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JavaScript is a dialect of the standardized ECMAScript programming language, primarily used for scripting web-pages.

Adapted from the SO Javascript tag.

The Basics

Javascript is probably the most ubiquitous programming language on the earth. It runs on virtually every OS, and a JS engine is included in virtually every mainstream web browser. Standalone Javascript engines or interpreters are available as well. Windows, for example, includes Javascript in its Windows Script Host, and Mozilla offers Rhino, an implementation of Javascript built in Java.

The Mozilla Developer Center offers good documentation on JavaScript.

JavaScript is most often used in the browser - see The Document Object Model.

ECMAScript, JavaScript & JScript

People often use the term "JavaScript" informally. The language and the term originated as a creation of Netscape.

ECMAScript was developed as a standardization of Netscape's JavaScript and Microsoft's independently-developed JScript. The canonical reference is The ECMA-262 Language Specification. While JavaScript and JScript aim to be compatible with ECMAScript, they also provide additional features not described in the ECMA specifications.

Most people today who use Javascript probably don't sweat the differences; they do not distinguish it from ECMAScript.