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{{Short description|High-level programming language}}
{{Distinguish|Java (programming language)|Javanese script|ECMAScript}}
{{Redirect|.js|the Microsoft dialect used in Internet Explorer|JScript|}}
{{For|the uses of JavaScript on Wikipedia|Wikipedia:WikiProject JavaScript|selfref=yes}}
{{pp-semi-indef|small=yes}}
{{Use dmy dates|date=March 2024}}
 
{{Infobox programming language
| name = JavaScript
| logo = <!-- Do not add the unofficial logo, per [[Talk:JavaScript/Archive 5#Why is the unofficial logo present?]] -->
| logo alt =
| screenshot = JavaScript code.png
| screenshot caption = Screenshot of JavaScript source code
| paradigmparadigms = [[Multi-paradigm]]: [[eventEvent-driven programming|event-driven]], [[functionalFunctional programming|functional]], [[imperativeImperative programming|imperative]], [[proceduralProcedural programming|procedural]], [[objectObject-oriented programming|object-oriented]]
| memory management = [[Garbage collection (computer science)|Garbage collected]]
| designer = [[Brendan Eich]] of [[Netscape]] initially; others have also contributed to the [[ECMAScript]] standard
| family = [[ECMAScript]]
| typing = [[Dynamic typing|Dynamic]], [[Weak typing|weak]], [[Duck typing|duck]]
| designers = [[Brendan Eich]] of [[Netscape]] first; then others contributed to [[ECMAScript]] standard
| implementations = [[V8 (JavaScript engine)|V8]], [[JavaScriptCore]], [[SpiderMonkey (JavaScript engine)|SpiderMonkey]], [[Chakra (JScript engine)|Chakra]]
| typing = [[Dynamic typing|Dynamic]], [[Strong and weak typing|weak]], [[Duck typing|duck]]
| influenced = [[ActionScript]], [[AssemblyScript]], [[CoffeeScript]], [[Dart (programming language)|Dart]], [[Haxe]], [[JS++]], [[Opa (programming language)|Opa]], [[TypeScript]], [[ArkTS]]
| scope = [[Scope (computer science)|lexical]]
| license =
| released = {{Start date and age|1995|12|04|df=y}}<ref name="press_release">{{Cite press release |url=https://wp.netscape.com/newsref/pr/newsrelease67.html |archive-url=https://web.archive.org/web/20070916144913/https://wp.netscape.com/newsref/pr/newsrelease67.html |archive-date=2007-09-16 |title=Netscape and Sun announce JavaScript, the Open, Cross-platform Object Scripting Language for Enterprise Networks and the Internet |date=December 4, 1995}}</ref>
| file extensions = {{flatlist|
* <code>.js</code>
* <code>.cjs</code>
* <code>.mjs</code><ref name="node.js ECMAScript Modules Specification">{{cite web|url=https://github.com/nodejs/node-eps/blob/master/002-es-modules.md|title=nodejs/node-eps|website=GitHub|access-date=2018-07-05|archive-date=2020-08-29|archive-url=https://web.archive.org/web/20200829024713/https://github.com/nodejs/node-eps/blob/master/002-es-modules.md|url-status=live}}</ref>
}}
| file format =
| wikibooks = JavaScript
| year = {{start date and age|1995|12|4}}<ref name="press_release">{{Cite press release |url=https://wp.netscape.com/newsref/pr/newsrelease67.html |archive-url=https://web.archive.org/web/20070916144913/https://wp.netscape.com/newsref/pr/newsrelease67.html |archive-date=2007-09-16 |title=Netscape and Sun announce JavaScript, the Open, Cross-platform Object Scripting Language for Enterprise Networks and the Internet |date=December 4, 1995}}</ref>
| latest release version = {{wikidata|property|edit|reference|P548=Q2804309|P348}}
| latest release date = {{start date and age|{{wikidata|qualifier|single|P548=Q2804309|P348|P577}}}}
| latest preview version = {{wikidata|property|edit|reference|P548=Q51930650|P348}}
| latest preview date = {{start date and age|{{wikidata|qualifier|single|P548=Q51930650|P348|P577}}}}
| license =
| influenced by = [[Java (programming language)|Java]],<ref name="looklikejava">{{cite book|last=Seibel|first=Peter|title=Coders at Work: Reflections on the Craft of Programming|isbn=9781430219484|quote="Eich: The immediate concern at Netscape was it must look like Java."|url=https://books.google.com/books?id=nneBa6-mWfgC&q=The+immediate+concern+at+Netscape+was+it+must+look+like+Java.&pg=PA141|access-date=December 25, 2018|date=September 16, 2009|publisher=Apress |archive-date=December 24, 2020|archive-url=https://web.archive.org/web/20201224233514/https://books.google.com/books?id=nneBa6-mWfgC&q=The+immediate+concern+at+Netscape+was+it+must+look+like+Java.&pg=PA141|url-status=live}}</ref><ref name="origin"/> [[Scheme (programming language)|Scheme]],<ref name="origin"/> [[Self (programming language)|Self]],<ref>{{cite web|url=https://brendaneich.com/2008/04/popularity/|title = Popularity – Brendan Eich}}</ref> [[AWK]],<ref>{{cite web|title=Brendan Eich: An Introduction to JavaScript, JSConf 2010|website=[[YouTube]]|quote="Eich: "function", eight letters, I was influenced by AWK."|url=https://www.youtube.com/watch?v=1EyRscXrehw|access-date=November 25, 2019|page=22m|archive-date=August 29, 2020|archive-url=https://web.archive.org/web/20200829024704/https://www.youtube.com/watch?v=1EyRscXrehw|url-status=live}}</ref> [[HyperTalk]]<ref>{{cite book |last=Eich |first=Brendan |authorlink1=Brendan Eich |chapter=Foreword |editor1-last=Goodman |editor1-first=Danny |editor1-link=Danny Goodman |year=1998 |title=JavaScript Bible |edition=3rd |publisher=[[John Wiley & Sons]] |isbn=0-7645-3188-3 |lccn=97078208 |oclc=38888873 |ol=712205M |url-access=registration |url=https://archive.org/details/javascriptbible000good}}</ref>
| file extensions = <code>.js</code> • <code>.mjs</code> • <code>.cjs</code><ref name="node.js ECMAScript Modules Specification">{{cite web|url=https://github.com/nodejs/node-eps/blob/master/002-es-modules.md|title=nodejs/node-eps|website=GitHub|access-date=2018-07-05|archive-date=2020-08-29|archive-url=https://web.archive.org/web/20200829024713/https://github.com/nodejs/node-eps/blob/master/002-es-modules.md|url-status=live}}</ref>
| file format =
| website = {{URL|https://ecma-international.org/publications-and-standards/standards/ecma-262/}}
| implementations = [[V8 (JavaScript engine)|V8]], [[JavaScriptCore]], [[SpiderMonkey]];<br/>[[Chakra (JavaScript engine)|Chakra]] (deprecated)
| influenced by = [[Java (programming language)|Java]],<ref name="looklikejava">{{cite book |last1=Seibel |first1=Peter |date=September 16, 2009 |title=Coders at Work: Reflections on the Craft of Programming |publisher=Apress |isbn=978-1-4302-1948-4 |quote=Eich: The immediate concern at Netscape was it must look like Java. |url=https://books.google.com/books?id=nneBa6-mWfgC&q=The+immediate+concern+at+Netscape+was+it+must+look+like+Java.&pg=PA141 |access-date=December 25, 2018 |url-status=live |archive-url=https://web.archive.org/web/20201224233514/https://books.google.com/books?id=nneBa6-mWfgC&q=The+immediate+concern+at+Netscape+was+it+must+look+like+Java.&pg=PA141 |archive-date=December 24, 2020}}</ref><ref name="origin"/> [[Scheme (programming language)|Scheme]],<ref name="origin"/> [[Self (programming language)|Self]],<ref>{{cite web |url=https://brendaneich.com/2008/04/popularity/ |title = Popularity – Brendan Eich}}</ref> [[AWK]],<ref>{{cite web |title=Brendan Eich: An Introduction to JavaScript, JSConf 2010 |website=[[YouTube]] |date=20 January 2013 |quote=Eich: "function", eight letters, I was influenced by AWK. |url=https://www.youtube.com/watch?v=1EyRscXrehw |access-date=November 25, 2019 |page=22m |url-status=live |archive-url=https://web.archive.org/web/20200829024704/https://www.youtube.com/watch?v=1EyRscXrehw |archive-date=August 29, 2020}}</ref> [[HyperTalk]]<ref>{{cite book |last1=Eich |first1=Brendan |author1-link=Brendan Eich |chapter=Foreword |editor1-last=Goodman |editor1-first=Danny |editor1-link=Danny Goodman |year=1998 |title=JavaScript Bible |edition=3rd |publisher=[[John Wiley & Sons]] |isbn=0-7645-3188-3 |lccn=97078208 |oclc=38888873 |ol=712205M |url-access=registration |url=https://archive.org/details/javascriptbible000good}}</ref>
| influenced = [[ActionScript]], [[ArkTS]], [[AssemblyScript]], [[CoffeeScript]], [[Dart (programming language)|Dart]], [[Haxe]], [[JS++]], [[Opa (programming language)|Opa]], [[TypeScript]]
| wikibooks = JavaScript
}}
 
'''JavaScript''' ('''JS'''){{efn|{{IPAc-en|ˈ|dʒ|ɑː|v|ə|s|k|r|ɪ|p|t}}|audio=LL-Q1860 (eng)-Flame, oftennot abbreviated as '''JS''',lame-JavaScript.wav}}}} is a [[programming language]] and core technology of the [[Worldweb Wide Webplatform]], alongside [[HTML]] and [[CSS]]. {{as<!--don't of|2024}},add 98.9dating or decimals: it'll remain 99% indefinitely-->Ninety-nine percent of [[website]]s on the [[World Wide Web]] use JavaScript on the [[Client (computing)|client]] side for [[Web page|webpage]] behavior,.<ref name="deployedstats">{{Citecite web |title=Usage Statistics of JavaScript as Client-side Programming Language on Websites, January 2024 |url=https://w3techs.com/technologies/details/cp-javascript |access-date=2024-0102-2927 |website=w3techs.comW3Techs }}</ref> often incorporating third-party [[Library (computing)|libraries]]. All major [[web browser]]s have a dedicated [[JavaScript engine]] to execute the [[Source code|code]] on [[User (computing)|users]]' devices.
 
[[Web browser]]s have a dedicated [[JavaScript engine]] that executes the client [[Source code|code]]. These engines are also utilized in some [[Server (computing)|servers]] and a variety of [[Application software|apps]]. The most popular [[runtime system]] for non-browser usage is [[Node.js]]{{cn|date=August 2025}}.
 
JavaScript is a [[High-level programming language|high-level]], often [[Just-in-time compilation|just-in-time compiledtime–compiled]] language that conforms to the [[ECMAScript]] standard.<ref name="tc39">{{cite web|title=ECMAScript® 2020 Language Specification|url=https://tc39.es/ecma262/#sec-overview|url-status=live|archive-url=https://web.archive.org/web/20200508053013/https://tc39.es/ecma262/#sec-overview|archive-date=2020-05-08|access-date=2020-05-08}}</ref> It has [[dynamic typing]], [[Prototype-based programming|prototype-based]] [[object-oriented programming|object-orientation]], and [[first-class function]]s. It is [[Programming paradigm|multi-paradigm]], supporting [[Event-driven programming|event-driven]], [[functional programming|functional]], and [[imperative programming|imperative]] [[programming paradigm|programming stylestyles]]s. It has [[application programming interface]]s (APIs) for working with text, dates, [[regular expression]]s, standard [[data structure]]s, and the [[Document Object Model]] (DOM).
 
The ECMAScript standard does not include any [[input/output]] (I/O), such as [[computer network|networking]], [[data storage|storage]], or [[computer graphics|graphics]] facilities. In practice, the web browser or other [[runtime system]] provides JavaScript APIs for I/O.
 
Although [[Java (programming language)|Java]] and JavaScript are similar in name and [[Syntax (programming languages)|syntax]], the two languages are distinct and differ greatly in design.
[[JavaScript engines]] were originally used only in web browsers, but are now core components of some [[Server (computing)|servers]] and a variety of [[Application software|applications]]. The most popular [[runtime system]] for this usage is [[Node.js]].
 
