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Not to be confused with Java (programming language).

JavaScript is an implementation of the ECMAScript language standard and is typically used to enable programmatic access to computational objects within a host environment. It can be characterized as a prototype-based object-oriented scripting language that is dynamic, weakly typed and has first-class functions. It is also considered a functional HTML Iconprogramming language like Scheme and OCaml because it has closures and supports higher-order functions.

JavaScript is primarily used in the form of client-side JavaScript, implemented as part of a web browser in order to provide enhanced user interfaces and dynamic websites. However, its use in applications outside web pages is also significant.

JavaScript uses syntax influenced by that of C syntax, also JavaScript copies many Java programming language names and naming conventions; but the two languages are otherwise unrelated and have very different semantics. The key design principles within JavaScript are taken from the Self and Scheme programming languages.

A Simple "Fish-Eye Menu" Created Using JavaScript


JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript. LiveScript was the official name for the language when it first shipped in beta releases of Netscape Navigator 2.0 in September 1995, but it was renamed JavaScript in a joint announcement with Sun Microsystems on December 4, 1995 when it was deployed in the Netscape browser version 2.0B3.

The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigator web browser. The final choice of name caused confusion, giving the impression that the language was a spin-off of the Java programming language, and the choice 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. It has also been claimed that the language's name is the result of a co-marketing deal between Netscape and Sun, in exchange for Netscape bundling Sun's Java runtime with their then-dominant browser.

JavaScript very quickly gained widespread success as a client-side scripting language for web pages. As a consequence, Microsoft developed a compatible dialect of the language, naming it JScript to avoid trademark issues. JScript added new date methods to fix the non-Y2K-friendly methods in JavaScript, which were based on java.util.Date. JScript was included in Internet Explorer 3.0, released in August 1996. The dialects are perceived to be so similar that the terms "JavaScript" and "JScript" are often used interchangeably. Microsoft, however, notes dozens of ways in which JScript is not ECMA-compliant.

In November, 1996 Netscape announced that it had submitted JavaScript to Ecma International for consideration as an industry standard, and subsequent work resulted in the standardized version named ECMAScript.

JavaScript has become one of the most popular programming languages on the web. Initially, however, many professional programmers denigrated the language because its target audience was web authors and other such "amateurs", among other reasons. The advent of Ajax returned JavaScript to the spotlight and brought more professional programming attention. The result was a proliferation of comprehensive frameworks and libraries, improved JavaScript programming practices, and increased usage of JavaScript outside of web browsers, as seen by the proliferation of server-side JavaScript platforms.

In January 2009 the CommonJS project was founded with the goal of specifying a common standard library mainly for JavaScript development outside the browser.


"JavaScript" is a trademark of Oracle Corporation. It is used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.


The following features are common to all conforming ECMAScript implementations, unless explicitly specified otherwise.

Imperative and structured

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 (instead, JavaScript has function-level scoping). JavaScript 1.7, however, supports block-level scoping with the let keyword. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, in which the semicolons that terminate statements can be omitted.

  • Dynamic

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.

object based

JavaScript is almost entirely object-based. JavaScript objects are associative arrays, augmented with prototypes (see below). Object property names are string keys: obj.x = 10 and obj["x"] = 10 are equivalent, the dot notation being syntactic sugar. Properties and their values can be added, changed, or deleted at run-time. Most properties of an object (and those on its prototype inheritance chain) can be enumerated using a loop. JavaScript has a small number of built-in objects such as Function and Date.

run-time evaluation

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

  • Functional

first-class functions

Functions are first-class; they are objects themselves. As such, they have properties and methods, such as length and call(); and they can be assigned to variables, passed as arguments, returned by other functions, and manipulated like any other object. Any reference to a function allows it to be invoked using the () operator.

nested functions

'Inner' or 'nested' functions are functions defined within another function. They are created each time the outer function is invoked. In addition to that, the scope of the outer function, including any constants, local variables and argument values, become part of the internal state of each inner function object, even after execution of the outer function concludes.


JavaScript allows nested functions to be created, with the lexical scope in force at their definition, and has a () operator to invoke them now or later. This combination of code that can be executed outside the scope in which it is defined, with its own scope to use during that execution, is called a closure in computer science.

  • Prototype-based


JavaScript uses prototypes instead of classes for 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 constructor's prototype property determines the object used for the new object's internal prototype. JavaScript's built-in constructors, such as Array, also have prototypes that can be modified.

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 called as a method of an object, the function's local this keyword is bound to that object for that invocation.

  • Miscellaneous

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". In fact, it relies on the environment to provide the ability to include/import scripts (e.g. HTML <script> elements). (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 access them through formal parameters and also through the local arguments object.

array and object literals

Like many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, these literals form 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.

  • Vendor-specific extensions

JavaScript is officially managed by Mozilla Foundation, and new language features are added periodically. However, only some non-Mozilla JavaScript engines support these new features:

  • property getter and setter functions (also supported by WebKit, Opera, ActionScript, and Rhino)
  • conditional catch clauses
  • iterator protocol adopted from Python
  • shallow generators/coroutines also adopted from Python
  • array comprehensions and generator expressions also adopted from Python
  • proper block scope via new let keyword
  • array and object destructuring (limited form of pattern matching)
  • concise function expressions (function(args) expr)
  • ECMAScript for XML (E4X), an extension that adds native XML support to ECMAScri

Syntax and Semantics

As of 2009, the latest version of the language is JavaScript 1.8.1. It is a superset of ECMAScript (ECMA-262) Edition 3. Extensions to the language, including partial E4X (ECMA-357) support and experimental features considered for inclusion into future ECMAScript editions, are documented here.

