JavaScript is an object-based scripting programming language based on the concept of prototypes. The language is best known for its use in websites, but is also used to enable scripting access to objects embedded in other applications. It was originally developed by Brendan Eich of Netscape Communications Corporation under the name Mocha, then LiveScript, and finally renamed to JavaScript. Like Java, JavaScript has a C-like syntax, but it has far more in common with the Self programming language than with Java.

As of 2005, the latest version of the language is JavaScript 1.5, which corresponds to ECMA-262 Edition 3. ECMAScript, in simple terms, is a standardized version of JavaScript. Mozilla versions since 1.8 Beta 1 also have partial support of E4X, which is a language extension dealing with XML, defined in the ECMA-357 standard.

The change of name from LiveScript to JavaScript happened at roughly the time when Netscape was including support for Java technology in its Netscape Navigator web browser. JavaScript was first introduced and deployed in the Netscape browser version 2.0B3 in December of 1995. The choice of name proved to be a source of much confusion. There is no real relationship between Java and JavaScript; their similarities are mostly in syntax (that is, both derived from C). Their semantics are quite different: notably, their object models are unrelated and largely incompatible. Also worth mentioning is Microsoft's own VBScript, which, like JavaScript, is mainly used in web pages. VBScript's syntax derives from Visual Basic, and is only available on Internet Explorer.

Due to the success of JavaScript, Microsoft developed a compatible language known as JScript. JScript was first supported in the Internet Explorer browser version 3.0 released in August, 1996. When web developers talk about using JavaScript in the IE browser, they usually mean JScript. The need for common specifications for the two languages was the basis of the ECMA 262 standard for ECMAScript (see external links below), three editions of which have been published since the work started in November 1996 (and which in turn set the stage for the standardization of C# a few years later). One term often related to JavaScript, the Document Object Model (DOM), is actually not part of the ECMAScript standard; it's rather a standard on its own, developed by the W3C, and closely related to XML.