== History ==
Although [[Java (programming language)|Java]] and JavaScript are similar in name, [[Syntax (programming languages)|syntax]], and respective [[Standard library|standard libraries]], the two languages are distinct and differ greatly in design.
=== Creation at Netscape ===
The first popular [[web browser]] with a [[graphical user interface]], [[Mosaic (web browser)|Mosaic]], was released in 1993. The lead developers of Mosaic then founded the [[Netscape]] corporation, which released a more polished browser, [[Netscape Navigator]], in 1994. This quickly became the most-used.<ref>{{cite web|url=https://www.mwdwebsites.com/nj-web-design-web-browsers.html|title=The Evolution of the Web Browsers|last=Enzer|first=Larry|date=August 31, 2018|website=Monmouth Web Developers|access-date=August 31, 2018|archive-date=August 31, 2018|archive-url=https://web.archive.org/web/20180831174847/https://www.mwdwebsites.com/nj-web-design-web-browsers.html|url-status=dead}}</ref>
 
During these formative years of the Web, [[web page]]s 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 a [[programming language]] to Navigator. They pursued two routes to achieve this: collaborating with [[Sun Microsystems]] to embed the [[Java (programming language)|Java]] language, while also hiring [[Brendan Eich]] to embed the [[Scheme (programming language)|Scheme]] language.<ref name="origin">{{cite web|url=https://exploringjs.com/es5/ch04.html|title=Chapter 4. How JavaScript Was Created|website=speakingjs.com|access-date=2017-11-21|archive-date=2020-02-27|archive-url=https://web.archive.org/web/20200227184037/https://speakingjs.com/es5/ch04.html|url-status=live}}</ref>
==History==
===Creation at Netscape===
The first popular [[web browser]] with a [[graphical user interface]], [[Mosaic (web browser)|Mosaic]], was released in 1993. Accessible to non-technical people, it played a prominent role in the rapid growth of the early [[World Wide Web]].<ref>{{cite news |date=March 17, 2011 |title=Bloomberg Game Changers: Marc Andreessen |work=[[Bloomberg L.P.|Bloomberg]] |publisher=Bloomberg |url=https://www.bloomberg.com/video/67758394 |url-status=live |access-date=December 7, 2011 |archive-url=https://web.archive.org/web/20120516093712/https://www.bloomberg.com/video/67758394/ |archive-date=May 16, 2012}}</ref> The lead developers of Mosaic then founded the [[Netscape]] corporation, which released a more polished browser, [[Netscape Navigator]], in 1994. This quickly became the most-used.<ref>{{cite web|url=https://www.mwdwebsites.com/nj-web-design-web-browsers.html|title=The Evolution of the Web Browsers|last=Enzer|first=Larry|date=August 31, 2018|website=Monmouth Web Developers|access-date=August 31, 2018|archive-date=August 31, 2018|archive-url=https://web.archive.org/web/20180831174847/https://www.mwdwebsites.com/nj-web-design-web-browsers.html|url-status=dead}}</ref>
 
The goal was a "language for the masses",<ref name="EichVideo" /> "to help nonprogrammers create dynamic, interactive [[website|Web sites]]".<ref>"Netscape Communications Corp.", Browser enhancements. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD</ref> 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 extant [[scripting language]]s.<ref name="looklikejava" /><ref name="origin" /> Although the new language and its [[Interpreter (computing)|interpreter]] implementation were called LiveScript when first shipped as part of a Navigator [[beta software|beta]] in September 1995, the name was changed to JavaScript for the official release in December.<ref name="origin" /><ref name="press_release" /><ref name="techvision">{{cite web | archive-url=https://web.archive.org/web/20080208124612/https://wp.netscape.com/comprod/columns/techvision/innovators_be.html | archive-date=February 8, 2008 | title=TechVision: Innovators of the Net: Brendan Eich and JavaScript | url=https://wp.netscape.com/comprod/columns/techvision/innovators_be.html }}</ref><ref name="wired2024">{{cite magazine |last1=Han |first1=Sheon |title=JavaScript Runs the World—Maybe Even Literally |url=https://www.wired.com/story/javascript-runs-the-world-maybe-literally/ |magazine=Wired |date=4 March 2024 |access-date=21 August 2024}}</ref>
During these formative years of the Web, [[web page]]s 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 a [[scripting language]] to Navigator. They pursued two routes to achieve this: collaborating with [[Sun Microsystems]] to embed the [[Java (programming language)|Java]] [[programming language]], while also hiring [[Brendan Eich]] to embed the [[Scheme (programming language)|Scheme]] language.<ref name="origin">{{cite web|url=https://exploringjs.com/es5/ch04.html|title=Chapter 4. How JavaScript Was Created|website=speakingjs.com|access-date=2017-11-21|archive-date=2020-02-27|archive-url=https://web.archive.org/web/20200227184037/https://speakingjs.com/es5/ch04.html|url-status=live}}</ref>
 
The choice of the ''JavaScript'' name has caused confusion, implying that it is directly related to Java. At the time, the [[Dot-com bubble|dot-com boom]] had begun and Java was a popular new language, so Eich considered the JavaScript name a marketing ploy by Netscape.<ref name="EichVideo">{{Citation|last=Fin JS|title=Brendan Eich&nbsp;– CEO of Brave|date=June 17, 2016|url=https://www.youtube.com/watch?v=XOmhtfTrRxc |website=YouTube |access-date=February 7, 2018 }}</ref>
The original goal was a "language for the masses",<ref name=":0" /> "to help [[programmer|nonprogrammers]] create dynamic, interactive [[website|Web sites]]".<ref>"Netscape Communications Corp.", Browser enhancements. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD</ref> In this vein, 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 extant [[Scripting language|scripting languages]].<ref name="looklikejava"/><ref name="origin"/> Although the new language and its [[Interpreter (computing)|interpreter]] implementation were called LiveScript when first shipped as part of a Navigator [[beta software|beta]] in September 1995, the name was changed to JavaScript for the official release in December.<ref name="origin"/><ref name="press_release" /><ref name="techvision">{{cite web | archive-url=https://web.archive.org/web/20080208124612/https://wp.netscape.com/comprod/columns/techvision/innovators_be.html | archive-date=February 8, 2008 | title=TechVision: Innovators of the Net: Brendan Eich and JavaScript | url=https://wp.netscape.com/comprod/columns/techvision/innovators_be.html }}</ref>
 
=== Adoption by Microsoft ===
The choice of the JavaScript name has caused confusion, implying that it is directly related to Java. At the time, the [[Dot-com bubble|dot-com boom]] had begun and Java was a popular new language, so Eich considered the JavaScript name a marketing ploy by Netscape.<ref name=":0">{{Citation|last=Fin JS|title=Brendan Eich&nbsp;– CEO of Brave|date=June 17, 2016|url=https://www.youtube.com/watch?v=XOmhtfTrRxc&t=2m5s|access-date=February 7, 2018|archive-date=February 10, 2019|archive-url=https://web.archive.org/web/20190210054957/https://www.youtube.com/watch?v=XOmhtfTrRxc&t=2m5s|url-status=live}}</ref>
[[Microsoft]] debuted [[Internet Explorer]] in 1995, leading to a [[browser war]] with Netscape. On the JavaScript front, Microsoft created its own [[Interpreter (computing)|interpreter]] called [[JScript]].<ref name="sjsch5">{{cite web |title=Chapter 5. Standardization: ECMAScript |url=https://speakingjs.com/es5/ch05.html |website=speakingjs.com |access-date=1 November 2021 |archive-date=1 November 2021 |archive-url=https://web.archive.org/web/20211101184346/http://speakingjs.com/es5/ch05.html |url-status=dead }}</ref>
 
Microsoft first released JScript in 1996, alongside initial support for [[CSS]] and extensions to [[HTML]]. Each of these [[implementation]]s was noticeably different from their counterparts in [[Netscape Navigator]].<ref name="O'Reilly-2001">{{cite web |url=https://archive.oreilly.com/pub/a/javascript/2001/04/06/js_history.html |title=JavaScript, How Did We Get Here? |last=Champeon |first=Steve |date=April 6, 2001 |website=oreilly.com |access-date=July 16, 2016 |archive-url=https://web.archive.org/web/20160719020828/https://archive.oreilly.com/pub/a/javascript/2001/04/06/js_history.html |archive-date=July 19, 2016 |url-status=dead}}</ref><ref>{{cite web |url=https://news.microsoft.com/1996/05/29/microsoft-internet-explorer-3-0-beta-now-available/ |title=Microsoft Internet Explorer 3.0 Beta Now Available |publisher=Microsoft |date=May 29, 1996 |website=microsoft.com |access-date=July 16, 2016 |archive-date=November 24, 2020 |archive-url=https://web.archive.org/web/20201124154053/https://news.microsoft.com/1996/05/29/microsoft-internet-explorer-3-0-beta-now-available/ |url-status=live }}</ref> 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.<ref name="O'Reilly-2001" /><ref>{{cite web |url=https://www.technologizer.com/2010/09/16/the-unwelcome-return-of-best-viewed-with-internet-explorer/ |title=The Unwelcome Return of "Best Viewed with Internet Explorer" |last=McCracken |first=Harry |date=September 16, 2010 |website=technologizer.com |access-date=July 16, 2016 |archive-date=June 23, 2018 |archive-url=https://web.archive.org/web/20180623192402/https://www.technologizer.com/2010/09/16/the-unwelcome-return-of-best-viewed-with-internet-explorer/ |url-status=live }}</ref>
===Adoption by Microsoft===
[[Microsoft]] debuted [[Internet Explorer]] in 1995, leading to a [[browser war]] with Netscape. On the JavaScript front, Microsoft [[Reverse engineering of software|reverse-engineered]] the Navigator [[Interpreter (computing)|interpreter]] to create its own, called [[JScript]].<ref name="sjsch5">{{cite web |title=Chapter 5. Standardization: ECMAScript |url=https://speakingjs.com/es5/ch05.html |website=speakingjs.com |access-date=1 November 2021 |archive-date=1 November 2021 |archive-url=https://web.archive.org/web/20211101184346/http://speakingjs.com/es5/ch05.html |url-status=dead }}</ref>
 
=== The rise of JScript ===
JavaScript was first released in 1996, alongside initial support for [[CSS]] and extensions to [[HTML]]. Each of these [[Implementation|implementations]] was noticeably different from their counterparts in Navigator.<ref name="O'Reilly-2001">{{cite web |url=https://archive.oreilly.com/pub/a/javascript/2001/04/06/js_history.html |title=JavaScript, How Did We Get Here? |last=Champeon |first=Steve |date=April 6, 2001 |website=oreilly.com |access-date=July 16, 2016 |archive-url=https://web.archive.org/web/20160719020828/https://archive.oreilly.com/pub/a/javascript/2001/04/06/js_history.html |archive-date=July 19, 2016 |url-status=dead}}</ref><ref>{{cite web |url=https://news.microsoft.com/1996/05/29/microsoft-internet-explorer-3-0-beta-now-available/ |title=Microsoft Internet Explorer 3.0 Beta Now Available |publisher=Microsoft |date=May 29, 1996 |website=microsoft.com |access-date=July 16, 2016 |archive-date=November 24, 2020 |archive-url=https://web.archive.org/web/20201124154053/https://news.microsoft.com/1996/05/29/microsoft-internet-explorer-3-0-beta-now-available/ |url-status=live }}</ref> 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.<ref name="O'Reilly-2001" /><ref>{{cite web |url=https://www.technologizer.com/2010/09/16/the-unwelcome-return-of-best-viewed-with-internet-explorer/ |title=The Unwelcome Return of "Best Viewed with Internet Explorer" |last=McCracken |first=Harry |date=September 16, 2010 |website=technologizer.com |access-date=July 16, 2016 |archive-date=June 23, 2018 |archive-url=https://web.archive.org/web/20180623192402/https://www.technologizer.com/2010/09/16/the-unwelcome-return-of-best-viewed-with-internet-explorer/ |url-status=live }}</ref>
{{Quote box|[[Brendan Eich]] later said of this period: "It's still kind of a [[sidekick]] language. It's considered slow or annoying. People do [[Pop-up ad|pop-ups]] or those scrolling messages in the old [[status bar]] at the bottom of your old [[web browser|browser]]."<ref name="EichVideo" />|width=30%}}
 
===The rise of JScript===
In November 1996, [[Netscape]] submitted JavaScript to [[Ecma International]], as the starting point for a standard specification that all browser vendors could conform to. This led to the official release of the first [[ECMAScript]] 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.<ref name="sjsch5" />
 
MeanwhileHowever, [[the effort to fully standardize the language was undermined by Microsoft]] gainedgaining an increasingly dominant position in the browser market. By the early 2000s, [[Internet Explorer]]'s market share reached 95%.<ref name="searchenginejournal.com">{{cite news |last1=Baker |first1=Loren |date=November 24, 2004 |url=https://www.searchenginejournal.com/mozilla-firefox-internet-browser-market-share-gains-to-74/1082/ | title=Mozilla Firefox Internet Browser Market Share Gains to 7.4% | first=Loren | last=Baker | work=Search Engine Journal | date=November 24, 2004 | access-date=May 8, 2021 | archive-date=May 7, 2021 | archive-url=https://web.archive.org/web/20210507013607/https://www.searchenginejournal.com/mozilla-firefox-internet-browser-market-share-gains-to-74/1082/ | url-status=live }}</ref> This meant that [[JScript]] became the de facto standard for [[client-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 EcmaECMA work. Thus ECMAScript 4 was mothballed.
 
