ECMAScript

ECMAScript

Paradigm	Multi-paradigm: prototype-based, functional, imperative, scripting
Appeared in	1997
Designed by	Brendan Eich, Ecma International
Typing discipline	duck, weak, dynamic
Dialects	JavaScript, ActionScript, JScript, QtScript, DMDScript, InScript
Influenced by	Self, HyperTalk, AWK, C, Perl, Python, Java

ECMAScript

File:Crystal source.png
Filename extension	.es
Internet media type	application/ecmascript
Developed by	Sun Microsystems, Ecma International
Initial release	June 1997
Latest release	Edition 5 / December 2009
Type of format	Scripting language
Extended from	JavaScript
Website	ECMA-262, ECMA-290, ECMA-327, ECMA-357

This article is part of the JavaScript series.

JavaScript

JavaScript syntax

JavaScript topics

ECMAScript is a scripting language, standardized by Ecma International in the ECMA-262 specification and ISO/IEC 16262. The language is widely used on the web, especially in the form of its three best-known dialects, JavaScript, ActionScript, and JScript.

History

JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, later LiveScript, and finally renamed to JavaScript. In December 1995, Sun Microsystems and Netscape announced JavaScript in a press release. In March 1996, Netscape Navigator 2.0 was out, featuring support for JavaScript.

Due 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 fix the non-Y2K-friendly methods in JavaScript, which were based on java.util.Date. JScript was included in 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 of June 1997.

ECMAScript is the name of the scripting language standardized in ECMA-262. Both JavaScript and JScript aim to be compatible with ECMAScript, while providing additional features not described in the Ecma specification.

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. Brendan Eich, the creator of JavaScript, is on record as saying that "ECMAScript was always an unwanted trade name that sounds like a skin disease."

Versions

There are four editions of ECMA-262 published. Work on a future edition codenamed "Harmony", is in progress.

Edition	Date published	Differences to the previous edition	Editor
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, with some of the work forming the basis of Fifth Edition and some forming the basis of 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
Harmony	Work in progress	Multiple new concepts and language features — see the section "Future development" below.

In June 2004, Ecma International published ECMA-357 standard, defining an extension to ECMAScript, known as E4X (ECMAScript for XML).

Ecma also defined a "Compact Profile" for ECMAScript — known as ES-CP, or ECMA 327 — which is designed for resource-constrained devices. Several of the dynamic features of ECMAScript (such as the "eval" function) are made optional, thus allowing the runtime to make more assumptions about the behaviour of programs and therefore make more performance trade-offs when running the code. The HD DVD standard was one place where the ECMAScript Compact Profile was used in favour of full ECMAScript in order to reduce processing and memory requirements on a device.

Features

Further information: ECMAScript features

The ECMAScript language includes structured, dynamic, functional, and prototype-based features, as officially summarized here.

Syntax

Further information: ECMAScript syntax

Dialects

ECMAScript is supported in many applications, especially web browsers, where it is commonly called JavaScript. Dialects sometimes include extensions to the language, or to the standard library and related APIs such as the W3C-specified DOM. This means that applications written in one dialect may be incompatible with another, unless they are written to use only a common subset of supported features and APIs.

Note that there is a distinction between a dialect and an implementation. A dialect of a language is significant variation of the language, while an implementation of a language/dialect executes a program written in that dialect.

Application/Implementation	Dialect and latest version	ECMAScript edition
Mozilla Firefox, the Gecko layout engine, SpiderMonkey, and Rhino	JavaScript 1.8.1	ECMA-262, edition 3
Google Chrome, the V8 engine	JavaScript	ECMA-262, edition 3
Internet Explorer, the Trident layout engine	JScript 5.8	ECMA-262, edition 3
Opera	ECMAScript	ECMA-262, edition 3
KHTML layout engine, KDE's Konqueror, and Apple Inc.'s Safari	JavaScript	ECMA-262, edition 3
Appweb Web Server, Samba 4	Ejscript 0.9.9	ECMA-262, edition 3
Microsoft .NET Framework	JScript .NET 8.0	ECMA-262, edition 3
Adobe Flash and Adobe Flex	ActionScript 3	ECMA-262, edition 3
Adobe Acrobat	JavaScript 1.7	ECMA-262, edition 3
General purpose scripting language	DMDScript 1.15	ECMA-262
OpenLaszlo Platform	JavaScript	ECMA-262, edition 3
CriScript, JScript for game platforms	CriScript 0.91.0	ECMA-262, edition 3
iCab	InScript 3.22 (abandoned)	ECMA-262, edition 3
Max/MSP	JavaScript 1.5	ECMA-262, edition 3
ANT Galio 3	JavaScript 1.5	ECMA-262, edition 3
KDE	QtScript	ECMA-262, edition 3
	Caja	ECMA-262, edition 3
	Objective-J	ECMA-262, edition 3
	WMLScript	ECMA-262, edition 3

