ECMAScript is a scripting programming language, standardized by Ecma International in the ECMA-262 specification. The language is widely used on the web, and is often referred to as JavaScript or JScript, after the two primary dialects of the specification.

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 wide-spread success of JavaScript as a client-side scripting language for web pages, Microsoft developed a compatible language known as JScript. JScript added new date methods to fix the Y2K non-compliant methods in JavaScript, which were based on java.util.Date. JScript was included in Internet Explorer 3.0, released in August 1996.

Netscape submitted JavaScript to Ecma International for standardization; 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 extra features not described in the ECMA specification.

The name "ECMAScript" was a compromise between the organizations involved in standardizing the language, particularly Netscape and Microsoft. 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 three editions of ECMA-262 published, and the work on the fourth edition is in progress.

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,thereforeallowing 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 is one place where the ECMAScript Compact Profile is used in favour of full ECMAScript in order to reduce processing and memory requirements on a device.

Features

Further information: ECMAScript features

Syntax

Further information: ECMAScript syntax

Dialects

ECMAScript is supported in many applications, particularly web browsers, where it is frequently called JavaScript. Dialects typically include their own, different standard libraries, of which some are standardized separately - such as the W3C-specified DOM. Some implementations, such as ActionScript used in Flash, have a completely different set of libraries. This means that applications written in one dialect of ECMAScript will not likely work in another, unless they are designed to be compatible.

Note (1): Gecko 1.8.1 has partial support of E4X and a few other features, see New in JavaScript 1.7.

Note (2): 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.

Note (3): In addition to supporting ECMA-262 edition 3, ActionScript 2 also included support of properties, methods, and mechanisms that were proposed in early draft specifications of as yet unseen versions of ECMAScript. It remains to be seen if ActionScript will stay in sync with future changes to the ECMAScript specifications.

Note (4): Adobe asserts it implements the preliminary edition 4 of ECMA-262

Note (5): As of version 4, OpenLaszlo uses standard ECMAScript Release 3 with some ECMAScript Release 4

Note (6): The present WebKit binaries, as of April 2007, also implement at least part of the Javascript 1.6 extras

Note (7): The Mozilla implementations, (SpiderMonkey in the C programming language and Rhino in the Java programming language), are used in several third-party programs, including the Yahoo! Widget Engine (Konfabulator) and the Macintosh system-level scripting language JavaScript OSA.

Note (8): Apple's Safari uses JavaScriptCore which is based on the KDE KJS library.

Version correspondence

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

Fourth edition

The ECMA-262 fourth edition is the first major update to ECMAScript since the third edition published in 1999. The specification (along with the reference implementation) is currently under development and is expected to be finished by October 2008 .

An overview of the language was released by the working group on October 22, 2007 and is available at http://www.ecmascript.org/es4/spec/overview.pdf

Features

The new version of the language is mostly backwards compatible with ECMAScript 3 (see below), while adding multiple new features, such as:

Classes

Structural types

Packages and namespaces

Optional type annotations and static typing

Generators and iterators

Destructuring assignment

JSON Encoding/Decoding

Algebraic data types

ECMAScript 4 intends 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 are fixed in edition 4 .

A document describing known incompatibilities between ES3 and ES4 is available from ecmascript.org

Implementations

Since the specification is not yet finished, there are no full implementations of the language at this time. However several implementations are in progress:

Tamarin, an open-source ECMAScript engine, will implement ES4. Mozilla plans to use Tamarin in Firefox 4.

TG1 is working on the reference implementation in SML/NJ and the work-in-progress is available from

According to Brendan Eich, there are several other "industry-scale implementations underway"

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 as it was less controversial, being only an update to Edition 3.

The upgrade is not without controversy. In late 2007, 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.