=== Growth and standardization ===
{{Plain image with caption|Logo of TC39.svg|Logo of [[Ecma International]] Technical Committee 39 who is responsible for maintaining the standard|150px}}
During the period of [[Internet Explorer]] dominance in the early 2000s, client-side scripting was stagnant. This started to change in 2004, when the successor of Netscape, [[Mozilla]], released the [[Firefox]] browser. Firefox was well received by many, taking significant market share from Internet Explorer.<ref>{{cite news |url=https://news.bbc.co.uk/2/hi/business/4508897.stm |title=The assault on software giant Microsoft |last=Weber |first=Tim |date=May 9, 2005 |work=[[BBC News]] |archive-url=https://web.archive.org/web/20170925233936/https://news.bbc.co.uk/2/hi/business/4508897.stm |archive-date=September 25, 2017}}</ref>
 
In 2005, Mozilla joined ECMA International, and work started on the [[ECMAScript for XML]] (E4X) standard. This led to Mozilla working jointly with [[Macromedia]] (later acquired by [[Adobe 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 the [[Tamarin (software)|Tamarin]] implementation as an [[Open-source model|open 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 term [[Ajax (programming)|Ajax]] and described a set of technologies, of which JavaScript was the backbone, to create [[web application]]s 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, including [[jQuery]], [[Prototype JavaScript Framework|Prototype]], [[Dojo Toolkit]], and [[MooTools]].
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In July 2008, these disparate parties came together for a conference in [[Oslo]]. 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 of [[ECMAScript]] 6]] in 2015.<ref>{{cite web |url=https://es6-features.org/ |title=ECMAScript 6: New Features: Overview and Comparison |website=es6-features.org |access-date=March 19, 2018 |archive-date=March 18, 2018 |archive-url=https://web.archive.org/web/20180318064130/https://es6-features.org/ |url-status=liveusurped }}</ref>
 
The creation of [[Node.js]] in 2009 by [[Ryan Dahl]] sparked a significant increase in the usage of JavaScript outside of web browsers. Node combines the [[V8 (JavaScript engine)|V8]] engine, an [[event loop]], and [[Input/output|I/O]] [[Application programming interface|APIs]], thereby providing a stand-alone JavaScript runtime system.<ref>[https://books.google.com/books?id=ZH6bpbcrlvYC&q=nodejs Professional Node.js: Building JavaScript Based Scalable Software] {{Webarchive|url=https://web.archive.org/web/20170324021220/https://books.google.com/books?id=ZH6bpbcrlvYC&printsec=frontcover&dq=nodejs&hl=en&sa=X#v=onepage&q=nodejs&f=false |date=2017-03-24 }}, John Wiley & Sons, 01-Oct-2012</ref><ref name="b3">[https://books.google.com/books?id=KGt-FxUEj48C&dq=nodejs&pg=PT24 Sams Teach Yourself Node.js in 24 Hours] {{Webarchive|url=https://web.archive.org/web/20170323192039/https://books.google.com/books?id=KGt-FxUEj48C&pg=PT24&dq=nodejs&hl=en&sa=X#v=onepage&q=nodejs&f=false |date=2017-03-23 }}, Sams Publishing, 05-Sep-2012</ref> As of 2018, Node had been used by millions of developers,<ref>{{cite web |last1=Lawton |first1=George |title=The secret history behind the success of npm and Node |url=https://www.theserverside.com/blog/Coffee-Talk-Java-News-Stories-and-Opinions/The-secret-history-behind-the-success-of-npm-and-Node |website=TheServerSide |access-date=2 August 2021 |date=19 July 2018 |archive-date=2 August 2021 |archive-url=https://web.archive.org/web/20210802165613/https://www.theserverside.com/blog/Coffee-Talk-Java-News-Stories-and-Opinions/The-secret-history-behind-the-success-of-npm-and-Node |url-status=live }}</ref> and [[npm (software)|npm]] had the most modules of any [[package manager]] in the world.<ref>{{cite web |last1=Brown |first1=Paul |title=State of the Union: npm |url=https://www.linux.com/news/state-union-npm/ |website=Linux.com |access-date=2 August 2021 |date=13 January 2017 |archive-date=2 August 2021 |archive-url=https://web.archive.org/web/20210802165614/https://www.linux.com/news/state-union-npm/ |url-status=live }}</ref>
 
The ECMAScript draft specification is currently{{As of?|date=September 2023}} maintained openly on [[GitHub]],<ref name=branscombe /> and editions are produced via regular annual snapshots.<ref name=branscombe>{{cite web |url=https://thenewstack.io/whats-new-es2016/ |title=JavaScript Standard Moves to Yearly Release Schedule; Here is What's New for ES16 |last=Branscombe |first=Mary |date=2016-05-04 |access-date=2021-01-15 |work=The New Stack |archive-date=2021-01-16 |archive-url=https://web.archive.org/web/20210116181757/https://thenewstack.io/whats-new-es2016/ |url-status=live }}</ref> Potential revisions to the language are vetted through a comprehensive proposal process.<ref>{{cite web|url=https://tc39.es/process-document/|title=The TC39 Process|publisher=Ecma International|work=tc39.es|access-date=2021-01-15|archive-date=2021-02-07|archive-url=https://web.archive.org/web/20210207105535/https://tc39.es/process-document/|url-status=live}}</ref><ref>{{cite web |url=https://github.com/tc39/proposals/blob/master/README.md |title=ECMAScript proposals |access-date=2021-01-15 |publisher=TC39 |archive-date=2020-12-04 |archive-url=https://web.archive.org/web/20201204221147/https://github.com/tc39/proposals/blob/master/README.md |url-status=live }}</ref> Now, instead of edition numbers, developers check the status of upcoming features individually.<ref name=branscombe />
 
The current JavaScript ecosystem has many [[List of JavaScript libraries|libraries]] and [[web framework|frameworks]], established programming practices, and substantial usage of JavaScript outside of web browsers.<ref name="wired2024" /> Plus, with the rise of [[single-page application]]s and other JavaScript-heavy websites, several [[source-to-source compiler|transpilers]] have been created to aid the development process.<ref name="transpilers" />
 
== Trademark ==
"JavaScript" is a [[trademark]] of [[Oracle Corporation]] in the United States.<ref>{{cite web |date=1997-05-06 |title=U.S. Trademark Serial No. 75026640 |url=https://tsdr.uspto.gov/#caseNumber=75026640&caseType=SERIAL_NO&searchType=statusSearch |url-status=live |archive-url=https://web.archive.org/web/20210713022850/https://tsdr.uspto.gov/#caseNumber=75026640&caseType=SERIAL_NO&searchType=statusSearch |archive-date=2021-07-13 |access-date=2021-05-08 |website=uspto.gov |publisher=[[United States Patent and Trademark Office]]}}</ref><ref>{{cite web |title=Legal Notices |url=https://www.oracle.com/legal/trademarks.html |url-status=live |archive-url=https://web.archive.org/web/20210605142505/https://www.oracle.com/legal/trademarks.html |archive-date=2021-06-05 |access-date=2021-05-08 |website=oracle.com |publisher=[[Oracle Corporation]]}}</ref> The trademark was originally issued to [[Sun Microsystems]] on 6 May 1997, and was transferred to Oracle when they acquired Sun in 2009.<ref>{{Cite news |title=Oracle to buy Sun in $7.4-bn deal | newspaper=The Economic Times |url=https://economictimes.indiatimes.com/tech/software/oracle-to-buy-sun-in-7-4-bn-deal/articleshow/4427747.cms | date = 21 April 2009}}</ref><ref>{{Cite news |title=Oracle urged again to give up JavaScript trademark |work=The Register |url=https://www.theregister.com/2024/09/17/oracle_urged_to_surrender_javascript_trademark/ |date=17 September 2024 |last1=Claburn |first1=Thomas |access-date=2 February 2025}}</ref>
 
A letter was circulated in September 2024, spearheaded by [[Ryan Dahl]], calling on Oracle to free the JavaScript trademark.<ref>{{Cite news|last=Krill|first=Paul|date=September 20, 2024|title=JavaScript community challenges Oracle's JavaScript trademark|url=https://www.infoworld.com/article/3532457/javascript-community-challenges-oracles-javascript-trademark.html|work=InfoWorld}}</ref> [[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 dominant [[client-side]] [[scripting language]] of the Web, with 9899% of all [[website]]s {{nowrap|(mid{{ndash}}2022)}} using it for this purpose.<ref name="deployedstats">{{cite web |date=2021-04-09 |title=Usage statistics of JavaScript as client-side programming language on websites |url=https://w3techs.com/technologies/details/cp-javascript/ |url-status=live |archive-url=https://wayback.archive-it.org/all/20220213043439/https://w3techs.com/technologies/details/cp%2Djavascript |archive-date=2022-02-13 |access-date=2021-04-09 |website=w3techs.com }}</ref> Scripts are embedded in or included from [[HTML]] documents and interact with the [[Document Object Model|DOM]].
 
All major [[web browser]]s have a built-in [[JavaScript engine]] that executes the [[Source code|code]] on the user's device.
 
=== Examples of scripted behavior ===
* Loading new [[web page]] content without reloading the page, via [[Ajax (programming)|Ajax]] or a [[WebSocket]]. For example, [[user (computing)|userusers]]s of [[social 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.
* Playing [[browser game]]s.
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* Storing and retrieving data on the user's device, via the [[web storage|storage]] or [[Indexed Database API|IndexedDB]] standards.
 
=== Web librariesLibraries and frameworks ===
By 2012, overOver 80% of websites useduse a third-party JavaScript [[Library (computing)|library]] or [[web framework]] foras part of their client-side scripting.<ref name="lib_usage">{{cite web|url=https://w3techs.com/technologies/overview/javascript_library|title=Usage statistics of JavaScript libraries for websites|website=w3techs.comW3Techs|access-date=2021-04-09|archive-date=2012-05-26|archive-url=https://archive.today/20120526001910/http://w3techs.com/technologies/overview/javascript_library/all|url-status=live}}</ref>
 
==== React ====
{{excerpt|React (JavaScript library)|only=paragraphs}}
 
==== Angular ====
{{Excerpt|Angular (web framework)|only=paragraphs}}
 
==== jQuery ====
{{Main|jQuery}}
 
In 2012, [[jQuery]] was by far the most popular client-side library, used by over 75% of websites.<ref name="lib_usage"/> While still in widespread use, its popularity has been surpassed by newer libraries and frameworks.
 
[[jQuery]] is by far the most-used.<ref name="lib_usage" /> Other notable ones include <!--alphabetically is fair here-->[[Angular (web framework)|Angular]], [[Bootstrap (front-end framework)|Bootstrap]], [[Lodash]], [[Modernizr]], [[React (software)|React]], [[Underscore.js|Underscore]], and [[Vue.js|Vue]].<ref name="lib_usage" /> Multiple options can be used in conjunction, such as jQuery and Bootstrap.<ref>{{cite web |title=Using jQuery with Bootstrap |url=https://clouddevs.com/jquery/web-projects-with-bootstrap/ |website=clouddevs.com |date=10 June 2019 |access-date=17 March 2024}}</ref>
==== Vanilla JS ====
The framework-like moniker "Vanilla JS" is just another name for plain old JavaScript. It has been coined for websites not using any libraries or frameworks at all, instead relying entirely on standard JavaScript functionality.<ref>{{cite web |date=2020-06-16 |title=Vanilla JS |url=https://vanilla-js.com/ |url-status=live |archive-url=https://web.archive.org/web/20200616052335/https://vanilla-js.com/ |archive-date=June 16, 2020 |access-date=June 17, 2020 |website=vanilla-js.com}}</ref> Example of usage: "What framework do you use?" "Oh, we use Vanilla JS."
 
However, the term "Vanilla JS" was coined for websites not using any libraries or frameworks at all, instead relying entirely on standard JavaScript functionality.<ref>{{cite web |date=2020-06-16 |title=Vanilla JS |url=https://vanilla-js.com/ |url-status=live |archive-url=https://web.archive.org/web/20200616052335/https://vanilla-js.com/ |archive-date=June 16, 2020 |access-date=June 17, 2020 |website=vanilla-js.com}}</ref>
The term is included here to avoid confusion.
 
