JavaScript#Delegative

The first popular web browser with a graphical user interface, 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.{{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}}

During these formative years of the Web, web pages could only be static, lacking the capability for dynamic behavior after the page was loaded in the browser. There was a desire in the flourishing web development scene to remove this limitation, so in 1995, Netscape decided to add a programming language to Navigator. They pursued two routes to achieve this: collaborating with Sun Microsystems to embed the Java language, while also hiring Brendan Eich to embed the Scheme language.{{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}}

The goal was a "language for the masses", "to help nonprogrammers create dynamic, interactive Web sites"."Netscape Communications Corp.", Browser enhancements. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 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 languages. Although the new language and its interpreter implementation were called LiveScript when first shipped as part of a Navigator beta in September 1995, the name was changed to JavaScript for the official release in December.{{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 }}{{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}}

The choice of the JavaScript name has caused confusion, implying that it is directly related to Java. At the time, the dot-com boom had begun and Java was a popular new language, so Eich considered the JavaScript name a marketing ploy by Netscape.{{Citation|last=Fin JS|title=Brendan Eich – CEO of Brave|date=June 17, 2016|url=https://www.youtube.com/watch?v=XOmhtfTrRxc |website=YouTube |access-date=February 7, 2018 }}

Microsoft debuted Internet Explorer in 1995, leading to a browser war with Netscape. On the JavaScript front, Microsoft created its own interpreter called JScript.{{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 }}

Microsoft first released JScript in 1996, alongside initial support for CSS and extensions to HTML. Each of these implementations was noticeably different from their counterparts in Netscape Navigator.{{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}}{{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 }} 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.{{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 }}

{{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-ups or those scrolling messages in the old status bar at the bottom of your old browser."|width=30%}}

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.

However, the effort to fully standardize the language was undermined by Microsoft gaining an increasingly dominant position in the browser market. By the early 2000s, Internet Explorer's market share reached 95%.{{cite news | 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 }} 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 ECMA work. Thus ECMAScript 4 was mothballed.

{{pic|Logo of TC39.svg|Logo of Technical Committee 39 ("TC39"), the committee of Ecma International that 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.{{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}}

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 implementation as an 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 and described a set of technologies, of which JavaScript was the backbone, to create web applications where data can be loaded in the background, avoiding the need for full page reloads. This sparked a renaissance period of JavaScript, spearheaded by open-source libraries and the communities that formed around them. Many new libraries were created, including jQuery, Prototype, Dojo Toolkit, and MooTools.

Google debuted its Chrome browser in 2008, with the V8 JavaScript engine that was faster than its competition.{{cite web|url=https://www.pcgameshardware.com/aid,687738/Big-browser-comparison-test-Internet-Explorer-vs-Firefox-Opera-Safari-and-Chrome-Update-Firefox-35-Final/Practice/|title=Big browser comparison test: Internet Explorer vs. Firefox, Opera, Safari and Chrome|work=PC Games Hardware|date=3 July 2009|publisher=Computec Media AG|access-date=June 28, 2010|archive-date=May 2, 2012|archive-url=https://web.archive.org/web/20120502043027/http://www.pcgameshardware.com/aid,687738/Big-browser-comparison-test-Internet-Explorer-vs-Firefox-Opera-Safari-and-Chrome-Update-Firefox-35-Final/Practice/|url-status=live}}{{cite web | url=https://lifehacker.com/lifehacker-speed-tests-safari-4-chrome-2-and-more-5286869 | title=Lifehacker Speed Tests: Safari 4, Chrome 2 | first=Kevin | last=Purdy | work=Lifehacker | date=June 11, 2009 | access-date=May 8, 2021 | archive-date=April 14, 2021 | archive-url=https://web.archive.org/web/20210414095403/https://lifehacker.com/lifehacker-speed-tests-safari-4-chrome-2-and-more-5286869 | url-status=live }} The key innovation was just-in-time compilation (JIT),{{cite web|url=https://brendaneich.com/2008/08/tracemonkey-javascript-lightspeed/|title=TraceMonkey: JavaScript Lightspeed, Brendan Eich's Blog|access-date=July 22, 2020|archive-date=December 4, 2015|archive-url=https://web.archive.org/web/20151204091540/https://brendaneich.com/2008/08/tracemonkey-javascript-lightspeed/|url-status=live}} so other browser vendors needed to overhaul their engines for JIT.{{cite magazine |title=Mozilla asks, 'Are we fast yet?' |url=https://www.wired.com/2010/09/mozilla-asks-are-we-fast-yet/ |magazine=Wired |access-date=January 18, 2019 |archive-date=June 22, 2018 |archive-url=https://web.archive.org/web/20180622213244/https://www.wired.com/2010/09/mozilla-asks-are-we-fast-yet/ |url-status=live }}