Version correspondence

The following table is based on and; items on the same line are approximately the same language.

JavaScript	JScript	ECMAScript
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: E4X, Array extras (e.g. Array.prototype.forEach), Array and String generics (New in JavaScript 1.6)
1.7 (Gecko 1.8.1, Firefox 2, October 2006)		Edition 3 plus all JavaScript 1.6 enhancements, plus Pythonic generators and array comprehensions ([a*a for (a in iter)]), block scope with let, destructuring assignment (var [a,b]=[1,2]) (New in JavaScript 1.7)
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 1.8)
	JScript .NET (ASP.NET; no IE release)	(JScript .NET is said to have been designed with the participation of other Ecma members)
JavaScript 2.0 (Work in progress)		Harmony (Work in progress; see the section "ECMAScript Harmony" below).

Future development

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 22, 2007.

As of August 2008, the ECMAScript 4th edition proposal has been scaled back into a project codenamed ECMAScript Harmony.

Features

Features under discussion for a future edition (originally "ECMAScript 4"; now ECMAScript Harmony) include:

• Classes
• A module system
• Optional type annotations and static typing, probably using a structural type system
• Generators and iterators
• Destructuring assignment
• Algebraic data types

The intent of these features is partly to better support "programming in the large", and to let programmers sacrifice some of the script's ability to be dynamic for performance. For example, Tamarin — the virtual machine for ActionScript developed and open sourced by Adobe — has JIT compilation support for certain classes of scripts.

Bug fixes and backwards compatibility

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.

History

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 E4X standard.

The update has not been without controversy. In late 2007, a debate between Eich, now 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," 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 challenging critics to give specific examples of incompatibility. He also 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.

ECMAScript, 5th Edition

Microsoft, Yahoo, 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 August 2008: Ecma TC39 announced it would focus work on the ECMAScript 3.1 (later renamed to ECMAScript, 5th Edition) project with full collaboration of all parties, and it would target 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.

ECMAScript Harmony

In the same announcement, Ecma TC39 also stated that the ECMAScript 4 proposal would be superseded by a new project, code-named ECMAScript Harmony. ECMAScript Harmony will include syntactic extensions, but the changes will be more modest than ECMAScript 4 in both semantic and syntactic innovation. Packages, namespaces and early binding from ECMAScript 4 are no longer included for planned releases. In addition, other goals and ideas from ECMAScript 4 are being rephrased to keep consensus in the committee; these include a notion of classes based on ECMAScript, 5th Edition (being an update to ECMAScript, 3rd edition). As of December 2009, there is no publicly announced release date for ECMAScript Harmony. Depending on Ecma, Harmony may end up being called ECMAScript, 6th edition.

See also

• ActionScript
• Comparison of layout engines (ECMAScript)
• Document Object Model
• E4X
• List of ECMAScript engines
• JavaScript

External links

• ECMAScript Official Website
• Ecma Standards
  • ECMA-262
    • ECMA-262 ECMAScript Language Specification 3rd edition (December 1999)
    • ECMAScript Language Specification, Edition 3 Final, 24-Mar-00
    • ECMA-262 ECMAScript Language Specification 5th edition (December 2009)
  • ECMA-290 ECMAScript Components Specification (June 1999)
  • ECMA-327 ECMAScript 3rd Edition Compact Profile (June 2001)
  • ECMA-357 ECMAScript for XML (E4X) Specification (June 2004)
• The World of ECMAScript : John Resig's map on ECMAScript