== Other usage{{anchor|Server-side_JavaScript|Uses_outside_web_pages}} ==
The use of JavaScript has expanded beyond its [[web browser]] roots. [[JavaScript engine]]s are now [[embedded<!--don't link Embedded system|embedded]] here--> in a variety of other software systems, both for [[server-side]] website deployments and non-browser [[application software|applications]].
 
Initial attempts at promoting server-side JavaScript usage were [[Netscape Enterprise Server]] and [[Microsoft]]'s [[Internet Information Services]],<ref>{{cite web |date=December 11, 1998 |title=Server-Side JavaScript Guide |url=https://docs.oracle.com/cd/E19957-01/816-6411-10/contents.htm |url-status=live |archive-url=https://web.archive.org/web/20210311173120/https://docs.oracle.com/cd/E19957-01/816-6411-10/contents.htm |archive-date=March 11, 2021 |access-date=May 8, 2021 |website=oracle.com |publisher=[[Oracle Corporation]]}}</ref><ref>{{cite web|last1=Clinick|first1=Andrew|title=Introducing JScript .NET|url=https://msdn.microsoft.com/en-us/library/ms974588.aspx|website=Microsoft Developer Network|publisher=Microsoft|access-date=April 10, 2018|date=July 14, 2000|quote=[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)|archive-date=November 10, 2017|archive-url=https://web.archive.org/web/20171110201649/https://msdn.microsoft.com/en-us/library/ms974588.aspx|url-status=live}}</ref> but they were small niches.<ref name="2009server" /> Server-side usage eventually started to grow in the late 2000s, with the creation of [[Node.js]] and [[List of server-side JavaScript implementations|other approaches]].<ref name="2009server">{{cite web |url=https://readwrite.com/2009/12/17/server-side_javascript_back_with_a_vengeance/ |title=Server-Side JavaScript, Back with a Vengeance |date=December 17, 2009 |website=readwrite.com |access-date=July 16, 2016 |author=Mahemoff, Michael |archive-date=June 17, 2016 |archive-url=https://web.archive.org/web/20160617030219/https://readwrite.com/2009/12/17/server-side_javascript_back_with_a_vengeance/ |url-status=live }}</ref>
 
[[Electron (software framework)|Electron]], [[Apache Cordova|Cordova]], [[React Native]], and other [[application framework]]s have been used to create many applications with behavior implemented in JavaScript. Other non-browser applications include [[Adobe Acrobat]] support for scripting [[PDF]] documents<ref>{{cite web |date=2009-08-07 |title=JavaScript for Acrobat |url=https://www.adobe.com/devnet/acrobat/javascript.html |url-status=live |archive-url=https://web.archive.org/web/20090807065130/https://www.adobe.com/devnet/acrobat/javascript.html |archive-date=August 7, 2009 |access-date=August 18, 2009 |website=adobe.com}}</ref> and [[GNOME Shell]] extensions written in JavaScript.<ref>{{cite web |last=treitter |date=2013-02-02 |title=Answering the question: "How do I develop an app for GNOME?" |url=https://treitter.livejournal.com/14871.html |url-status=live |archive-url=https://web.archive.org/web/20130211032900/https://treitter.livejournal.com/14871.html |archive-date=2013-02-11 |access-date=2013-02-07 |website=livejournal.com}}</ref>
 
[[Oracle Corporation|Oracle]] used to provide [[Nashorn (JavaScript engine)| Nashorn]], a JavaScript interpreter, as part of their [[JDK|Java Development Kit (JDK)]] API library along with '''jjs''' 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 the [[OpenJDK]] which allows one to create and reference Java objects in JavaScript code and add runtime scripting in JavaScript to applications written in Java.<ref> {{cite web |last=Ponge |first=Julien |date=2018-04-19 |title=Oracle Nashorn: A Next-Generation JavaScript Engine for the JVM |website=oracle.com |url=https://www.oracle.com/technical-resources/articles/java/jf14-nashorn.html |publisher=Oracle Corporation |access-date=Feb 17, 2025}} </ref><ref>{{cite web |author=<!-- not stated --> |title=Migration Guide from Nashorn to GraalJS |url=https://www.graalvm.org/latest/reference-manual/js/NashornMigrationGuide/ |website=graalvm.org | access-date=Feb 17, 2025}} </ref><ref>{{cite web |author=<!-- not stated --> |title=GraalJS |url=https://www.graalvm.org/latest/reference-manual/js/ |website=|publisher=GraalVM |access-date=Feb 17, 2025}}</ref><ref>{{cite web |author=<!-- not stated --> |title=Java Interoperability | url=https://docs.oracle.com/en/graalvm/enterprise/21/docs/reference-manual/js/JavaInteroperability/|website=oracle.com|publisher=Oracle |access-date=Feb 17, 2025}}</ref>
JavaScript has recently begun to appear in some [[embedded system]]s, usually by leveraging Node.js.<ref>{{cite web |title=Tessel 2... Leverage all the libraries of Node.JS to create useful devices in minutes with Tessel. |url=https://tessel.io/ |url-status=live |archive-url=https://web.archive.org/web/20210526212559/https://tessel.io/ |archive-date=2021-05-26 |access-date=2021-05-08 |website=tessel.io}}</ref><ref>{{cite web |title=Node.js Raspberry Pi GPIO Introduction |url=https://www.w3schools.com/nodejs/nodejs_raspberrypi_gpio_intro.asp |url-status=live |archive-url=https://web.archive.org/web/20210813192938/https://www.w3schools.com/nodejs/nodejs_raspberrypi_gpio_intro.asp |archive-date=2021-08-13 |access-date=2020-05-03 |website=w3schools.com}}</ref><ref>{{cite web |title=Espruino&nbsp;– JavaScript for Microcontrollers |url=https://www.espruino.com/ |url-status=live |archive-url=https://web.archive.org/web/20200501010722/https://www.espruino.com/ |archive-date=2020-05-01 |access-date=2020-05-03 |website=espruino.com}}</ref>
 
JavaScript has been used in some [[embedded system]]s, usually by leveraging Node.js.<ref>{{cite web |title=Tessel 2... Leverage all the libraries of Node.JS to create useful devices in minutes with Tessel. |url=https://tessel.io/ |url-status=live |archive-url=https://web.archive.org/web/20210526212559/https://tessel.io/ |archive-date=2021-05-26 |access-date=2021-05-08 |website=tessel.io}}</ref><ref>{{cite web |title=Node.js Raspberry Pi GPIO Introduction |url=https://www.w3schools.com/nodejs/nodejs_raspberrypi_gpio_intro.asp |url-status=live |archive-url=https://web.archive.org/web/20210813192938/https://www.w3schools.com/nodejs/nodejs_raspberrypi_gpio_intro.asp |archive-date=2021-08-13 |access-date=2020-05-03 |website=w3schools.com}}</ref><ref>{{cite web |title=Espruino&nbsp;– JavaScript for Microcontrollers |url=https://www.espruino.com/ |url-status=live |archive-url=https://web.archive.org/web/20200501010722/https://www.espruino.com/ |archive-date=2020-05-01 |access-date=2020-05-03 |website=espruino.com}}</ref>
==Execution system ==
 
== Execution ==
===Just-in-time compilation===
==== JavaScript engine= ===
{{Excerpt|JavaScript engine}}
 
=== Run-timeRuntime environmentsystem ===
A JavaScript engine must be embedded within a [[runtime system]] (such as a [[web browser]] or a standalone system) to enable scripts to interact with the broader environment. The runtime system includes the necessary APIs for [[input/output]] operations, such as [[computer network|networking]], [[data storage|storage]], and [[computer graphics|graphics]], and provides the ability to import scripts.
JavaScript typically relies on a run-time environment (e.g., a [[web browser]]) to provide objects and methods by which scripts can interact with the environment (e.g., a web page [[Document Object Model|DOM]]). These environments are single-[[Thread (computing)|threaded]]. JavaScript also relies on the run-time environment to provide the ability to include/import scripts (e.g., [[HTML]] <code><nowiki><script></nowiki></code> elements). This is not a language feature per se, but it is common in most JavaScript implementations. JavaScript processes [[Message (computer science)|messages]] from a [[Queue (abstract data type)|queue]] one at a time. JavaScript calls a [[Subroutine|function]] associated with each new message, creating a [[call stack]] frame with the function's [[Parameter (computer programming)|arguments]] and [[local variable]]s. 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 the [[event loop]], described as "run to completion" because each message is fully processed before the next message is considered. However, the language's [[Concurrency (computer science)|concurrency model]] describes the event loop as [[Asynchronous I/O|non-blocking]]: program [[input/output]] is performed using [[Event (computing)|events]] and [[Callback (computer programming)|callback functions]]. This means, for instance, that JavaScript can process a mouse click while waiting for a database query to return information.<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop|title=Concurrency model and Event Loop|website=Mozilla Developer Network|access-date=August 28, 2015|archive-date=September 5, 2015|archive-url=https://web.archive.org/web/20150905045241/https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop|url-status=live}}</ref>
 
JavaScript is a single-[[Thread (computing)|threaded]] language. The runtime processes [[Message (computer science)|messages]] from a [[Queue (abstract data type)|queue]] one at a time, and it calls a [[Subroutine|function]] associated with each new message, creating a [[call stack]] frame with the function's [[Parameter (computer programming)|arguments]] and [[local variable]]s. 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 the [[event loop]], described as "run to completion" because each message is fully processed before the next message is considered. However, the language's [[Concurrency (computer science)|concurrency model]] describes the event loop as [[Asynchronous I/O|non-blocking]]: program I/O is performed using [[Event (computing)|events]] and [[Callback (computer programming)|callback functions]]. This means, for example, that JavaScript can process a mouse click while waiting for a database query to return information.<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop|title=Concurrency model and Event Loop|website=Mozilla Developer Network|access-date=August 28, 2015|archive-date=September 5, 2015|archive-url=https://web.archive.org/web/20150905045241/https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop|url-status=live}}</ref>
====Examples====
=====Node.js=====
{{Excerpt|Node.js|only=paragraphs|paragraph=1}}
 
The notable standalone runtimes are [[Node.js]], [[Deno (software)|Deno]], and [[Bun (software)|Bun]].
=====Deno=====
{{Excerpt|Deno (software)|only=paragraphs|paragraph=1}}
 
== 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.
 
=== Imperative and structured ===
{{Main|Structured programming}}
 
JavaScript supports much of the [[structured programming]] syntax from [[C (computer language)|C]] (e.g., <code>if</code> statements, <code>while</code> loops, <code>switch</code> statements, <code>do while</code> loops, etc.). One partial exception is [[scope (computer science)|scoping]]: originally JavaScript only had [[function scoping]] with <code>var</code>; [[block scoping]] was added in ECMAScript 2015 with the keywords <code>let</code> and <code>[[const (computer programming)|const]]</code>. Like C, JavaScript makes a distinction between [[Expression (computer science)|expressions]] and [[Statement (computer science)|statements]]. One syntactic difference from C is [[Defensive semicolon|automatic semicolon insertion]], which allow semicolons (which terminate statements) to be omitted.<ref name="Flanagan2006">{{cite book|last=Flanagan|first=David|title=JavaScript: The Definitive Guide: The Definitive Guide|url=https://books.google.com/books?id=2weL0iAfrEMC|date=August 17, 2006|publisher="[[O'Reilly Media, Inc."]]|isbn=978-0-596-55447-7|page=16|access-date=March 29, 2019|archive-date=August 1, 2020|archive-url=https://web.archive.org/web/20200801065235/https://books.google.com/books?id=2weL0iAfrEMC|url-status=live}}</ref>
 
=== Weakly typed ===
{{MainFurther|Weakly typed}}
 
JavaScript is [[Strong and weak typing|weakly typed]], which means certain types are implicitly cast depending on the operation used.<ref name="casting_rules">{{cite web |last=Korolev |first=Mikhail |date=2019-03-01 |title=JavaScript quirks in one image from the Internet |url=https://dev.to/mkrl/javascript-quirks-in-one-image-from-the-internet-52m7 |url-status=live |archive-url=https://web.archive.org/web/20191028204723/https://dev.to/mkrl/javascript-quirks-in-one-image-from-the-internet-52m7 |archive-date=October 28, 2019 |access-date=October 28, 2019 |website=The DEV Community |language=en}}</ref>
Line 165 ⟶ 152:
* The binary <code>+</code> 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 <code>-</code> operator always casts both operands to a number
* Both unary operators (<code>+</code>, <code>-</code>) always cast the operand to a number. However, <code>+</code> always casts to <code>Number</code> ([[Double-precision floating-point format|binary64]]) while <code>-</code> preserves <code>BigInt</code> ([[Arbitrary-precision arithmetic|integer]])<ref>{{cite web | url=https://github.com/tc39/proposal-bigint/blob/master/ADVANCED.md#dont-break-asmjs | title=Proposal-bigint/ADVANCED.md at master · tc39/Proposal-bigint | website=[[GitHub]] }}</ref>
* Both unary operators (<code>+</code>, <code>-</code>) always cast the operand to a number
 
Values are cast to strings like the following:<ref name="casting_rules" />
Line 216 ⟶ 203:
|}
 
Often also mentioned is <code>{} + []</code> resulting in <code>0</code> (number). This is misleading: the <code>{}</code> 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 <code>+</code> operator. If you wrap the expression is wrapped in parentheses - <code>({} + [])</code> the curly brackets are interpreted as an empty object and the result of the expression is <code>"[object Object]"</code> as expected.<ref name="casting_rules" />
 
=== Dynamic ===
{{Main|Dynamic Programmingprogramming}}
 
==== Typing ====
Line 230 ⟶ 217:
JavaScript includes an <code>[[eval]]</code> function that can execute statements provided as strings at run-time.
 