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.

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.{{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=usurped }}

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 engine, an event loop, and I/O APIs, thereby providing a stand-alone JavaScript runtime system.[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[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 As of 2018, Node had been used by millions of developers,{{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 }} and npm had the most modules of any package manager in the world.{{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 }}

The ECMAScript draft specification is currently maintained openly on GitHub, and editions are produced via regular annual snapshots.{{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 }} Potential revisions to the language are vetted through a comprehensive proposal process.{{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}}{{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 }} Now, instead of edition numbers, developers check the status of upcoming features individually.

The current JavaScript ecosystem has many libraries and frameworks, established programming practices, and substantial usage of JavaScript outside of web browsers. Plus, with the rise of single-page applications and other JavaScript-heavy websites, several transpilers have been created to aid the development process.

"JavaScript" is a trademark of Oracle Corporation in the United States.{{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}}{{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}} The trademark was originally issued to Sun Microsystems on 6 May 1997, and was transferred to Oracle when they acquired Sun in 2009.{{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}}{{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}}

A letter was circulated in September 2024, spearheaded by Ryan Dahl, calling on Oracle to free the JavaScript trademark.{{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}} Brendan Eich, the original creator of JavaScript, was among the over 14,000 signatories who supported the initiative.

JavaScript is the dominant client-side scripting language of the Web, with 99% of all websites using it for this purpose. Scripts are embedded in or included from HTML documents and interact with the DOM.

All major web browsers have a built-in JavaScript engine that executes the code on the user's device.

• Loading new web page content without reloading the page, via Ajax or a WebSocket. For example, users 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 games.
• Controlling the playback of streaming media.
• Generating pop-up ads or alert boxes.
• Validating input values of a web form before the data is sent to a web server.
• Logging data about the user's behavior then sending it to a server. The website owner can use this data for analytics, ad tracking, and personalization.
• Redirecting a user to another page.
• Storing and retrieving data on the user's device, via the storage or IndexedDB standards.

Over 80% of websites use a third-party JavaScript library or web framework as part of their client-side scripting.{{cite web|url=https://w3techs.com/technologies/overview/javascript_library|title=Usage statistics of JavaScript libraries for websites|website=W3Techs|access-date=2021-04-09}}

jQuery is by far the most-used. Other notable ones include Angular, Bootstrap, Lodash, Modernizr, React, Underscore, and Vue. Multiple options can be used in conjunction, such as jQuery and Bootstrap.{{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}}

However, the term "Vanilla JS" was coined for websites not using any libraries or frameworks at all, instead relying entirely on standard JavaScript functionality.{{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}}

The use of JavaScript has expanded beyond its web browser roots. JavaScript engines are now embedded in a variety of other software systems, both for server-side website deployments and non-browser applications.

