Last updated
Paradigm Multi-paradigm: prototype-based, functional, imperative
Designed by Brendan Eich, Ecma International
First appeared1997;23 years ago (1997)
Typing discipline weak, dynamic
Major implementations
JavaScript, SpiderMonkey, V8, ActionScript, JScript, QtScript, InScript, Google Apps Script
Influenced by
Self, HyperTalk, AWK, C, CoffeeScript, Perl, Python, Java, Scheme
Crystal source.png
Filename extensions .es
Internet media type application/ecmascript
Developed by Sun Microsystems,
Ecma International
Initial releaseJune 1997;22 years ago (1997-06)
Latest release
Edition 10
(June 2019;8 months ago (2019-06))
Type of format Scripting language
Website ECMA-262, ECMA-290,
ECMA-327, ECMA-357,

ECMAScript (or ES) [1] is a scripting-language specification standardized by Ecma International. It was created to standardize JavaScript to help foster multiple independent implementations. JavaScript has remained the most widely used implementation of ECMAScript since the standard was first published, with other implementations including JScript and ActionScript. [2] ECMAScript is commonly used for client-side scripting on the World Wide Web, and it is increasingly being used for writing server applications and services using Node.js.



The ECMAScript specification is a standardized specification of a scripting language developed by Brendan Eich of Netscape; initially it was named Mocha, later LiveScript, and finally JavaScript. [3] In December 1995, Sun Microsystems and Netscape announced JavaScript in a press release. [4] In November 1996, Netscape announced a meeting of the Ecma International standards organization to advance the standardization of JavaScript. [5] The first edition of ECMA-262 was adopted by the Ecma General Assembly in June 1997. Several editions of the language standard have been published since then. The name "ECMAScript" was a compromise between the organizations involved in standardizing the language, especially Netscape and Microsoft, whose disputes dominated the early standards sessions. Eich commented that "ECMAScript was always an unwanted trade name that sounds like a skin disease." [6] ECMAScript has been formalised through operational semantics by work at Stanford University and the Department of Computing, Imperial College London for security analysis and standardization. [7]

While both JavaScript and JScript aim to be compatible with ECMAScript, they also provide additional features not described in the ECMA specifications. [8] [9]


There are ten editions of ECMA-262 published. Work on version 10 of the standard was finalized in June 2019. [10]

EditionDate publishedNameChanges from prior editionEditor
1June 1997First edition Guy L. Steele Jr.
2June 1998Editorial changes to keep the specification fully aligned with ISO/IEC 16262 international standard Mike Cowlishaw
3December 1999Added regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and other enhancementsMike Cowlishaw
4AbandonedFourth Edition was abandoned, due to political differences concerning language complexity. Many features proposed for the Fourth Edition have been completely dropped; some were incorporated into the sixth edition.
5December 2009Adds "strict mode," a subset intended to provide more thorough error checking and avoid error-prone constructs. Clarifies many ambiguities in the 3rd edition specification, and accommodates behaviour of real-world implementations that differed consistently from that specification. Adds some new features, such as getters and setters, library support for JSON, and more complete reflection on object properties. [11] Pratap Lakshman, Allen Wirfs-Brock
5.1June 2011This edition 5.1 of the ECMAScript standard is fully aligned with third edition of the international standard ISO/IEC 16262:2011.Pratap Lakshman, Allen Wirfs-Brock
6June 2015 [12] ECMAScript 2015 (ES2015)See 6th Edition - ECMAScript 2015 Allen Wirfs-Brock
7June 2016 [13] ECMAScript 2016 (ES2016)See 7th Edition - ECMAScript 2016 Brian Terlson
8June 2017 [14] ECMAScript 2017 (ES2017)See 8th Edition - ECMAScript 2017 Brian Terlson
9June 2018 [15] ECMAScript 2018 (ES2018)See 9th Edition - ECMAScript 2018 Brian Terlson
10June 2019 [10] ECMAScript 2019 (ES2019)See 10th Edition - ECMAScript 2019 Brian Terlson, Bradley Farias, Jordan Harband

In June 2004, Ecma International published ECMA-357 standard, defining an extension to ECMAScript, known as ECMAScript for XML (E4X). Ecma also defined a "Compact Profile" for ECMAScript – known as ES-CP, or ECMA 327 – that was designed for resource-constrained devices, which was withdrawn in 2015. [16]

