JavaScript
JavaScript is a scripting language most often used for client-side web development. It was the originating dialect of the ECMAScript standard. It 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 (for example Microsoft Gadgets in Windows Vista Sidebar).
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 code can run locally in a user’s browser (rather than on a remote 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 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 ever 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 (also at Netscape) is a JavaScript implementation in Java. Like SpiderMonkey, Rhino is ECMA-262 Edition 3 compliant.
By far, the most common host environment for JavaScript is a web browser. Web browsers typically use the public API to create “host objects” responsible for reflecting the DOM into JavaScript. The web server is another common application of the engine. A JavaScript webserver would expose host objects representing a HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages.
Tags: DOM, JavaScript, scripting language