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* ECMAScript

* v * t * e

ECMASCRIPT (or ES) is a trademarked scripting-language specification standardized by Ecma International in ECMA-262 and ISO/IEC 16262. It was created to standardize JavaScript
, so as to foster multiple independent implementations. JavaScript
has remained the best-known implementation of ECMAScript since the standard was first published, with other well-known implementations including JScript and ActionScript
. Coders commonly use ECMAScript for client-side scripting on the World Wide Web
World Wide Web
, and it is increasingly being used for writing server applications and services using Node.js .


* 1 History

* 1.1 Versions * 1.2 4th Edition (abandoned) * 1.3 5th Edition * 1.4 6th Edition - ECMAScript 2015 * 1.5 7th Edition - ECMAScript 2016 * 1.6 8th Edition - ECMAScript 2017 * 1.7 ES.Next

* 2 Features * 3 Syntax * 4 Implementations * 5 Version correspondence * 6 Conformance tests * 7 See also * 8 References * 9 External links


The ECMAScript specification is a standardized specification of a scripting language developed by Brendan Eich
Brendan Eich
of Netscape ; initially it was named Mocha, later LiveScript, and finally JavaScript. In December 1995, Sun Microsystems
Sun Microsystems
and Netscape announced JavaScript
in a press release. In March 1996, Netscape Navigator
Netscape Navigator
2.0 was released, featuring support for JavaScript.

Owing to the widespread success of JavaScript
as a client-side scripting language for Web pages, Microsoft
developed a compatible dialect of the language, naming it JScript to avoid trademark issues. JScript added new date methods to alleviate the Year 2000 problem caused by the JavaScript
methods that were based on the Java Date class. JScript was included in Internet Explorer
Internet Explorer
3.0, released in August 1996.

Netscape delivered JavaScript
to Ecma International for standardization and the work on the specification, ECMA-262, began in November 1996. 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 ."

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


There are seven editions of ECMA-262 published. Work on version 7 of the standard, was finalized in June 2016.


1 June 1997 First edition Guy L. Steele Jr.

2 June 1998 Editorial changes to keep the specification fully aligned with ISO/IEC 16262 international standard Mike Cowlishaw

3 December 1999 Added regular expressions , better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and other enhancements Mike Cowlishaw

4 Abandoned Fourth Edition was abandoned, due to political differences concerning language complexity. Many features proposed for the Fourth Edition have been completely dropped; some are proposed for ECMAScript Harmony.

5 December 2009 Adds "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. Pratap Lakshman , Allen Wirfs-Brock

5.1 June 2011 This 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

6 June 2015 The Sixth Edition, initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015 (ES2015) adds significant new syntax for writing complex applications, including classes and modules, 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 and generator expressions, arrow functions, binary data, typed arrays, collections (maps, sets and weak maps), promises , number and math enhancements, reflection, and proxies (metaprogramming for virtual objects and wrappers). As the first " ECMAScript Harmony" specification, it is also known as "ES6 Harmony." Allen Wirfs-Brock

7 June 2016 The Seventh Edition, also known as ECMAScript 2016, intended to continue the themes of language reform, code isolation, control of effects and library/tool enabling from ES2015, includes two new features: the exponentiation operator (**) and Array.prototype.includes. Brian Terlson

8 June 2017 New features proposed include concurrency and atomics, zero-copy binary data transfer, more number and math enhancements, syntactic integration with promises (await/async), observable streams, SIMD types, better metaprogramming with classes, class and instance properties, operator overloading, value types (first-class primitive-like objects), records and tuples, and traits.

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.


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. An overview of the language was released by the working group on October 23, 2007.

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

* classes , * a module system , * optional type annotations and static typing , probably using a structural type system , * generators and iterators , * destructuring assignment , and * algebraic data types .

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. These fixes and others, and support for JSON
encoding/decoding, have been folded into the ECMAScript, 5th Edition specification.

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
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," and that stakeholders who opposed the changes were being "hidden from view". 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. He also pointed out that Microsoft
Silverlight and Adobe AIR
Adobe AIR
rely on C# and ActionScript
3 respectively, both of which are larger and more complex than ECMAScript Edition 3.