=== Object-orientation (prototype-based) ===
Prototypal inheritance in JavaScript is described by [[Douglas Crockford]] as:
{{Blockquote
Line 236 ⟶ 223:
}}
 
In JavaScript, an [[Object (computer science)|object]] is an [[associative array]], augmented with a prototype (see below); each key provides the name for an object [[Property (programming)|property]], and there are two syntactical ways to specify such a name: dot notation (<code>obj.x&nbsp; =&nbsp; 10</code>) and bracket notation (<code>obj['"x'"]&nbsp; =&nbsp; 10</code>). A property may be added, rebound, or deleted at run-time. Most [[property (programming)|properties]] of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using a <code>for...in</code> loop.
 
==== Prototypes ====
{{Main|Prototype-based programming}}
 
JavaScript uses [[prototype-based programming|prototypes]] where many other object-oriented languages use [[Class (computer science)|classes]] for [[Inheritance (object-oriented programming)|inheritance]].,<ref>{{cite web|title=Inheritance and the prototype chain|url=https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Inheritance_and_the_prototype_chain|work=[[Mozilla]] Developer Network |access-date=April 6, 2013|archive-date=April 25, 2013|archive-url=https://web.archive.org/web/20130425144207/https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Inheritance_and_the_prototype_chain|url-status=live}}</ref> Itbut isit's still possible to simulate manymost class-based features with prototypesthe inprototype JavaScriptsystem.<ref>{{cite book|last=Herman|first=David|title=Effective JavaScript|year=2013|publisher=Addison-Wesley|isbn=978-0-321-81218-6|page=83 |url=https://books.google.com/books?id=Nz9iAwAAQBAJ&pg=PA83 }}</ref> Additionally, [[ECMAScript |ECMAScript version 6]] (released June 2015) introduced the keywords '''class''', '''extends''' and '''super''', which serve as syntactic sugar to abstract the underlying prototypal inheritance system with a more conventional interface. Constructors are declared by specifying a method named '''constructor''', and all classes are automatically subclasses of the base class Object, similarly to Java. <syntaxhighlight lang="javascript">
class Person {
constructor(name) {
this.name = name;
}
}
 
class Student extends Person {
==== Functions as object constructors ====
constructor(name, id) {
super(name);
this.id = id;
}
}
 
const bob = new Student("Robert", 12345);
console.log(bob.name); // Robert
</syntaxhighlight>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 a [[number sign]] (#), and [[Polymorphism (computer science)|polymorphism]] is not directly supported, although it can be emulated by manually calling different functions depending on the number and type of arguments provided.<ref name="JavaScriptNext">{{cite book |last=Ghandi |first=Raju |date=2019 |title=JavaScript Next|___location=New York City |publisher=Apress Media |pages=159–171 |isbn=978-1-4842-5394-6}}</ref>
 
==== Functions as object constructors ====
Functions double as object constructors, along with their typical role. Prefixing a function call with ''new'' will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the <code>Object</code> prototype).<ref name="Haverbeke2024">{{Cite book |title=Eloquent JavaScript |last=Haverbeke |first=Marijn |publisher=[[No Starch Press]] |isbn=978-1-71850-411-0 |edition=4th |___location=San Francisco |publication-date=September 2024 |pages=[https://eloquentjavascript.net/Eloquent_JavaScript.pdf#section*.204 97–98] |language=en |url=https://eloquentjavascript.net/Eloquent_JavaScript.pdf |archive-url=https://web.archive.org/web/20250312193854/https://eloquentjavascript.net/Eloquent_JavaScript.pdf |archive-date=2025-03-12 |url-status=live}}</ref> ECMAScript 5 offers the <code>Object.create</code> method, allowing explicit creation of an instance without automatically inheriting from the <code>Object</code> prototype (older environments can assign the prototype to <code>null</code>).<ref>{{cite web|last=Katz|first=Yehuda|title=Understanding "Prototypes" in JavaScript|date=12 August 2011|url=https://yehudakatz.com/2011/08/12/understanding-prototypes-in-javascript/|access-date=April 6, 2013|archive-date=5 April 2013|archive-url=https://web.archive.org/web/20130405154842/https://yehudakatz.com/2011/08/12/understanding-prototypes-in-javascript/|url-status=live}}</ref> The constructor's <code>prototype</code> 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.<syntaxhighlight lang="javascript">// This code is completely equivalent to the previous snippet
function Person(name) {
this.name = name;
}
 
function Student(name, id) {
Person.call(this, name);
this.id = id;
}
 
var bob = new Student("Robert", 12345);
Functions double as object constructors, along with their typical role. Prefixing a function call with ''new'' will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the <code>Object</code> prototype).<ref>{{cite book|last=Haverbeke|first=Marijn|title=Eloquent JavaScript|year=2011|publisher=No Starch Press|isbn=978-1-59327-282-1|pages=95–97 |url=https://books.google.com/books?id=9U5I_tskq9MC&pg=PA95 }}</ref> ECMAScript 5 offers the <code>Object.create</code> method, allowing explicit creation of an instance without automatically inheriting from the <code>Object</code> prototype (older environments can assign the prototype to <code>null</code>).<ref>{{cite web|last=Katz|first=Yehuda|title=Understanding "Prototypes" in JavaScript|date=12 August 2011|url=https://yehudakatz.com/2011/08/12/understanding-prototypes-in-javascript/|access-date=April 6, 2013|archive-date=5 April 2013|archive-url=https://web.archive.org/web/20130405154842/https://yehudakatz.com/2011/08/12/understanding-prototypes-in-javascript/|url-status=live}}</ref> The constructor's <code>prototype</code> 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. JavaScript's built-in constructors, such as <code>Array</code> or <code>Object</code>, also have prototypes that can be modified. While it is possible to modify the <code>Object</code> prototype, it is generally considered bad practice because most objects in JavaScript will inherit methods and properties from the <code>Object</code> prototype, and they may not expect the prototype to be modified.<ref>{{cite book|last=Herman|first=David|title=Effective JavaScript|year=2013|publisher=Addison-Wesley|isbn=978-0-321-81218-6|pages=125–127 |url=https://books.google.com/books?id=Nz9iAwAAQBAJ&pg=PA125 }}</ref>
console.log(bob.name); // Robert</syntaxhighlight>JavaScript's built-in classes, such as <code>Array</code> and <code>Object</code>, also have prototypes that can be modified. However, it's generally considered bad practice to [[Monkey patch|modify 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.<ref>{{cite book |last=Herman |first=David |url=https://books.google.com/books?id=Nz9iAwAAQBAJ&pg=PA125 |title=Effective JavaScript |publisher=Addison-Wesley |year=2013 |isbn=978-0-321-81218-6 |pages=125–127}}</ref>
 
==== Functions as methods ====
Line 253 ⟶ 266:
Unlike in many object-oriented languages, in JavaScript there is no distinction between a function definition and a [[method (computer science)|method]] definition. Rather, the distinction occurs during function calling. When a function is called as a method of an object, the function's local ''this'' keyword is bound to that object for that invocation.
 
=== Functional ===
{{Main|Functional programming}}
 
JavaScript [[Subroutine|functionfunctions]]s are [[first-class function|first-class]]; a function is considered to be an object.<ref>{{cite web|title=Function&nbsp;– JavaScript|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function|access-date=2021-10-30|website=[[MDN Web Docs]]|language=en-US}}</ref> As such, a function may have properties and methods, such as <code>.call()</code> and <code>.bind()</code>.<ref>{{cite web | url=https://es5.github.com/#x15.3.4-toc | title=Properties of the Function Object | publisher=Es5.github.com | access-date=May 26, 2013 | archive-date=January 28, 2013 | archive-url=https://web.archive.org/web/20130128185825/https://es5.github.com/#x15.3.4-toc | url-status=live }}</ref>
 
==== Lexical closure ====
{{Main|Closure (computer programming)}}
 
A ''nested'' function is a function defined within another function. It is created each time the outer function is invoked.
 
In addition, each nested function forms a [[Closure (computer programming)|lexical closure]]: the [[Scope (programming)#Lexical scoping vs. dynamic scoping|lexical 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.{{Sfn|Flanagan|2006|p=141}}
 
==== Anonymous function ====
Line 270 ⟶ 283:
JavaScript also supports [[anonymous function]]s.
 
=== Delegative ===
{{Main|Delegation (object-oriented programming)}}
JavaScript supports implicit and explicit [[Delegation (object-oriented programming)|delegation]].
Line 277 ⟶ 290:
{{Main|Role-oriented programming|Traits (computer science)|Mixin}}
 
JavaScript natively supports various function-based implementations of [[Role-oriented programming|Role]]<ref>[https://peterseliger.blogspot.de/2014/04/the-many-talents-of-javascript.html#the-many-talents-of-javascript-for-generalizing-role-oriented-programming-approaches-like-traits-and-mixins The many talents of JavaScript for generalizing Role-Oriented Programming approaches like Traits and Mixins] {{Webarchive|url=https://web.archive.org/web/20171005050713/https://peterseliger.blogspot.de/2014/04/the-many-talents-of-javascript.html#the-many-talents-of-javascript-for-generalizing-role-oriented-programming-approaches-like-traits-and-mixins |date=2017-10-05 }}, Peterseliger.blogpsotblogspot.de, April 11, 2014.</ref> patterns like [[Traits (computer science)|Traits]]<ref>[https://soft.vub.ac.be/~tvcutsem/traitsjs/ Traits for JavaScript] {{Webarchive|url=https://web.archive.org/web/20140724052500/https://soft.vub.ac.be/~tvcutsem/traitsjs/ |date=2014-07-24 }}, 2010.</ref><ref>{{cite web |url=https://cocktailjs.github.io/ |title=Home &#124; CocktailJS |website=Cocktailjs.github.io |access-date=February 24, 2017 |archive-date=February 4, 2017 |archive-url=https://web.archive.org/web/20170204083608/https://cocktailjs.github.io/ |url-status=live }}</ref> and [[Mixin]]s.<ref>{{cite web |url-status=live |first1=Angus |last1=Croll |url=https://javascriptweblog.wordpress.com/2011/05/31/a-fresh-look-at-javascript-mixins/ |title=A fresh look at JavaScript Mixins |archive-url=https://web.archive.org/web/20200415004603/https://javascriptweblog.wordpress.com/2011/05/31/a-fresh-look-at-javascript-mixins/ |archive-date=2020-04-15 |date=May 31, 2011 |website= JavaScript, JavaScript… }}</ref> Such a function defines additional behavior by at least one method bound to the <code>this</code> keyword within its <code>function</code> body. A Role then has to be delegated explicitly via <code>call</code> or <code>apply</code> 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 cover [[Object composition|composition]] 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. Thus [[Inheritance (object-oriented programming)|inheritance]] in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.
 
