JScript is Microsoft's dialect of the ECMAScript standard that is used in Microsoft's Internet Explorer.
JScript is implemented as an Active Scripting engine. This means that it can be "plugged in" to OLE Automation applications that support Active Scripting, such as Internet Explorer, Active Server Pages, and Windows Script Host. It also means such applications can use multiple Active Scripting languages, e.g., JScript, VBScript or PerlScript.
JScript was first supported in the Internet Explorer 3.0 browser released in August 1996. Its most recent version is JScript 9.0, included in Internet Explorer 9.
JScript 10.0 is a separate dialect, also known as JScript.NET, which adds several new features from the abandoned fourth edition of the ECMAScript standard. It must be compiled for.NET Framework version 2 or version 4, but static type annotations are optional.

Comparison to JavaScript

As explained by Douglas Crockford in his talk titled The JavaScript Programming Language on YUI Theater,
However, JScript supports conditional compilation, which allows a programmer to selectively execute code within block comments. This is an extension to the ECMAScript standard that is not supported in other JavaScript implementations, thus making the above statement not completely true, although conditional compilation is no longer supported in Internet Explorer 11 Standards mode.
Other internal implementation differences between JavaScript and JScript, at some point in time, are noted on the Microsoft Developer Network. Although, the default type value for the script element in Internet Explorer is JavaScript, while JScript was its alias. In an apparent transition from JScript to JavaScript, online, the Microsoft Edge Developer Guide refers to the Mozilla MDN web reference library as its definitive documentation. As of October 2017, Microsoft MSDN pages for scripting in Internet Explorer are being redirected there as well. This information may not include JScript specific objects, such as Enumerator, which are listed in the JavaScript language reference on Microsoft Docs. Those provide additional features that are not included in the ECMA Standards, whether they are supported in the Edge browser or its predecessor.


JScript (COM Classic)

The original JScript is an Active Scripting engine. Like other Active Scripting languages, it is built on the COM/OLE Automation platform and provides scripting capabilities to host applications.
This is the version used when hosting JScript inside a Web page displayed by Internet Explorer, in an HTML application before IE9, as well as in classic ASP, Windows Script Host scripts and other Automation environments.
JScript is sometimes referred to as "classic JScript" or "Active Scripting JScript" to differentiate it from newer.NET-based versions.
Some versions of JScript are available for multiple versions of Internet Explorer and Windows. For example, JScript 5.7 was introduced with Internet Explorer 7.0 and is also installed for Internet Explorer 6.0 with Windows XP Service Pack 3, while JScript 5.8 was introduced with Internet Explorer 8.0 and is also installed with Internet Explorer 6.0 on Windows Mobile 6.5.
Microsoft's implementation of ECMAScript 5th Edition in Windows 8 Consumer Preview is called JavaScript and the corresponding Visual Studio 11 Express Beta includes a “completely new”, full-featured JavaScript editor with IntelliSense enhancements for HTML5 and ECMAScript 5 syntax, “VSDOC” annotations for multiple overloads, simplified DOM configuration, brace matching, collapsible outlining and “go to definition”.
VersionDateIntroduced withBased onSimilar JavaScript version
1.0Aug 1996Internet Explorer [|3].0Netscape JavaScript1.0
2.0Jan 1997Windows IIS 3.0Netscape JavaScript1.1
3.0Oct 1997Internet Explorer 4.0ECMA-262 1st edition1.3
4.0Visual Studio 6.0 ECMA-262 1st edition1.3
5.0Mar 1999Internet Explorer 5.0ECMA-262 2nd edition1.4
5.1Internet Explorer 5.01ECMA-262 2nd edition1.4
5.5Jul 2000Internet Explorer 5.5 & Windows CE 4.2ECMA-262 3rd edition1.5
5.6Oct 2001Internet Explorer 6.0 & Windows CE 5.0ECMA-262 3rd edition1.5
5.7Nov 2006Internet Explorer 7.0ECMA-262 3rd edition + ECMA-327 1.5
5.8Mar 2009Internet Explorer 8.0 & Internet Explorer Mobile 6.0ECMA-262 3rd edition + ECMA-327 + JSON 31.5

JScript is also available on Windows CE. The Windows CE version lacks Active Debugging.