Initial attempts at promoting server-side JavaScript usage were Netscape Enterprise Server and Microsoft's Internet Information Services,{{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}}{{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}} but they were small niches. Server-side usage eventually started to grow in the late 2000s, with the creation of Node.js and other approaches.{{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 }}

Electron, Cordova, React Native, and other application frameworks have been used to create many applications with behavior implemented in JavaScript. Other non-browser applications include Adobe Acrobat support for scripting PDF documents{{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}} and GNOME Shell extensions written in JavaScript.{{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}}

Oracle used to provide Nashorn, a JavaScript interpreter, as part of their 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. {{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}} {{cite web |author= |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}} {{cite web |author= |title=GraalJS |url=https://www.graalvm.org/latest/reference-manual/js/ |website=|publisher=GraalVM |access-date=Feb 17, 2025}}{{cite web |author= |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}}

JavaScript has been used in some embedded systems, usually by leveraging Node.js.{{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}}{{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}}{{cite web |title=Espruino – 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}}

{{Excerpt|JavaScript engine}}

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 networking, storage, and graphics, and provides the ability to import scripts.

JavaScript is a single-threaded language. The runtime processes messages from a queue one at a time, and it calls a function associated with each new message, creating a call stack frame with the function's arguments and local variables. The call stack shrinks and grows based on the function's needs. When the call stack is empty upon function completion, JavaScript proceeds to the next message in the queue. This is called 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 model describes the event loop as non-blocking: program I/O is performed using events and callback functions. This means, for example, that JavaScript can process a mouse click while waiting for a database query to return information.{{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}}

The notable standalone runtimes are Node.js, Deno, and Bun.

The following features are common to all conforming ECMAScript implementations unless explicitly specified otherwise.

{{Main|Structured programming}}

JavaScript supports much of the structured programming syntax from C (e.g., if statements, while loops, switch statements, do while loops, etc.). One partial exception is scoping: originally JavaScript only had function scoping with var; block scoping was added in ECMAScript 2015 with the keywords let and const. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, which allow semicolons (which terminate statements) to be omitted.{{cite book|last=Flanagan|first=David|title=JavaScript: 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}}

{{Main|Weakly typed}}

JavaScript is weakly typed, which means certain types are implicitly cast depending on the operation used.{{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}}

• The binary + operator casts both operands to a string unless both operands are numbers. This is because the addition operator doubles as a concatenation operator
• The binary - operator always casts both operands to a number
• Both unary operators (+, -) always cast the operand to a number. However, + always casts to Number (binary64) while - preserves BigInt (integer){{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 }}

Values are cast to strings like the following:

• Strings are left as-is
• Numbers are converted to their string representation
• Arrays have their elements cast to strings after which they are joined by commas (,)
• Other objects are converted to the string [object Object] where Object is the name of the constructor of the object

Values are cast to numbers by casting to strings and then casting the strings to numbers. These processes can be modified by defining toString and valueOf functions on the prototype for string and number casting respectively.

JavaScript has received criticism for the way it implements these conversions as the complexity of the rules can be mistaken for inconsistency.{{cite web |date=2012 |title=Wat |url=https://www.destroyallsoftware.com/talks/wat |url-status=live |archive-url=https://web.archive.org/web/20191028204723/https://www.destroyallsoftware.com/talks/wat |archive-date=October 28, 2019 |access-date=October 28, 2019 |website=Destroy All Software |first1=Gary |last1=Bernhardt }} For example, when adding a number to a string, the number will be cast to a string before performing concatenation, but when subtracting a number from a string, the string is cast to a number before performing subtraction.

Often also mentioned is {} + [] resulting in 0 (number). This is misleading: the {} is interpreted as an empty code block instead of an empty object, and the empty array is cast to a number by the remaining unary + operator. If the expression is wrapped in parentheses - ({} + []) – the curly brackets are interpreted as an empty object and the result of the expression is "[object Object]" as expected.

{{Main|Dynamic Programming}}

{{Main|Dynamic typing}}

JavaScript is dynamically typed like most other scripting languages. A type is associated with a value rather than an expression. For example, a variable initially bound to a number may be reassigned to a string.{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures|title=JavaScript data types and data structures |date=February 16, 2017|website=MDN |access-date=February 24, 2017|archive-date=March 14, 2017|archive-url=https://web.archive.org/web/20170314230542/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures|url-status=live}} JavaScript supports various ways to test the type of objects, including duck typing.{{Sfn|Flanagan|2006|pp=176–178}}

{{Main|eval}}

JavaScript includes an eval function that can execute statements provided as strings at run-time.