=== Miscellaneous ===
==== Zero-based numbering ====
JavaScript is a [[Zero-based numbering#Usage in programming languages|zero-index]] language.
Line 300 ⟶ 313:
{{Main|Regular expression}}
 
JavaScript supports [[regular expression]]s for text searches and manipulation.{{r|n=Haverbeke2024|p=139}}
In a manner similar to [[Perl]], JavaScript also supports [[regular expression]]s, which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.<ref>{{cite book | last=Haverbeke | first=Marijn | title=Eloquent JavaScript | year=2011 | publisher=No Starch Press|isbn=978-1-59327-282-1|pages=139–149 |url=https://books.google.com/books?id=9U5I_tskq9MC&pg=PA139 }}</ref>
 
==== Promises and Async/await ====
JavaScript supports [[futures and promises|promises]] and [[Async/await]] for handling asynchronous operations.{{cn|date=August 2023}}
 
===== Promises =====
{{Main|Futures and promises}}
 
A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. Recently, the JavaScript specification introducedsupplies 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 =====
Line 315 ⟶ 325:
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, some [[JavaScript engine]]s supported these non-standard features:
* conditional <code>catch</code> clauses (like Java)
* [[List comprehension|array comprehensions]] and generator expressions (like Python)
* concise function expressions (<code>function(args) expr</code>; this experimental syntax predated arrow functions)
* [[ECMAScript for XML]] (E4X), an extension that adds native XML support to ECMAScript (unsupported in Firefox since version 21<ref>{{cite web|title=E4X – Archive of obsolete content |url=https://developer.mozilla.org/en-US/docs/Archive/Web/E4X|website=Mozilla Developer Network|publisher=Mozilla Foundation|date=February 14, 2014|access-date=July 13, 2014|archive-date=July 24, 2014|archive-url=https://web.archive.org/web/20140724100129/https://developer.mozilla.org/en-US/docs/Archive/Web/E4X|url-status=dead}}</ref>)
 
== Syntax ==
{{Main|JavaScript syntax}}
 
===Simple examples===
[[Variable (computer science)|Variables]] in JavaScript can be defined using either the <code>var</code>,<ref>{{cite web | url=https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/var | title=var – JavaScript | publisher=The [[Mozilla Developer Network]] | access-date=December 22, 2012 | archive-date=December 23, 2012 | archive-url=https://web.archive.org/web/20121223162713/https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/var | url-status=live }}</ref> <code>let</code><ref name="moz_let">{{cite web |title=let |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let |website=MDN web docs |publisher=Mozilla |access-date=June 27, 2018 |ref=moz_let |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528140803/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let |url-status=live }}</ref> or <code>const</code><ref name="moz_const">{{cite web |title=const |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const |website=MDN web docs |publisher=Mozilla |access-date=June 27, 2018 |ref=moz_const |archive-date=June 28, 2018 |archive-url=https://web.archive.org/web/20180628044054/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const |url-status=live }}</ref> keywords. Variables defined without keywords will be defined at the global scope.
 
Arrow functions were first introduced in [[w:ECMAScript#6th Edition – ECMAScript 2015|6th 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.
<syntaxhighlight lang="javascript" start="1">
// Declares a function-scoped variable named `x`, and implicitly assigns the
Line 365 ⟶ 375:
Note the [[Comment (computer programming)|comments]] in the examples above, all of which were preceded with two [[Slash (punctuation)|forward slashes]].
 
More examples can be found at the [[wikibooks:JavaScript/Syntax examples|Wikibooks page on JavaScript syntax examples]].
There is no built-in [[Input/output]] functionality in JavaScript, instead it is provided by the run-time environment. The ECMAScript specification in edition 5.1 mentions that "there are no provisions in this specification for input of external data or output of computed results".<ref>{{cite web| url=https://www.ecma-international.org/ecma-262/5.1/#sec-4| title=ECMAScript Language Specification – ECMA-262 Edition 5.1| publisher=[[Ecma International]]| access-date=December 22, 2012| archive-date=November 26, 2012| archive-url=https://web.archive.org/web/20121126044218/https://ecma-international.org/ecma-262/5.1/#sec-4| url-status=live}}</ref>
However, most runtime environments have a <code>console</code> object that can be used to print output.<ref>{{cite web|title=console|url=https://developer.mozilla.org/en-US/docs/DOM/console|work=Mozilla Developer Network|publisher=[[Mozilla]]|access-date=April 6, 2013|archive-date=February 28, 2013|archive-url=https://web.archive.org/web/20130228112150/https://developer.mozilla.org/en-US/docs/DOM/console|url-status=live}}</ref> Here is a minimalist [["Hello, World!" program]] in JavaScript in a runtime environment with a console object:
 
== Security ==
<syntaxhighlight lang="javascript">
console.log("Hello, World!");
</syntaxhighlight>
In HTML documents, a program like this is required for an output:
<syntaxhighlight lang="javascript">
// Text nodes can be made using the "write" method.
// This is frowned upon, as it can overwrite the document if the document is fully loaded.
document.write('foo');
 
// Elements can be made too. First, they have to be created in the DOM.
const myElem = document.createElement('span');
 
// Attributes like classes and the id can be set as well
myElem.classList.add('foo');
myElem.id = 'bar';
 
// After setting this, the tag will look like this: `<span class="foo" id="bar" data-attr="baz"></span>`
myElem.setAttribute('data-attr', 'baz'); // Which could also be written as `myElem.dataset.attr = 'baz'`
 
// Finally append it as a child element to the <body> in the HTML
document.body.appendChild(myElem);
 
// Elements can be imperatively grabbed with querySelector for one element, or querySelectorAll for multiple elements that can be looped with forEach
document.querySelector('.class'); // Selects the first element with the "class" class
document.querySelector('#id'); // Selects the first element with an `id` of "id"
document.querySelector('[data-other]'); // Selects the first element with the "data-other" attribute
document.querySelectorAll('.multiple'); // Returns an Array-like NodeList of all elements with the "multiple" class
</syntaxhighlight>A simple [[Recursion (computer science)|recursive]] function to calculate the [[factorial]] of a [[natural number]]:
 
<syntaxhighlight lang="javascript">
function factorial(n) {
// Checking the argument for legitimacy. Factorial is defined for positive integers.
if (isNaN(n)) {
console.error("Non-numerical argument not allowed.");
return NaN; // The special value: Not a Number
}
if (n === 0)
return 1; // 0! = 1
if (n < 0)
return undefined; // Factorial of negative numbers is not defined.
if (n % 1) {
console.warn(`${n} will be rounded to the closest integer. For non-integers consider using gamma function instead.`);
n = Math.round(n);
}
// The above checks need not be repeated in the recursion, hence defining the actual recursive part separately below.
 
// The following line is a function expression to recursively compute the factorial. It uses the arrow syntax introduced in ES6.
const recursivelyCompute = a => a > 1 ? a * recursivelyCompute(a - 1) : 1; // Note the use of the ternary operator `?`.
return recursivelyCompute(n);
}
 
factorial(3); // Returns 6
</syntaxhighlight>
 
An [[anonymous function]] (or lambda):
 
<syntaxhighlight lang="javascript">
const counter = function() {
let count = 0;
return function() {
return ++count;
}
};
 
const x = counter();
x(); // Returns 1
x(); // Returns 2
x(); // Returns 3
</syntaxhighlight>
 
This example shows that, in JavaScript, [[Closure (computer programming)|function closures]] capture their non-local variables by reference.
 
Arrow functions were first introduced in [[ECMAScript#6th Edition – ECMAScript 2015|6th 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.
 
Example of arrow function:
 
<syntaxhighlight lang="javascript">
// Arrow functions let us omit the `function` keyword.
// Here `long_example` points to an anonymous function value.
const long_example = (input1, input2) => {
console.log("Hello, World!");
const output = input1 + input2;
 
return output;
};
 
// If there are no braces, the arrow function simply returns the expression
// So here it's (input1 + input2)
const short_example = (input1, input2) => input1 + input2;
 
long_example(2, 3); // Prints "Hello, World!" and returns 5
short_example(2, 5); // Returns 7
 
// If an arrow function has only one parameter, the parentheses can be removed.
const no_parentheses = input => input + 2;
 
no_parentheses(3); // Returns 5
 
// An arrow function, like other function definitions, can be executed in the same statement as they are created.
// This is useful when writing libraries to avoid filling the global scope, and for closures.
let three = ((a, b) => a + b) (1, 2);
 
const generate_multiplier_function = a => (b => isNaN(b) || !b ? a : a*=b);
const five_multiples = generate_multiplier_function(5); // The supplied argument "seeds" the expression and is retained by a.
five_multiples(1); // Returns 5
five_multiples(3); // Returns 15
five_multiples(4); // Returns 60
</syntaxhighlight>
 
In JavaScript, [[Object (computer science)|objects]] can be created as [[Instance (computer science)|instances]] of a [[class (computer science)|class]].
 
Object class example:
<syntaxhighlight lang="javascript">
class Ball {
 
constructor(radius) {
this.radius = radius;
this.area = Math.PI * ( radius ** 2 );
}
 
// Classes (and thus objects) can contain functions known as methods
show() {
console.log(this.radius);
}
};
 
const myBall = new Ball(5); // Creates a new instance of the ball object with radius 5
myBall.radius++; // Object properties can usually be modified from the outside
myBall.show(); // Using the inherited "show" function logs "6"
</syntaxhighlight>
 
In JavaScript, [[Object (computer science)|objects]] can be instantiated directly from a function.
 
Object functional example:
<syntaxhighlight lang="javascript">
function Ball(radius) {
 
const area = Math.PI * ( radius ** 2 );
const obj = { radius, area };
 
// Objects are mutable, and functions can be added as properties.
obj.show = () => console.log(obj.radius);
return obj;
};
 
const myBall = Ball(5); // Creates a new ball object with radius 5. No "new" keyword needed.
myBall.radius++; // The instance property can be modified.
myBall.show(); // Using the "show" function logs "6" - the new instance value.
</syntaxhighlight>
 
[[Variadic function]] demonstration (<code>arguments</code> is a special [[variable (programming)|variable]]):<ref>{{cite web|title=arguments|url=https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments|work=Mozilla Developer Network|publisher=[[Mozilla]]|access-date=April 6, 2013|archive-date=April 13, 2013|archive-url=https://web.archive.org/web/20130413230225/https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments|url-status=live}}</ref><!-- What about spread syntax? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax -->
 
<syntaxhighlight lang="javascript">
function sum() {
let x = 0;
 
for (let i = 0; i < arguments.length; ++i)
x += arguments[i];
 
return x;
}
 
sum(1, 2); // Returns 3
sum(1, 2, 3); // Returns 6
 
// As of ES6, using the rest operator.
function sum(...args) {
return args.reduce((a, b) => a + b);
}
 
sum(1, 2); // Returns 3
sum(1, 2, 3); // Returns 6
</syntaxhighlight>
 
[[Immediately-invoked function expression]]s are often used to create closures. Closures allow gathering properties and methods in a namespace and making some of them private:
 
<syntaxhighlight lang="javascript">
let counter = (function() {
let i = 0; // Private property
 
return { // Public methods
get: function() {
alert(i);
},
set: function(value) {
i = value;
},
increment: function() {
alert(++i);
}
};
})(); // Module
 
counter.get(); // Returns 0
counter.set(6);
counter.increment(); // Returns 7
counter.increment(); // Returns 8
</syntaxhighlight>
[[Generator (computer programming)|Generator]] objects (in the form of generator functions) provide a function which can be called, exited, and re-entered while maintaining internal context (statefulness).<ref>{{Cite web |title=function* - JavaScript {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function* |access-date=2022-09-27 |website=developer.mozilla.org |language=en-US}}</ref><syntaxhighlight lang="javascript">
function* rawCounter() {
yield 1;
yield 2;
}
 
function* dynamicCounter() {
let count = 0;
while (true) {
// It is not recommended to utilize while true loops in most cases.
yield ++count;
}
}
 
// Instances
const counter1 = rawCounter();
const counter2 = dynamicCounter();
 
// Implementation
counter1.next(); // {value: 1, done: false}
counter1.next(); // {value: 2, done: false}
counter1.next(); // {value: undefined, done: true}
 
counter2.next(); // {value: 1, done: false}
counter2.next(); // {value: 2, done: false}
counter2.next(); // {value: 3, done: false}
// ...infinitely
</syntaxhighlight>
 
JavaScript can export and import from modules:<ref name="moz_modules">{{cite web |title=JavaScript modules |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules |website=MDN Web Docs |publisher=Mozilla |access-date=28 July 2022 |archive-url=https://web.archive.org/web/20220717083604/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules |archive-date=17 July 2022 |url-status=live}}</ref>
 
Export example:
<syntaxhighlight lang="javascript">
/* mymodule.js */
// This function remains private, as it is not exported
let sum = (a, b) => {
return a + b;
}
 
// Export variables
export let name = 'Alice';
export let age = 23;
 
// Export named functions
export function add(num1, num2) {
return num1 + num2;
}
 
// Export class
export class Multiplication {
constructor(num1, num2) {
this.num1 = num1;
this.num2 = num2;
}
 
add() {
return sum(this.num1, this.num2);
}
}
</syntaxhighlight>
 
Import example:
<syntaxhighlight lang="javascript">
// Import one property
import { add } from './mymodule.js';
console.log(add(1, 2));
//> 3
 