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
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. 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. On December 3, 2009, ECMA-262 5th edition was published.


The 6th edition, officially known as ECMAScript 2015, was finalized in June 2015. This update adds significant new syntax for writing complex applications, including classes and modules, 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 and generator expressions, arrow functions, binary data, typed arrays, collections (maps, sets and weak maps), promises , number and math enhancements, reflection, and proxies (metaprogramming for virtual objects and wrappers). The complete list is extensive.

Browser support for ES2015 is still incomplete. However, ES2015 code can be transpiled into ES5 code, which has more consistent support across browsers. Transpiling adds an extra step to build processes whereas polyfills allow adding extra functionalities by including another JavaScript


The 7th edition, officially known as ECMAScript 2016, was finalized in June 2016. New features include the exponentiation operator (**), Array.prototype.includes (not to be confused with ClassList.contains).


The 8th edition, officially known as ECMAScript 2017, was finalized in June 2017. Includes await /async, which works using generators and promises.


ES.Next is a dynamic name that refers to whatever the next version is at time of writing. ES.Next features are more correctly called proposals, because, by definition, the specification has not been finalized yet.


THIS SECTION NEEDS EXPANSION. You can help by adding to it . (February 2017)

Main article: JavaScript
§ Features

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


THIS SECTION IS EMPTY. You can help by adding to it . (February 2017)

Main article: ECMAScript syntax


Main article: List of ECMAScript engines

ECMAScript is supported in many applications, especially Web browsers , where it is implemented by JavaScript, or, in the case of Internet Explorer, JScript. Implementations sometimes include extensions to the language, or to the standard library and related application programming interfaces (API) such as the World Wide Web
World Wide Web
Consortium (W3C) specified Document Object Model
Document Object Model
(DOM). This means that applications written in one implementation may be incompatible with another, unless they are written to use only a common subset of supported features and APIs.


SpiderMonkey Firefox
, the Gecko layout engine , Adobe Acrobat
Adobe Acrobat

V8 Google Chrome
Google Chrome
, Node.js
, Opera , MarkLogic . 2016 and features from 2017

JavaScriptCore (Nitro) WebKit
, Safari , Qt 5 2017

Chakra Microsoft
Edge 5.1 and features from 2015, 2016 and 2017

JerryScript Resource constrained IoT devices, Pebble 5.1

JScript 9.0 Internet Explorer
Internet Explorer
, the Trident layout engine 5.1

Nashorn Java 9 6

Nashorn Java 5.1

Rhino Java Platform, Standard Edition 3

Carakan (deprecated) Opera 12 5.1

5.1 and features from 2015

Ejscript Appweb Web Server, Samba 4 2015

JScript .NET Microsoft
.NET Framework
.NET Framework

Adobe Flash
Adobe Flash
, Adobe Flex
Adobe Flex
, Adobe AIR
Adobe AIR

ExtendScript Adobe Creative Suite products: InDesign , Illustrator , Photoshop , Bridge , After Effects , Premiere Pro 3

InScript iCab 3

Max/MSP engine Max 3

QtScript (deprecated) KDE SC 4 3



* ^ Adobe Acrobat
Adobe Acrobat
X uses the SpiderMonkey 1.8 engine: JavaScript for Acrobat * ^ According to the widely used compatibility table, Firefox supports the vast majority, but not all, of the features introduced in ECMAScript 2015. See Tracking bug for ECMAScript 6 in SpiderMonkey for missing features. * ^ According to the widely used compatibility table, Chrome supports the vast majority, but not all, of the features introduced in ECMAScript 2015. * ^ Microsoft
states that Edge "supports most ES2015 features," supporting 81% of the specification as of May 2015 and 96% as of April 2017. * ^ Opera's implementation includes some JavaScript
and JScript extensions: ECMAScript support in Opera Presto 2.3 * ^ Microsoft
asserts that JScript 8.0 supports "almost all of the features of the ECMAScript Edition 3 Language Specification", but does not list the unsupported features.