Prototypal inheritance in JavaScript is described by Douglas Crockford as:

{{Blockquote

|You make prototype objects, and then ... make new instances. Objects are mutable in JavaScript, so we can augment the new instances, giving them new fields and methods. These can then act as prototypes for even newer objects. We don't need classes to make lots of similar objects... Objects inherit from objects. What could be more object oriented than that?{{cite web|last=Crockford|first=Douglas|title=Prototypal Inheritance in JavaScript|url=https://javascript.crockford.com/prototypal.html|access-date=20 August 2013|archive-date=13 August 2013|archive-url=https://web.archive.org/web/20130813163035/https://javascript.crockford.com/prototypal.html|url-status=live}}

}}

In JavaScript, an object is an associative array, augmented with a prototype (see below); each key provides the name for an object property, and there are two syntactical ways to specify such a name: dot notation (obj.x = 10) and bracket notation (obj["x"] = 10). A property may be added, rebound, or deleted at run-time. Most properties of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using a for...in loop.

{{Main|Prototype-based programming}}

JavaScript uses prototypes where many other object-oriented languages use classes for inheritance,{{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}} but it's still possible to simulate most class-based features with the prototype system.{{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 }} Additionally, 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.

class Person {

constructor(name) {

this.name = name;

}

}

class Student extends Person {

constructor(name, id) {

super(name);

this.id = id;

}

}

const bob = new Student("Robert", 12345);

console.log(bob.name); // Robert

Though the underlying object mechanism is still based on prototypes, the newer syntax is similar to other object oriented languages. Private variables are declared by prefixing the field name with a number sign (#), and polymorphism is not directly supported, although it can be emulated by manually calling different functions depending on the number and type of arguments provided.{{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}}

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 Object prototype).{{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}} ECMAScript 5 offers the Object.create method, allowing explicit creation of an instance without automatically inheriting from the Object prototype (older environments can assign the prototype to null).{{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}} The constructor's prototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the function used as a constructor.// This code is completely equivalent to the previous snippet

function Person(name) {

this.name = name;

}

function Student(name, id) {

Person.call(this, name);

this.id = id;

}

var bob = new Student("Robert", 12345);

console.log(bob.name); // RobertJavaScript's built-in classes, such as Array and Object, also have prototypes that can be modified. However, it's generally considered bad practice to 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.{{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}}

{{Main|Method (computer science)}}

Unlike in many object-oriented languages, in JavaScript there is no distinction between a function definition and a 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.

{{Main|Functional programming}}

JavaScript functions are first-class; a function is considered to be an object.{{cite web|title=Function – 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}} As such, a function may have properties and methods, such as .call() and .bind().{{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 }}

{{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 lexical closure: the 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}}

{{Main|Anonymous function}}

JavaScript also supports anonymous functions.

{{Main|Delegation (object-oriented programming)}}

JavaScript supports implicit and explicit delegation.

{{Main|Role-oriented programming|Traits (computer science)|Mixin}}

JavaScript natively supports various function-based implementations of Role[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.blogspot.de, April 11, 2014. patterns like Traits[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.{{cite web |url=https://cocktailjs.github.io/ |title=Home | 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 }} and Mixins.{{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… }} Such a function defines additional behavior by at least one method bound to the this keyword within its function body. A Role then has to be delegated explicitly via call or apply to objects that need to feature additional behavior that is not shared via the prototype chain.

Whereas explicit function-based delegation does cover 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 in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.

JavaScript is a zero-index language.

{{Main|Variadic function}}

An indefinite number of parameters can be passed to a function. The function can access them through formal parameters and also through the local arguments object. Variadic functions can also be created by using the [https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind bind] method.