// Import multiple properties
import { name, age } from './mymodule.js';
console.log(name, age);
//> "Alice", 23
 
// Import all properties from a module
import * from './module.js'
console.log(name, age);
//> "Alice", 23
console.log(add(1,2));
//> 3
</syntaxhighlight>
 
===More advanced example===
This sample code displays various JavaScript features.
<!-- Don't try to standardize any styling in this example—its purpose is to showcase language syntax, not provide some standard role model. -->
 
<syntaxhighlight lang="javascript">
/* Finds the lowest common multiple (LCM) of two numbers */
function LCMCalculator(x, y) { // constructor function
if (isNaN(x*y)) throw new TypeError("Non-numeric arguments not allowed.");
const checkInt = function(x) { // inner function
if (x % 1 !== 0)
throw new TypeError(x + "is not an integer");
 
return x;
};
 
this.a = checkInt(x)
// semicolons ^^^^ are optional, a newline is enough
this.b = checkInt(y);
}
// The prototype of object instances created by a constructor is
// that constructor's "prototype" property.
LCMCalculator.prototype = { // object literal
constructor: LCMCalculator, // when reassigning a prototype, set the constructor property appropriately
gcd: function() { // method that calculates the greatest common divisor
// Euclidean algorithm:
let a = Math.abs(this.a), b = Math.abs(this.b), t;
 
if (a < b) {
// swap variables
// t = b; b = a; a = t;
[a, b] = [b, a]; // swap using destructuring assignment (ES6)
}
 
while (b !== 0) {
t = b;
b = a % b;
a = t;
}
 
// Only need to calculate GCD once, so "redefine" this method.
// (Actually not redefinition—it's defined on the instance itself,
// so that this.gcd refers to this "redefinition" instead of LCMCalculator.prototype.gcd.
// Note that this leads to a wrong result if the LCMCalculator object members "a" and/or "b" are altered afterwards.)
// Also, 'gcd' === "gcd", this['gcd'] === this.gcd
this['gcd'] = function() {
return a;
};
 
return a;
},
 
// Object property names can be specified by strings delimited by double (") or single (') quotes.
"lcm": function() {
// Variable names do not collide with object properties, e.g., |lcm| is not |this.lcm|.
// not using |this.a*this.b| to avoid FP precision issues
let lcm = this.a / this.gcd() * this.b;
 
// Only need to calculate lcm once, so "redefine" this method.
this.lcm = function() {
return lcm;
};
 
return lcm;
},
 
// Methods can also be declared using ES6 syntax
toString() {
// Using both ES6 template literals and the (+) operator to concatenate values
return `LCMCalculator: a = ${this.a}, b = ` + this.b;
}
};
 
// Define generic output function; this implementation only works for Web browsers
function output(x) {
document.body.appendChild(document.createTextNode(x));
document.body.appendChild(document.createElement('br'));
}
 
// Note: Array's map() and forEach() are defined in JavaScript 1.6.
// They are used here to demonstrate JavaScript's inherent functional nature.
[
[25, 55],
[21, 56],
[22, 58],
[28, 56]
].map(function(pair) { // array literal + mapping function
return new LCMCalculator(pair[0], pair[1]);
}).sort((a, b) => a.lcm() - b.lcm()) // sort with this comparative function; => is a shorthand form of a function, called "arrow function"
.forEach(printResult);
 
function printResult(obj) {
output(obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());
}
</syntaxhighlight>
 
The following output should be displayed in the browser window.
<syntaxhighlight lang="output">
LCMCalculator: a = 28, b = 56, gcd = 28, lcm = 56
LCMCalculator: a = 21, b = 56, gcd = 7, lcm = 168
LCMCalculator: a = 25, b = 55, gcd = 5, lcm = 275
LCMCalculator: a = 22, b = 58, gcd = 2, lcm = 638
</syntaxhighlight>
 
==Security==
{{See also|Browser security}}
JavaScript and the [[Document Object Model|DOM]] 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 a [[Sandbox (computer security)|sandbox]] in which they can only perform Web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by the [[same-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.
Line 759 ⟶ 385:
[[Content Security Policy]] is the main intended method of ensuring that only trusted code is executed on a Web page.
 
=== Cross-site vulnerabilitiesscripting ===
{{Main|Cross-site scripting|Cross-site request forgery}}
 
====Cross-site scripting====
{{Main|Cross-site scripting}}
 
A common JavaScript-related security problem is [[cross-site scripting]] (XSS), a violation of the [[same-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. AOne important solution to XSS vulnerabilities is to use ''[[HTML escaping'' whenever displaying untrusted datasanitization]].
 
Some browsers include partial protection against ''reflected'' 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.
Line 771 ⟶ 394:
XSS vulnerabilities can also occur because of implementation mistakes by browser authors.<ref>{{cite web |url=https://www.mozillazine.org/talkback.html?article=4392 |title=Mozilla Cross-Site Scripting Vulnerability Reported and Fixed&nbsp;– MozillaZine Talkback |website=Mozillazine.org |access-date=February 24, 2017 |archive-date=July 21, 2011 |archive-url=https://web.archive.org/web/20110721230916/http://www.mozillazine.org/talkback.html?article=4392 |url-status=live }}</ref>
 
==== Cross-site request forgery= ===
{{Main|Cross-site request forgery}}
 
Another cross-site vulnerability is [[cross-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 <{{code><nowiki>|<script></nowiki></code>}} tag on an attacker's site exploits a page on the victim's site that returns private information such as [[JSON]] or JavaScript. Possible solutions include:
 
* requiring an authentication token in the [[POST (HTTP)|POST]] and [[GET (HTTP)|GET]] 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 authorshould cannotnot 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 be [[obfuscated code|obfuscated]], 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.<ref>{{cite web |last1=Kottelin |first1=Thor |title=Right-click "protection"? Forget about it |url=https://blog.anta.net/2008/06/17/right-click-%E2%80%9Cprotection%E2%80%9D-forget-about-it/ |website=blog.anta.net |access-date=28 July 2022 |archive-url=https://web.archive.org/web/20110809195359/https://blog.anta.net/2008/06/17/right-click-%E2%80%9Cprotection%E2%80%9D-forget-about-it/ |archive-date=9 August 2011 |date=17 June 2008}}</ref>
* It is considered very bad practice to embed sensitive information such as passwords in JavaScript because it can be extracted by an attacker.<ref>{{cite web |last1=Rehorik |first1=Jan |title=Why You Should Never Put Sensitive Data in Your JavaScript |url=https://www.serviceobjects.com/blog/why-you-should-never-put-sensitive-data-in-your-javascript/ |website=ServiceObjects Blog |date=29 November 2016 |publisher=ServiceObjects |access-date=June 3, 2019 |archive-date=June 3, 2019 |archive-url=https://web.archive.org/web/20190603142957/https://www.serviceobjects.com/blog/why-you-should-never-put-sensitive-data-in-your-javascript/ |url-status=live }}</ref>
* [[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 as [[npm (software)|npm]] 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.<ref name="jslibs">{{citation |last1=Lauinger |first1=Tobias |last2=Chaabane |first2=Abdelberi |last3=Arshad |first3=Sajjad |last4=Robertson |first4=William |last5=Wilson |first5=Christo |last6=Kirda |first6=Engin |title=Thou Shalt Not Depend on Me: Analysing the Use of Outdated JavaScript Libraries on the Web |url=https://www.ccs.neu.edu/home/arshad/publications/ndss2017jslibs.pdf |website=Northeastern University |access-date=28 July 2022 |archive-url=https://web.archive.org/web/20170329045344/https://www.ccs.neu.edu/home/arshad/publications/ndss2017jslibs.pdf |archive-date=29 March 2017 |doi = 10.14722/ndss.2017.23414 |date = December 21, 2016|arxiv=1811.00918 |isbn=978-1-891562-46-4 |s2cid=17885720 |url-status=dead}}</ref> "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."<ref name="jslibs" /> 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.<ref>{{cite news |work=Quartz |url=https://qz.com/646467/how-one-programmer-broke-the-internet-by-deleting-a-tiny-piece-of-code/ |title=How one programmer broke the internet by deleting a tiny piece of code |first=Keith |last=Collins |date=March 27, 2016 |access-date=February 22, 2017 |archive-date=February 22, 2017 |archive-url=https://web.archive.org/web/20170222200836/https://qz.com/646467/how-one-programmer-broke-the-internet-by-deleting-a-tiny-piece-of-code/ |url-status=live }}</ref><ref>SC Magazine UK, [https://www.scmagazineuk.com/developers-11-lines-of-deleted-code-breaks-the-internet/article/532050/ Developer's 11 lines of deleted code 'breaks the internet'] {{Webarchive|url=https://web.archive.org/web/20170223041434/https://www.scmagazineuk.com/developers-11-lines-of-deleted-code-breaks-the-internet/article/532050/ |date=February 23, 2017 }}</ref>
 
=== Browser and plugin coding errors ===
{{Further|Buffer overflow}}
JavaScript provides an interface to a wide range of browser capabilities, some of which may have flaws such as [[buffer overflow]]s. 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's [[API]] with superuser privileges.
Line 800 ⟶ 424:
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.<ref>{{cite web |url=https://blogs.msdn.com/ie/archive/2006/02/09/528963.aspx |title=Protected Mode in Vista IE7 – IEBlog |website=Blogs.msdn.com |date=February 9, 2006 |access-date=February 24, 2017 |archive-date=January 23, 2010 |archive-url=https://web.archive.org/web/20100123103719/https://blogs.msdn.com/ie/archive/2006/02/09/528963.aspx |url-status=live }}</ref> [[Google Chrome]] similarly confines its page renderers to their own "[[Sandbox (computer security)|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.
 
Line 807 ⟶ 431:
[[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 (like [[VBScript]]) a theoretically viable vector for a [[Trojan horse (computing)|Trojan horse]], although JavaScript Trojan horses are uncommon in practice.<ref>For one example of a rare JavaScript Trojan Horse, see Symantec Corporation, [https://www.symantec.com/security_response/writeup.jsp?docid=2003-100111-0931-99 JS.Seeker.K] {{Webarchive|url=https://web.archive.org/web/20110913210848/http://www.symantec.com/security_response/writeup.jsp?docid=2003-100111-0931-99 |date=2011-09-13 }}</ref>{{failed verification|date=March 2017}}
 
=== Hardware vulnerabilities ===
In 2015, a JavaScript-based proof-of-concept implementation of a [[rowhammer]] attack was described in a paper by security researchers.<ref>{{cite arXiv | eprint = 1507.06955 | title = Rowhammer.js: A Remote Software-Induced Fault Attack in JavaScript | date = July 24, 2015 | last1 = Gruss | first1 = Daniel | last2 = Maurice | first2 = Clémentine | last3 = Mangard | first3 = Stefan | class = cs.CR }}</ref><ref>{{cite news |work=Motherboard |publisher=[[Vice (magazine)|Vice]] |title=Rowhammer.js Is the Most Ingenious Hack I've Ever Seen |first=Alix |last=Jean-Pharuns |date=July 30, 2015 |url=https://motherboardwww.vice.com/en_usen/article/9akpwz/rowhammerjs-is-the-most-ingenious-hack-ive-ever-seen/ |access-date=January 26, 2018 |archive-date=January 27, 2018 |archive-url=https://web.archive.org/web/20180127084042/https://motherboard.vice.com/en_us/article/9akpwz/rowhammerjs-is-the-most-ingenious-hack-ive-ever-seen |url-status=live }}</ref><ref>{{cite web|website=[[Ars Technica]]|title=DRAM 'Bitflipping' exploit for attacking PCs: Just add JavaScript|first=Dan|last=Goodin|date=August 4, 2015|url=https://arstechnica.com/information-technology/2015/08/dram-bitflipping-exploit-for-attacking-pcs-just-add-javascript/|access-date=January 26, 2018|archive-date=January 27, 2018|archive-url=https://web.archive.org/web/20180127143154/https://arstechnica.com/information-technology/2015/08/dram-bitflipping-exploit-for-attacking-pcs-just-add-javascript/|url-status=live}}</ref><ref>{{cite web | url = https://www.slate.com/articles/technology/bitwise/2015/07/rowhammer_security_exploit_why_a_new_security_attack_is_truly_terrifying.html | title = Rowhammer security exploit: Why a new security attack is truly terrifying | date = July 28, 2015 | access-date = July 29, 2015 | first = David | last = Auerbach | author-link = David Auerbach | website = slate.com | archive-date = July 30, 2015 | archive-url = https://web.archive.org/web/20150730004023/https://www.slate.com/articles/technology/bitwise/2015/07/rowhammer_security_exploit_why_a_new_security_attack_is_truly_terrifying.html | url-status = live }}</ref>
 
In 2017, a JavaScript-based attack via browser was demonstrated that could bypass [[Address space layout randomization|ASLR]]. It is called "ASLR⊕Cache" or AnC.<ref>[https://www.vusec.net/projects/anc/ AnC] {{Webarchive|url=https://web.archive.org/web/20170316055626/https://www.vusec.net/projects/anc/ |date=2017-03-16 }} VUSec, 2017</ref><ref>[https://arstechnica.com/security/2017/02/new-aslr-busting-javascript-is-about-to-make-drive-by-exploits-much-nastier/ New ASLR-busting JavaScript is about to make drive-by exploits much nastier] {{Webarchive|url=https://web.archive.org/web/20170316024419/https://arstechnica.com/security/2017/02/new-aslr-busting-javascript-is-about-to-make-drive-by-exploits-much-nastier/ |date=2017-03-16 }} Ars Technica, 2017</ref>
Line 814 ⟶ 438:
In 2018, the paper that announced the [[Spectre (security vulnerability)|Spectre]] attacks against Speculative Execution in Intel and other processors included a JavaScript implementation.<ref>[https://spectreattack.com/spectre.pdf Spectre Attack] {{Webarchive|url=https://web.archive.org/web/20180103225843/https://spectreattack.com/spectre.pdf |date=2018-01-03 }} Spectre Attack</ref>
 
== Development tools ==
Important tools have evolved with the language.
 
