"Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid." – Albert Einstein

JavaScript



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


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
As in most scripting languages, types are associated with values, not variables. For example, a variable x could be bound to a number, then later rebound to a string. JavaScript supports various ways to test the type of an object, including duck typing.[7]
objects as associative arrays

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
JavaScript includes an eval function that can execute statements provided as strings at run-time.

Function-level programming

first-class functions
Functions are first-class; they are objects themselves. As such, they have properties and can be passed around and interacted with like any other object.

inner functions and closures
Inner functions (functions defined within other functions) are created each time the outer function is invoked, and variables of the outer functions for that invocation continue to exist as long as the inner functions still exist, even after that invocation is finished (e.g. if the inner function was returned, it still has access to the outer function’s variables) — this is the mechanism behind closures within JavaScript.

Prototype-based

prototypes
JavaScript uses prototypes instead of classes for defining object properties, including methods, and inheritance. It is possible to simulate many class-based features with prototypes in JavaScript.

functions as object constructors
Functions double as object constructors along with their typical role. Prefixing a function call with new creates a new object and calls that function with its local this keyword bound to that object for that invocation. The function’s prototype property determines the new object’s prototype.

functions as methods
Unlike many object-oriented languages, there is no distinction between a function definition and a method definition. Rather, the distinction occurs during function calling; a function can be called as a method. When a function is invoked as a method of an object, the function’s local this keyword is bound to that object for that invocation.

Others

run-time environment
JavaScript typically relies on a run-time environment (e.g. in a web browser) to provide objects and methods by which scripts can interact with “the outside world”. (This is not a language feature per se, but it is common in most JavaScript implementations.)
variadic functions

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
Like many scripting languages, arrays and objects (associative arrays in other languages) can be created with a succinct shortcut syntax. The object literal in particular is the basis of the JSON data format.

regular expressions
JavaScript also supports regular expressions in a manner similar to Perl, which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.


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 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 page

document.write(‘Hello World!’);

Your browser either does not support JavaScript, or you have JavaScript turned off.

–>

JavaScript Versions
Version Release date Equivalent to Netscape
Navigator
Mozilla
Firefox
Internet

Explorer

Opera Safari
1.0 March 1996 2.0 3.0
1.1 August 1996 3.0
1.2 June 1997 4.0-4.05
1.3 October 1998 ECMA-262 1st edition / ECMA-262 2nd edition 4.06-4.7x 4.0
1.4 Netscape
Server
1.5 November 2000 ECMA-262 3rd edition 6.0 1.0 5.5 (JScript 5.5),
6 (JScript 5.6),
7 (JScript 5.7),
8 (JScript 6)
1.6 November 2005 1.5 + Array extras + Array and String generics + E4X 1.5
1.7 October 2006 1.6 + Pythonic generators + Iterators + let 2.0 3.x
1.8 June 2008 1.7 + Generator expressions + Expression closures 3.0
1.9 1.8 + New Features 3.1
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