JavaScript Use in web pages

The most common use of JavaScript is to write functions that are embedded in or included from HTML pages and that interact with the Document Object Model (DOM) of the page. Some simple examples of this usage are:
Opening or popping up a new window with programmatic control over the size, position, and attributes of the new window (e.g. whether the menus, toolbars, etc., are visible).
Validating input values of a web form to make sure that they are acceptable before being submitted to the server.
Changing images as the mouse cursor moves over them: This effect is often used to draw the user's attention to important links displayed as graphical elements.
Transmitting information about the user's reading habits and browsing activities to various websites. Web pages frequently do this for web analytics, ad tracking, personalization or other purposes.
Because JavaScript code can run locally in a user's browser (rather than on a remote server), the browser can respond to user actions quickly, making an application more responsive. Furthermore, JavaScript code can detect user actions which HTML alone cannot, such as individual keystrokes. Applications such as Gmail take advantage of this: much of the user-interface logic is written in JavaScript, and JavaScript dispatches requests for information (such as the content of an e-mail message) to the server. The wider trend of Ajax programming similarly exploits this strength.
A JavaScript engine (also known as JavaScript interpreter or JavaScript implementation) is an interpreter that interprets JavaScript source code and executes the script accordingly. The first JavaScript engine was created by Brendan Eich at Netscape Communications Corporation, for the Netscape Navigator web browser. The engine, code-named SpiderMonkey, is implemented in C. It has since been updated (in JavaScript 1.5) to conform to ECMA-262 Edition 3. The Rhino engine, created primarily by Norris Boyd (formerly of Netscape; now at Google) is a JavaScript implementation in Java. Rhino, like SpiderMonkey, is ECMA-262 Edition 3 compliant.
A web browser is by far the most common host environment for JavaScript. Web browsers typically use the public API to create "host objects" responsible for reflecting the Document Object Model (DOM) into JavaScript. The web server is another common application of the engine. A JavaScript webserver would expose host objects representing an HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages.

Because JavaScript is the only language that the most popular browsers share support for, it has become a target language for many frameworks in other languages, even though JavaScript was never intended to be such a language. Despite the performance limitations inherent to its dynamic nature, the increasing speed of JavaScript engines has made the language a surprisingly feasible compilation target.