Line 822 ⟶ 446:
* Many [[text editor]]s have syntax highlighting support for JavaScript code.
 
== Related technologies ==
===Static program analysis===
=== Java ===
A common misconception is that JavaScript is directly related to [[Java (programming language)|Java]].<!-- While one is intentionally similar to the other, they're different. (Previously the misconception sentence said "JavaScript is similar to Java" but that's not a misconception) --> Both indeed have a C-like syntax (the C language being their most immediate common ancestor language). They are also typically [[Sandbox (computer security)|sandboxed]], 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's {{code|lang=javascript|code=Math}} and {{code|lang=javascript|code=Date}} objects are based on classes from Java 1.0.<ref name="popularity">{{cite web | url=https://brendaneich.com/2008/04/popularity/ | title=Popularity | first=Brendan | last=Eich | author-link=Brendan Eich | date=April 3, 2008 | access-date=January 19, 2012 | archive-date=July 3, 2011 | archive-url=https://web.archive.org/web/20110703020955/https://brendaneich.com/2008/04/popularity/ | url-status=live }}</ref>
 
Both languages first appeared in 1995, but Java was developed by [[James Gosling]] of Sun Microsystems and JavaScript by [[Brendan Eich]] of Netscape Communications.
====ESLint====
{{Excerpt|ESLint}}
 
The differences between the two languages are more prominent than their similarities. Java has [[static typing]], while JavaScript's typing is [[Dynamic typing|dynamic]]. Java is loaded from [[Compiler|compiled]] bytecode, while JavaScript is loaded as human-readable source code. Java's objects are [[Class-based programming|class-based]], while JavaScript's are [[Prototype-based programming|prototype-based]]. Finally, Java did not support functional programming until Java 8, while JavaScript has done so from the beginning, being influenced by [[Scheme (programming language)|Scheme]].
====JSLint====
{{Excerpt|JSLint}}
 
=== Code incompatibilityJSON ===
[[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.
Being a just-in-time compiled, or even an interpreted language, JavaScript code does not run in all types of runtime environments. Depending on the browser and JavaScript-engine support for the many editions of the ECMAScript standard, there are as many variations of the JavaScript language. Since [[HTML#HTML 4|HTML 4]]'s deprecation of the ''language-''attribute of ''script''-tags, code can no longer detect JavaScript version support. Instead the [[User-Agent header|User-Agent String]] recognizes the specific browser in use, although often this is not sufficient to ensure compatibility.
 
=== Transpilers ===
Developers are expected to gracefully degrade incompatible code and use feature detection techniques per normative programming guidelines. This is especially important when compatibility with the previous standardization of ES5.1 is not established(ISO/IEC 16262:2011, withdrawn in May 2018). Transpiler-preprocessors like [[Babel (transcompiler)|Babel]] are also available for automated conversion of ES6+ code. Certain browsers, runtime environments or even installed versions may be designated as a system requirement for specific implementations.
Many websites are JavaScript-heavy, so [[transpiler]]s have been created to convert code written in other languages, which can aid the development process.<ref name="transpilers">{{cite web |title=List of languages that compile to JS |first=Jeremy |last=Ashkenas |website=[[GitHub]] |author-link=Jeremy Ashkenas |url=https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS |access-date=February 6, 2020 |archive-date=January 31, 2020 |archive-url=https://web.archive.org/web/20200131233044/https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS |url-status=live }}</ref>
 
[[TypeScript]] and [[CoffeeScript]] are two notable languages that transpile to JavaScript.
==Related technologies==
===Java===
{{Main|Java}}
A common misconception is that JavaScript is the same as [[Java (programming language)|Java]]. <!-- While one is intentionally similar to the other, they're different. (Previously the misconception sentence said "JavaScript is similar to Java" but that's not a misconception) --> Both indeed have a C-like syntax (the C language being their most immediate common ancestor language). They are also typically [[Sandbox (computer security)|sandboxed]] (when used inside a browser), 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's {{code|lang=javascript|code=Math}} and {{code|lang=javascript|code=Date}} objects are based on classes from Java 1.0.<ref name="popularity">{{cite web | url=https://brendaneich.com/2008/04/popularity/ | title=Popularity | first=Brendan | last=Eich | author-link=Brendan Eich | date=April 3, 2008 | access-date=January 19, 2012 | archive-date=July 3, 2011 | archive-url=https://web.archive.org/web/20110703020955/https://brendaneich.com/2008/04/popularity/ | url-status=live }}</ref>
 
=== WebAssembly ===
[[Java (programming language)|Java]] and JavaScript both first appeared in 1995, but Java was developed by [[James Gosling]] of Sun Microsystems and JavaScript by [[Brendan Eich]] of Netscape Communications.
[[WebAssembly]] is a newer language with a [[bytecode]] format designed to complement JavaScript, especially the performance-critical portions of [[web page]] scripts. All of the major [[JavaScript engine]]s support WebAssembly,<ref>{{cite web|url=https://visualstudiomagazine.com/articles/2017/11/06/edge-webassembly.aspx|title=Edge Browser Switches WebAssembly to 'On' -- Visual Studio Magazine|website=Visual Studio Magazine|access-date=2018-02-09|archive-date=2018-02-10|archive-url=https://web.archive.org/web/20180210002432/https://visualstudiomagazine.com/articles/2017/11/06/edge-webassembly.aspx|url-status=live}}</ref> which runs in the same [[Sandbox (computer security)|sandbox]] as regular JavaScript code.
 
The differences between the two languages are more prominent than their similarities. Java has [[static typing]], while JavaScript's typing is [[Dynamic typing|dynamic]]. Java is loaded from [[Compiler|compiled]] bytecode, while JavaScript is loaded as human-readable source code. Java's objects are [[Class-based programming|class-based]], while JavaScript's are [[Prototype-based programming|prototype-based]]. Finally, Java did not support functional programming until Java 8, while JavaScript has done so from the beginning, being influenced by [[Scheme (programming language)|Scheme]].
 
===JSON===
{{Excerpt|JSON}}
 
===TypeScript===
{{Main|TypeScript}}
TypeScript (TS) is a strictly-typed variant of JavaScript. TS differs by introducing type annotations to variables and functions, and introducing a type language to describe the types within JS. Otherwise TS shares much the same featureset as JS, to allow it to be easily transpiled to JS for running client-side, and to interoperate with other JS code.<ref>{{cite web|url=https://www.typescriptlang.org/ |title=TypeScript: JavaScript With Syntax For Types |publisher=Typescriptlang.org |date= |accessdate=2022-08-12}}</ref>
 
===WebAssembly===
{{Main|WebAssembly}}
Since 2017, web browsers have supported [[WebAssembly]], a binary format that enables a [[JavaScript engine]] to execute performance-critical portions of [[web page]] scripts close to native speed.<ref>{{cite web|url=https://visualstudiomagazine.com/articles/2017/11/06/edge-webassembly.aspx|title=Edge Browser Switches WebAssembly to 'On' -- Visual Studio Magazine|website=Visual Studio Magazine|access-date=2018-02-09|archive-date=2018-02-10|archive-url=https://web.archive.org/web/20180210002432/https://visualstudiomagazine.com/articles/2017/11/06/edge-webassembly.aspx|url-status=live}}</ref> WebAssembly code runs in the same [[Sandbox (computer security)|sandbox]] as regular JavaScript code.
 
[[asm.js]] is a subset of JavaScript that served as the forerunner of WebAssembly.<ref>{{cite web | url=https://asmjs.org/faq.html | title=frequently asked questions | publisher=asm.js | access-date=April 13, 2014 | archive-date=June 4, 2014 | archive-url=https://web.archive.org/web/20140604012024/https://asmjs.org/faq.html | url-status=live }}</ref>
 
== Notes ==
{{anchor|transpilers}}
{{notelist}}
 
===Transpilers= References ==
{{Main|Transpiler}}
JavaScript is the dominant client-side language of the Web, and many websites are script-heavy. Thus [[transpiler]]s have been created to convert code written in other languages, which can aid the development process.<ref name="transpilers">{{cite web |title=List of languages that compile to JS |first=Jeremy |last=Ashkenas |website=[[GitHub]] |author-link=Jeremy Ashkenas |url=https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS |access-date=February 6, 2020 |archive-date=January 31, 2020 |archive-url=https://web.archive.org/web/20200131233044/https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS |url-status=live }}</ref>
 
===Ajax===
{{Excerpt|Ajax (programming)}}
 
==References==
{{Reflist}}
 
== Further reading ==
==Sources==
* {{cite news |last=Dere |first=Mohan |date=2017-12-21|title=How to integrate create-react-app with all the libraries you need to make a great app |language=en-US |work=freeCodeCamp |url=https://medium.freecodecamp.org/integrating-create-react-app-redux-react-router-redux-observable-bootstrap-altogether-216db97e89a3 |access-date=2018-06-14}}
* {{cite news |last=Panchal |first=Krunal |date=2022-04-26 |title=Angular vs React Detailed Comparison |language=en-US |work=Groovy Web |url=https://www.groovyweb.co/blog/angular-vs-react-detail-comparison |access-date=2023-06-05}}
 
==Further reading==
{{See also|ECMAScript#External_links|l1=ECMAScript Specification Documents}}
* {{cite book|last=Flanagan, |first=David. ''|title=JavaScript: The Definitive Guide''. |edition=7th edition. |___location=Sebastopol, California: |publisher=O'Reilly, |year=2020. {{|isbn|=978-1-491-95202-3}}.
* {{Cite book |title=Eloquent JavaScript |last=Haverbeke |first=Marijn |publisher=[[No Starch Press]] |isbn=978-1-71850-411-0 |edition=4th |___location=San Francisco |publication-date=2024 |url=https://eloquentjavascript.net/Eloquent_JavaScript.pdf |archive-url=https://web.archive.org/web/20250312193854/https://eloquentjavascript.net/Eloquent_JavaScript.pdf |archive-date=2025-03-12 |url-status=live}}
* Haverbeke, Marijn. ''Eloquent JavaScript''. 3rd edition. No Starch Press, 2018. 472 pages. {{ISBN|978-1593279509}}.<small>[https://eloquentjavascript.net/ ''(download)'']</small>
* {{cite book|last=Zakas, |first=Nicholas. ''|title=Principles of Object-Oriented JavaScript'', |edition=1st edition. |publisher=No Starch Press, |year=2014. 120 pages. {{ISBN|isbn=978-1593275402}}.
 
== External links ==
{{sisterSister project links|commons=Category:JavaScript|n=no|v=Topic:JavaScript|q=no|s=no|m=no|voy=no|species=no|mw=JavaScript}}
{{Spoken Wikipedia|En-JavaScript.ogg|date=August 20, 2013}}
* [https://javascript.info/ The Modern JavaScript Tutorial]. A community maintained continuously updated collection of tutorials on the entirety of the language.
* {{cite web |url=https://www.pldi21.org/prerecorded_hopl.12.html |title=JavaScript: The First 20 Years |access-date=2022-02-06}}
* {{Curlie|Computers/Programming/Languages/JavaScript/}}
* {{cite web |url = https://www.pldi21.org/prerecorded_hopl.12.html |title = JavaScript: The First 20 Years |access-date = 2022-02-06}}
 
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