JavaScript is a scripting language most often used for client-side web development. Its standardized name is ECMAScript, though "JavaScript" is much more commonly used. "JavaScript" is actually Netscape Communications Corporation's (and now the Mozilla Foundation's) implementation of the ECMAScript standard.
JavaScript is a dynamic, weakly typed, prototype-based language with first-class functions. JavaScript was influenced by many languages and was designed to have a similar look to Java, but be easier for non-programmers to work with. The language is best known for its use in websites (as client-side JavaScript), but is also used to enable scripting access to objects embedded in other applications.
Javascript
Despite the name, JavaScript is unrelated to the Java programming language; though both have a common debt to C syntax. The language was renamed from LiveScript in a co-marketing deal between Netscape and Sun in exchange for Netscape bundling Sun's Java runtime with their browser, which was dominant at the time. JavaScript semantics is much more similar to the Self programming language.
"JavaScript" is a registered trademark of Sun Microsystems, Inc. It was used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.[http://www.sun.com/suntrademarks/]
As of 2006, the latest version of the language is JavaScript 1.7. The previous version 1.6 corresponded to ECMA-262 Edition 3 like JavaScript 1.5, except for Array extras, and Array and String generics. ECMAScript, in simple terms, is a standardized version of JavaScript. The ECMA-357 standard specifies E4X, a language extension dealing with XML.
JavaScript is a prototype-based scripting language with a syntax loosely based on C. Like C, the language has no input or output constructs of its own. Where C relies on standard I/O libraries, a JavaScript engine relies on a host environment into which it is embedded. There are many such host environment applications, of which web technologies are the best-known examples. These are examined first.
Use in Web pages
The primary use of JavaScript is to write functions that are embedded in or included from HTML pages and 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 'look' of the new window (i.e. whether the menus, toolbars, etc. are visible).
Validation of web form input values to make sure that they will be accepted before they are 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.
Because JavaScript runs on the client rather than the server, it can respond to user actions quickly, making an application feel more responsive. Furthermore, JavaScript code can detect user actions which HTML alone cannot, such as individual keystrokes. Applications such as Gmail attempt to 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 seeks to exploit JavaScript's strengths.
Compatibility considerations
DOM interfaces are not part of the ECMAScript standard, or of JavaScript itself. Officially, they are defined by a separate standardization effort by the W3C; in practice, browser implementations differ from the standards and from each other, and not all browsers execute JavaScript.
To deal with these differences, JavaScript authors can attempt to write standards-compliant code which will also be executed correctly by most browsers; failing that, they can write code that checks for the presence of certain browser features and behaves differently if they are not available.[Peter-Paul Koch, Object detection] In some cases, two browsers may both implement a feature but with different behavior, and authors may find it practical to detect what browser is running and change their script's behavior to match.[Peter-Paul Koch, Mission Impossible - mouse position, Browser detect]
Furthermore, scripts will not work for all users. For example, a user may:
use an old or rare browser with incomplete or unusual DOM support,
use a PDA or mobile phone browser which cannot execute JavaScript,
have JavaScript execution disabled as a security precaution,
or be visually or otherwise disabled and use a speech browser
To support these users, Web authors can try to create pages which degrade gracefully on user agents (browsers) which do not support the page's JavaScript.
Security
JavaScript and the DOM provide the potential for malicious authors to deliver scripts to run on a client computer via the Web. Browser authors contain this risk using two restrictions. First, scripts run in a sandbox in which they can only perform Web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by the same origin policy: scripts from one Web site do not have access to information such as usernames, passwords, or cookies sent to another site. Most JavaScript-related security bugs are breaches of either the same origin policy or the sandbox.
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