JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript. The change of name from LiveScript to JavaScript roughly coincided with Netscape adding 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 1995. The naming has caused confusion, giving the impression that the language is a spin-off of Java, and it has been characterized by many as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new web-programming language. Microsoft named its dialect of the language JScript to avoid trademark issues. JScript was first supported in Internet Explorer version 3.0, released in August 1996, and it included Y2K compliant date functions, unlike those based on java.util.Date in JavaScript at the time. The dialects are perceived to be so similar that the terms “JavaScript” and “JScript” are often used interchangeably (including in this article). Microsoft, however, notes dozens of ways in which JScript is not ECMA compliant. Netscape submitted JavaScript to Ecma International for standardization resulting in the standardized version named ECMAScript |
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Features Structured programming JavaScript supports all the structured programming syntax in C (e.g., if statements, while loops, switch statements, etc.). One partial exception is scoping: C-style block-level scoping is not supported. However, JavaScript 1.7 supports block-level scoping with the let keyword. Like C, JavaScript makes a distinction between expressions and statements. Dynamic programming dynamic typing JavaScript is heavily object-based. Objects are associative arrays, augmented with prototypes (see below). Object property names are associative array keys: obj.x = 10 and obj[“x”] = 10 are equivalent, the dot notation being merely syntactic sugar. Properties and their values can be added, changed, or deleted at run-time. The properties of an object can also be enumerated via a for…in loop. run-time evaluation Function-level programming first-class functions inner functions and closures Prototype-based prototypes functions as object constructors functions as methods Others run-time environment An indefinite number of parameters can be passed to a function. The function can both access them through formal parameters and the local arguments object. array and object literals regular expressions |
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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:
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 an HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages. A minimal example of a web page containing JavaScript (using HTML 4.01 syntax) would be: <!– simple pagedocument.write(‘Hello World!’); Your browser either does not support JavaScript, or you have JavaScript turned off. –> |
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