Managed JScript

Managed JScript is an implementation of JScript for the Dynamic Language Runtime, it is part of Microsoft's dynamic languages for.NET along with IronRuby, IronPython, and Dynamic Visual Basic.
Unlike JScript.NET which is less dynamic than the original JScript but provides CLS compatibility, Managed JScript is designed on top of the DLR and provides the features needed for scripting scenarios.
While it is primarily designed to be used within Silverlight and ASP.NET at this time, it can also easily be embedded within any.NET application.
Two builds of Managed JScript exists, one for the Desktop CLR and one for the CoreCLR
VersionDateIntroduced withBased onPlatform Futures ECMA-262 3rd editionDesktop CLR 2.0
1.1.20625.02007Microsoft Silverlight 1.1 Alpha ECMA-262 3rd editionCoreCLR 1.1

Managed JScript is not supported in the.NET Compact Framework.

JScript "Chakra" (JsRT)

JScript "Chakra" is based on the JScript version, but it has been redesigned to improve performance in Internet Explorer 9 at the expense of proper Active Scripting engine compatibility. It requires a specific Microsoft JavaScript Hosting API for proper use. Therefore, it is installed side by side with JScript 5.x, and is only used by Internet Explorer 9 and later as well as JsRT hosts, while other Active Scripting hosts keep using the 5.x version when requesting the JScript engine.
VersionDateIntroduced withBased onSimilar JavaScript version
Chakra 9.0Mar 2011Internet Explorer 9.0ECMA-262 5th edition1.8.1
Chakra 10.0Sep 2012Internet Explorer 10.0ECMA-262 5.1 edition
Chakra 11.0Oct 2013Internet Explorer 11.0ECMA-262 6th edition
Chakra EdgeJul 2015Edge / Windows 10ECMA-262 5.1 to 9 edition

There are two versions of the Chakra engine, the original one used by Internet Explorer 9 and later, and sometimes referred to as "jscript9.dll" or "legacy Chakra engine", and a second one used by Microsoft Edge browser and sometimes referred to as "new Chakra engine", "Edge engine" or "Chakra.dll".
Both Chakra versions can be used by other applications using the JsRT API and can be installed side by side.
See separate page about new Chakra engine.

JScript .NET (CLI)

JScript.NET is a Microsoft.NET implementation of JScript. It is a CLI language and thus inherits very powerful features, but lacks many features of the original JScript language, making it inappropriate for many scripting scenarios.
JScript.NET can be used for ASP.NET pages and for complete.NET applications, but the lack of support for this language in Microsoft Visual Studio places it more as an upgrade path for classic ASP using classic JScript than as a new first-class language.
VersionPlatformDateIntroduced withBased on
7.0Desktop CLR 1.02002-01-05Microsoft.NET Framework 1.0ECMA-262 3rd edition
7.1Desktop CLR 1.12003-04-01Microsoft.NET Framework 1.1ECMA-262 3rd edition
8.0Desktop CLR 2.02005-11-07Microsoft.NET Framework 2.0ECMA-262 3rd edition
10.0Desktop CLR 4.02010-08-03Microsoft.NET Framework 4.0ECMA-262 3rd edition

JScript.NET is not supported in the.NET Compact Framework.
Note: JScript.NET versions are not related to classic JScript versions. JScript.NET is a separate product. Even though JScript.NET is not supported within the Visual Studio IDE, its versions are in sync with other.NET languages versions that follow their corresponding Visual Studio versions.
.NET Framework 3.0 and 3.5 are built on top of 2.0 and do not include the newer JScript.NET release.