a library with at least one known vulnerability.[91] "The median lag between the oldest library version used on each website and the newest available version of that library is 1,177 days in ALEXA, and development of some libraries still in active use ceased years ago."[91] Another possibility is that the maintainer of a library may remove the library entirely. This occurred in March 2016 when Azer Koçulu removed his repository from npm. This caused tens of thousands of programs and websites depending upon his libraries to break.[92][93]
JavaScript provides an interface to a wide range of browser capabilities, some of which may have flaws such asbuffer overflows. These flaws can allow attackers to write scripts that would run any code they wish on the user's system. This code is not by any means limited to another JavaScript application. For example, a buffer overrun exploit can allow an attacker to gain access to the operating system'sAPI with superuser privileges.
These flaws have affected major browsers including Firefox,[94] Internet Explorer,[95] and Safari.[96]
Plugins, such as video players,Adobe Flash, and the wide range ofActiveX controls enabled by default in Microsoft Internet Explorer, may also have flaws exploitable via JavaScript (such flaws have been exploited in the past).[97][98]
In Windows Vista, Microsoft has attempted to contain the risks of bugs such as buffer overflows by running the Internet Explorer process with limited privileges.[99]Google Chrome similarly confines its page renderers to their own "sandbox".
Sandbox implementation errors
Web browsers are capable of running JavaScript outside the sandbox, with the privileges necessary to, for example, create or delete files. Such privileges are not intended to be granted to code from the Web.
Incorrectly granting privileges to JavaScript from the Web has played a role in vulnerabilities in both Internet Explorer[100] and Firefox.[101] In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.[102]
Microsoft Windows allows JavaScript source files on a computer's hard drive to be launched as general-purpose, non-sandboxed programs (see:Windows Script Host). This makes JavaScript (likeVBScript) a theoretically viable vector for aTrojan horse, although JavaScript Trojan horses are uncommon in practice.[103][failed verification]
Hardware vulnerabilities
In 2015, a JavaScript-based proof-of-concept implementation of arowhammer attack was described in a paper by security researchers.[104][105][106][107]
In 2017, a JavaScript-based attack via browser was demonstrated that could bypassASLR. It is called "ASLR⊕Cache" or AnC.[108][109]
In 2018, the paper that announced theSpectre attacks against Speculative Execution in Intel and other processors included a JavaScript implementation.[110]
Some browsers have built-inprofilers. Stand-alone profiling libraries have also been created, such as benchmark.js and jsbench.[111][112]
Manytext editors have syntax highlighting support for JavaScript code.
Related technologies
Java
A common misconception is that JavaScript is directly related toJava. Both indeed have a C-like syntax (the C language being their most immediate common ancestor language). They are also typicallysandboxed, and JavaScript was designed with Java's syntax and standard library in mind. In particular, all Java keywords were reserved in original JavaScript, JavaScript's standard library follows Java's naming conventions, and JavaScript'sMath andDate objects are based on classes from Java 1.0.[113]
Both languages first appeared in 1995, but Java was developed byJames Gosling of Sun Microsystems and JavaScript byBrendan Eich of Netscape Communications.
The differences between the two languages are more prominent than their similarities. Java hasstatic typing, while JavaScript's typing isdynamic. Java is loaded fromcompiled bytecode, while JavaScript is loaded as human-readable source code. Java's objects areclass-based, while JavaScript's areprototype-based. Finally, Java did not support functional programming until Java 8, while JavaScript has done so from the beginning, being influenced byScheme.
JSON
JSON is a data format derived from JavaScript; hence the name JavaScript Object Notation. It is a widely used format supported by many other programming languages.
Transpilers
Many websites are JavaScript-heavy, sotranspilers have been created to convert code written in other languages, which can aid the development process.[35]
WebAssembly is a newer language with abytecode format designed to complement JavaScript, especially the performance-critical portions ofweb page scripts. All of the majorJavaScript engines support WebAssembly,[114] which runs in the samesandbox as regular JavaScript code.
asm.js is a subset of JavaScript that served as the forerunner of WebAssembly.[115]
^Clinick, Andrew (14 July 2000)."Introducing JScript .NET".Microsoft Developer Network. Microsoft.Archived from the original on 10 November 2017. Retrieved10 April 2018.[S]ince the 1996 introduction of JScript version 1.0 ... we've been seeing a steady increase in the usage of JScript on the server—particularly in Active Server Pages (ASP)
^Gruss, Daniel; Maurice, Clémentine; Mangard, Stefan (24 July 2015). "Rowhammer.js: A Remote Software-Induced Fault Attack in JavaScript".arXiv:1507.06955 [cs.CR].