Items on the same line are approximately the same language.


1.0 (Netscape 2.0, March 1996) 1.0 (IE 3.0 – early versions, August 1996)

1.1 (Netscape 3.0, August 1996) 2.0 (IE 3.0 – later versions, January 1997)

1.2 (Netscape 4.0-4.05, June 1997)

1.3 (Netscape 4.06-4.7x, October 1998) 3.0 (IE 4.0, Oct 1997) Edition 1 (June 1997) / Edition 2 (June 1998)

1.4 (Netscape Server only) 4.0 (Visual Studio 6, no IE release)

5.0 (IE 5.0, March 1999)

5.1 (IE 5.01)

1.5 (Netscape 6.0, Nov 2000; also later Netscape and Mozilla
releases) 5.5 (IE 5.5, July 2000) Edition 3 (December 1999)

5.6 (IE 6.0, October 2001)

1.6 (Gecko 1.8, Firefox
1.5, November 2005)

Edition 3, with some compliant enhancements: ECMAScript for XML (E4X), Array extras (e.g. Array.prototype.forEach), Array and String generics (New in JavaScript

1.7 (Gecko 1.8.1, Firefox
2, October 2006)

Edition 3 plus all JavaScript
1.6 enhancements, plus Pythonic generators and array comprehensions (), block scope with let, destructuring assignment (var =) (New in JavaScript

1.8 (Gecko 1.9, Firefox
3, June 2008)

Edition 3 plus all JavaScript
1.7 enhancements, plus expression closures (function(x) x * x), generator expressions, and more (New in JavaScript

JScript .NET ( ASP.NET ; no IE release) ( JScript .NET is said to have been designed with the participation of other Ecma members)


In 2010, Ecma International started developing a standards test for Ecma 262 ECMAScript. 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.

Development of test262 is a project of 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 already contains more than 11,000 tests and is being developed further as of 2013 .

The following table shows current conformance results of browser products. Lower scores are better, although scores can not be compared, as tests are not weighted. Also, be aware that Test262 itself is likely to contain bugs that may impact a browser's score. So browsers with a score significantly lower than the current test suite bug count may not necessarily do better than those with a higher one. That may be particularly true when several browsers have a higher score in their current development builds as compared to their last released version.

Results of test262 (suite version: ES5, suite date: 2014-09-18) PRODUCT LATEST STABLE TEST262 FAILED PREVIEW/BETA TEST262 FAILED ALPHA TEST262 FAILED NIGHTLY TEST262 FAILED

Google Chrome
Google Chrome
46.0.2490.86 m 7002207000000000000♠207/11552 47.0.2526.69 beta-m 7002208000000000000♠208/11552 48.0.2564.10 dev-m 7002203000000000000♠203/11552 48.0.2569.0 canary 7002203000000000000♠203/11552

42.0 7002260000000000000♠260/11552 43.0 Beta 5 7002260000000000000♠260/11552 44.0a2 (20151120004044) 7002260000000000000♠260/11552 45.0a1 (20151120030227) 7002260000000000000♠260/11552

ESR 38.4.0 7002228000000000000♠228/11552

Internet Explorer
Internet Explorer
11.0.25 (11.0.9600.18097) 7000800000000000000♠8/11552

Maxthon 7001180000000000000♠18/11552

Opera 33.0.1990.115 7002207000000000000♠207/11552 beta 34.0.2036.3 7002208000000000000♠208/11552 developer 35.0.2052.0 7002210000000000000♠210/11552

12.17 (classic) 7001110000000000000♠11/11552

Safari 7.1 (9537.85) 7000700000000000000♠7/11552


* ActionScript
* Comparison of layout engines (ECMAScript) * Dart (programming language)
Dart (programming language)
* Document Object Model
Document Object Model
(DOM) * ECMAScript for XML (E4X) * JavaScript
* JScript * List of ECMAScript engines * Qt Meta (or Modeling) Language ( QML ) * Server-side JavaScript
* TypeScript
* Gulp.js


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* Official website * The World of ECMAScript: John