{{Main|Associative arrays|Object literal}}

Like in many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, these literals form the basis of the JSON data format.

{{Main|Regular expression}}

JavaScript supports regular expressions for text searches and manipulation.{{r|n=Haverbeke2024|p=139}}

JavaScript supports promises and Async/await for handling asynchronous operations.{{citation needed|date=August 2023}}

{{Main|Futures and promises}}

A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. JavaScript supplies combinator methods, which allow developers to combine multiple JavaScript promises and do operations based on different scenarios. The methods introduced are: Promise.race, Promise.all, Promise.allSettled and Promise.any.

{{Main|Async/await}}

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.

Historically, some JavaScript engines supported these non-standard features:

• array comprehensions and generator expressions (like Python)
• concise function expressions (function(args) expr; 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{{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}})

{{Main|JavaScript syntax}}

Variables in JavaScript can be defined using either the var,{{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 }} 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 }} or 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 }} keywords. Variables defined without keywords will be defined at the global scope.

Arrow functions were first introduced in 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.

// Declares a function-scoped variable named `x`, and implicitly assigns the

// special value `undefined` to it. Variables without value are automatically

// set to undefined.

// var is generally considered bad practice and let and const are usually preferred.

var x;

// Variables can be manually set to `undefined` like so

let x2 = undefined;

// Declares a block-scoped variable named `y`, and implicitly sets it to

// `undefined`. The `let` keyword was introduced in ECMAScript 2015.

let y;

// Declares a block-scoped, un-reassignable variable named `z`, and sets it to

// a string literal. The `const` keyword was also introduced in ECMAScript 2015,

// and must be explicitly assigned to.

// The keyword `const` means constant, hence the variable cannot be reassigned

// as the value is `constant`.

const z = "this value cannot be reassigned!";

// Declares a global-scoped variable and assigns 3. This is generally considered

// bad practice, and will not work if strict mode is on.

t = 3;

// Declares a variable named `myNumber`, and assigns a number literal (the value

// `2`) to it.

let myNumber = 2;

// Reassigns `myNumber`, setting it to a string literal (the value `"foo"`).

// JavaScript is a dynamically-typed language, so this is legal.

myNumber = "foo";

Note the comments in the examples above, all of which were preceded with two forward slashes.

More examples can be found at the Wikibooks page on JavaScript syntax examples.

{{See also|Browser security}}

JavaScript and the 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 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.

There are subsets of general JavaScript—ADsafe, Secure ECMAScript (SES)—that provide greater levels of security, especially on code created by third parties (such as advertisements).{{cite web | url=https://www.adsafe.org/ | title=Making JavaScript Safe for Advertising | publisher=ADsafe | access-date=2021-05-08 | archive-date=2021-07-06 | archive-url=https://web.archive.org/web/20210706153324/https://www.adsafe.org/ | url-status=live }}{{cite web | url=https://code.google.com/p/es-lab/wiki/SecureEcmaScript | title=Secure ECMA Script (SES) | access-date=May 26, 2013 | archive-date=May 15, 2013 | archive-url=https://web.archive.org/web/20130515073412/https://code.google.com/p/es-lab/wiki/SecureEcmaScript | url-status=live }} Closure Toolkit is another project for safe embedding and isolation of third-party JavaScript and HTML.{{cite web|title=Google Caja Project|url=https://developers.google.com/caja/|url-status=live|website=Google|access-date=2021-07-09|archive-date=2021-01-22|archive-url=https://web.archive.org/web/20210122083321/https://developers.google.com/caja/}}

Content Security Policy is the main intended method of ensuring that only trusted code is executed on a Web page.

{{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. One important solution to XSS vulnerabilities is HTML sanitization.

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.

XSS vulnerabilities can also occur because of implementation mistakes by browser authors.{{cite web |url=https://www.mozillazine.org/talkback.html?article=4392 |title=Mozilla Cross-Site Scripting Vulnerability Reported and Fixed – 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 }}

{{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