4th Edition (abandoned)

The proposed fourth edition of ECMA-262 (ECMAScript 4 or ES4) would have been the first major update to ECMAScript since the third edition was published in 1999. The specification (along with a reference implementation) was originally targeted for completion by October 2008. [17] An overview of the language was released by the working group on October 23, 2007. [18]

By August 2008, the ECMAScript 4th edition proposal had been scaled back into a project codenamed ECMAScript Harmony. Features under discussion for Harmony at the time included

The intent of these features was partly to better support programming in the large , and to allow sacrificing some of the script's ability to be dynamic to improve performance. For example, Tamarin – the virtual machine for ActionScript, developed and open-sourced by Adobe – has just-in-time compilation (JIT) support for certain classes of scripts.

In addition to introducing new features, some ES3 bugs were proposed to be fixed in edition 4. [19] [20] These fixes and others, and support for JSON encoding/decoding, have been folded into the ECMAScript, 5th Edition specification. [21]

Work started on Edition 4 after the ES-CP (Compact Profile) specification was completed, and continued for approximately 18 months where slow progress was made balancing the theory of Netscape's JavaScript 2 specification with the implementation experience of Microsoft's JScript .NET. After some time, the focus shifted to the ECMAScript for XML (E4X) standard. The update has not been without controversy. In late 2007, a debate between Eich, later the Mozilla Foundation's CTO, and Chris Wilson, Microsoft's platform architect for Internet Explorer, became public on a number of blogs. Wilson cautioned that because the proposed changes to ECMAScript made it backwards incompatible in some respects to earlier versions of the language, the update amounted to "breaking the Web," [22] and that stakeholders who opposed the changes were being "hidden from view". [23] Eich responded by stating that Wilson seemed to be "repeating falsehoods in blogs" and denied that there was attempt to suppress dissent and challenged critics to give specific examples of incompatibility. [24] He pointed out that Microsoft Silverlight and Adobe AIR rely on C# and ActionScript 3 respectively, both of which are larger and more complex than ECMAScript Edition 3. [25]

5th Edition

Yahoo, Microsoft, Google, and other 4th edition dissenters formed their own subcommittee to design a less ambitious update of ECMAScript 3, tentatively named ECMAScript 3.1. This edition would focus on security and library updates with a large emphasis on compatibility. After the aforementioned public sparring, the ECMAScript 3.1 and ECMAScript 4 teams agreed on a compromise: the two editions would be worked on, in parallel, with coordination between the teams to ensure that ECMAScript 3.1 remains a strict subset of ECMAScript 4 in both semantics and syntax.

However, the differing philosophies in each team resulted in repeated breakages of the subset rule, and it remained doubtful that the ECMAScript 4 dissenters would ever support or implement ECMAScript 4 in the future. After over a year since the disagreement over the future of ECMAScript within the Ecma Technical Committee 39, the two teams reached a new compromise in July 2008: Brendan Eich announced that Ecma TC39 would focus work on the ECMAScript 3.1 (later renamed to ECMAScript, 5th Edition) project with full collaboration of all parties, and vendors would target at least two interoperable implementations by early 2009. [26] [27] In April 2009, Ecma TC39 published the "final" draft of the 5th edition and announced that testing of interoperable implementations was expected to be completed by mid-July. [28] On December 3, 2009, ECMA-262 5th edition was published. [29]

6th Edition - ECMAScript 2015

The 6th edition, initially known as ECMAScript 6 (ES6) then and later renamed to ECMAScript 2015, was finalized in June 2015. [12] [30] This update adds significant new syntax for writing complex applications, including class declarations (classFoo{...}), ES6 modules like import*asmoduleNamefrom"...";exportconstFoo, but defines them semantically in the same terms as ECMAScript 5 strict mode. Other new features include iterators and for...of loops, Python-style generators, arrow function expression (()=>{...}), let keyword for local declarations, const keyword for constant variable declarations, binary data, typed arrays, new collections (maps, sets and WeakMap), promises, number and math enhancements, reflection, proxies (metaprogramming for virtual objects and wrappers) and template literals for strings. [31] [32] The complete list is extensive. [33] [34] As the first "ECMAScript Harmony" specification, it is also known as "ES6 Harmony."

7th Edition - ECMAScript 2016

The 7th edition, officially known as ECMAScript 2016, was finalized in June 2016. [13] The major standard language features include block-scoping of variables and functions, destructuring patterns (of variables), proper tail calls, exponentiation operator ** for numbers, await, async keywords for asynchronous programming. [13] [35]

8th Edition - ECMAScript 2017

The 8th edition, officially known as ECMAScript 2017, was finalized in June 2017. [14] Includesasync/awaitconstructions, which work using generators and promises. [36] ECMAScript 2017 (ES2017), the eighth edition, includes features for concurrency and atomics, syntactic integration with promises (async/await). [36] [14]

9th Edition - ECMAScript 2018

The 9th edition, officially known as ECMAScript 2018, was finalized in June 2018. [15] New features include rest/spread operators for variables (three dots: ...identifier), asynchronous iteration, Promise.prototype.finally() and additions to RegExp. [15]

10th Edition - ECMAScript 2019

The 10th edition, officially known as ECMAScript 2019, was published in June 2019. [10] Added features include, but are not limited to, Array.prototype.flat, Array.prototype.flatMap, changes to Array.sort and Object.fromEntries. [10]


ES.Next is a dynamic name that refers to whatever the next version is at the time of writing. ES.Next features are finished proposals (aka "stage 4 proposals") as listed at finished proposal that are not part of a ratified specification. The language committee follows a "living spec" model so these changes are part of the standard and ratification is a formality. [ citation needed ]


The ECMAScript language includes structured, dynamic, functional, and prototype-based features. [37]

Imperative and structured

ECMAScript JavaScript supports C style structured programming. However, there exist some dissimilarities between both languages implementation of scoping. Until ECMAScript 2015, JavaScript supported only function scoping using the keyword var. ECMAScript 2015 added the keywords let and const allowing JavaScript to support both block scoping as well as function scoping. JavaScript supports automatic semicolon insertion, meaning that semicolons that are normally used to terminate a statement in C may be omitted in JavaScript. [38]

Weakly typed

ECMAScript JavaScript is weakly typed. This means that certain types are assigned implicitly based on the operation being performed. However, there are several quirks in JavaScript's implementation of the conversion of a variable from one type to another. These quirks have drawn criticism from many developers.[ citation needed ]


ECMAScript JavaScript is dynamically typed. Thus, a type is associated with a value rather than an expression. ECMAScript JavaScript supports various ways to test the type of objects, including duck typing. [39]


Since ES 2015, transpiling JavaScript has become very common. Transpilation is a source-to-source compilation in which the newer versions of JavaScript are used in the user's source code and the transpiler rewrites them so that they are compliant with the current specification. Usually, transpilers transpile down to ES3 to maintain compatibility with all versions of browsers. The settings to transpiling to a specific version can be configured according to need. Transpiling adds an extra step to the build process and is sometimes done to avoid needing polyfills. Polyfills allow using functionalities from newer ECMA versions in older environments that lack them. Polyfills do this at runtime in the interpreter, such as the user's browser or on the server. Instead, transpiling rewrites the ECMA code itself during the build phase of development, before it reaches the interpreter.


In 2010, Ecma International started developing a standards test for Ecma 262 ECMAScript. [40] Test262 is an ECMAScript conformance test suite that can be used to check how closely a JavaScript implementation follows the ECMAScript 5th Edition Specification. The test suite contains thousands of individual tests, each of which tests some specific requirements of the ECMAScript specification. The development of Test262 is a project of the Ecma Technical Committee 39 (TC39). The testing framework and individual tests are created by member organizations of TC39 and contributed to Ecma for use in Test262.

Important contributions were made by Google (Sputnik testsuite) and Microsoft who both contributed thousands of tests. The Test262 testsuite consisted of 38014 tests as of January 2020. [41] ECMAScript specifications through ES7 are well-supported in major web browsers. The table below shows the conformance rate for current versions of software with respect to the most recent editions of ECMAScript.

Scripting engineReference application(s)Conformance [42]
ES5 [43] ES6 [44] ES7 [45] Newer (2016+) [45] [46]
Chakra Microsoft Edge 18100%96%100%48%
SpiderMonkey Firefox 67100%98%100%83%
Chrome V8 Google Chrome 75, Opera 62100%98%100%98%
JavaScriptCore (Nitro) Safari 12.199%99%100%87%

See also

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ISO Standard
Ecma Standards