Simple examples

A simple recursive function:

   function factorial (n){
       if (n == 0)
           return 1;
           return n * factorial(n-1);

Anonymous function (or lambda) syntax:

    function add(i, j){

        var add_pri = function(x, y){
                          return x + y

        return add_pri(i, j);


    function showclosure(){

        var inc = makeinc(1);

        inc(); //1
        inc(); //2
        inc(); //3

    function makeinc(initalValue){

        var count = initalValue

        return function(){
            return count++;

Variadic function demonstration: (This will alert with 1 then 2 then 3. arguments is a special variable)

    function unlimited_args(){

        for( var i = 0; i < arguments.length; i++ ) {


Example - syntax and semantics

This sample code showcases various JavaScript features. The example can be executed with the following steps: (1) Copy the code to a file with extension .html. (2) Use Mozilla Firefox or Google Chrome to open the file.

      <head><title>LCM Calculator</title></head>
      <body style="font-family:'Courier New'">
      <script type="text/javascript">
    /* Finds the lowest common multiple of two numbers */
    function LCMCalculator(x, y) { // constructor function
        function checkInt(x) { // inner function
            if (x % 1 != 0)
                throw new TypeError(x + " is not an integer"); // exception throwing
            return x;
    //semicolons are optional (but beware since this may cause consecutive lines to be
    //erroneously treated as a single statement)

    this.a = checkInt(x)
    this.b = checkInt(y)

    // The prototype of object instances created by a constructor is
    // that constructor's "prototype" property.

    LCMCalculator.prototype = { // object literal
        gcd : function() { // method that calculates the greatest common divisor
            // Euclidean algorithm:

            var a = Math.abs(this.a), b = Math.abs(this.b), t;
            if (a < b) {
                t = b; b = a; a = t; // swap variables
            while (b !== 0) {
                t = b;
                b = a % b;
                a = t;
            // Only need to calculate gcd once, so "redefine" this method.
            // (Actually not redefinition - it's defined on the instance itself,
            // so that this.gcd refers to this "redefinition" instead of                LCMCalculator.prototype.gcd.)
            // Also, 'gcd' == "gcd", this['gcd'] == this.gcd

            this['gcd'] = function() { return a; };
            return a;

         "lcm" /* can use strings here */: function() {
             // Variable names don't collide with object properties, e.g. |lcm| is not                 |this.lcm|.
             // not using |this.a * this.b| to avoid FP precision issues

             var lcm = this.a / this.gcd() * this.b;
             // Only need to calculate lcm once, so "redefine" this method.
             this.lcm = function() { return lcm; };
             return lcm;

         toString : function() {
             return "LCMCalculator: a = " + this.a + ", b = " + this.b;

    // Note: Array's map() and forEach() are predefined in JavaScript 1.6.
    // They are currently not available in the JScript engine built into
    // Microsoft Internet Explorer, but are implemented in Firefox, Chrome, etc.
    // They are used here to demonstrate JavaScript's inherent functional nature.

    [[25,55],[21,56],[22,58],[28,56]].map(function(pair) { // array literal + mapping function
        return new LCMCalculator(pair[0], pair[1]);
.sort(function(a, b) { // sort with this comparative function
        return a.lcm() - b.lcm();
.forEach(function(obj) {
        /* Note: print() is a JS builtin function available in Mozilla's js CLI;
         * It is functionally equivalent to Java's System.out.println().
         * Within a web browser, print() is a very different function
         * (opens the "Print Page" dialog),
         * so use something like document.write() or alert() instead.
         // print (obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());
         // alert (obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());

         document.write(obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm() + "<br>");
    (Message from JavaScript example) <br>
    Your browser either does not support JavaScript, or you have JavaScript turned off.

The following output should be displayed in the browser window.

    LCMCalculator: a = 28, b = 56, gcd = 28, lcm = 56
    LCMCalculator: a = 21, b = 56, gcd = 7, lcm = 168
    LCMCalculator: a = 25, b = 55, gcd = 5, lcm = 275
    LCMCalculator: a = 22, b = 58, gcd = 2, lcm = 638

If Internet Explorer is used, the example will generate an error. Hence the example illustrates the point that the JScript interpreter in Internet Explorer executes code differently from the JavaScript interpreters in other browsers. (See comments in the source code for details on the relevant differences for this example.)

Use In Web Pages

The primary 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.

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 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.

Example - Use In Web Pages

A minimal example of a standards-conforming web page containing JavaScript (using HTML 4.01 syntax) would be the following:


head><title>simple page</title></head>
script type="text/javascript">
          document.write('Hello World!');
    Your browser either does not support JavaScript, or you have JavaScript turned off.

Compatibility Considerations

Since JavaScript runs in widely varying environments, an important part of testing and debugging it is testing across browsers.

The DOM interfaces for manipulating web pages 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. 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. Programmers may also use libraries or toolkits which take browser differences into account.

Furthermore, scripts may not work for some 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,
  • use a speech browser due to, for example, a visual disability.

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. In particular, the page should remain usable albeit without the extra features that the JavaScript would have added.


Assuming that the user has not disabled its execution, client-side web JavaScript should be written to enhance the experiences of visitors with visual or physical disabilities, and certainly should avoid denying information to these visitors.

Screen readers, used by the blind and partially sighted, can be JavaScript-aware and so may access and read the page DOM after the script has altered it. The HTML should be as concise, navigable and semantically rich as possible whether the scripts have run or not. JavaScript should not be totally reliant on mouse-specific events so as to deny its benefits to users who either cannot use a mouse or who choose to favor the keyboard for whatever reason. Equally, although hyperlinks and webforms can be navigated and operated from the keyboard, accessible JavaScript should not require keyboard events either. There are device-independent events such as onfocus and onchange that are preferable in most cases.

JavaScript should not be used in a way that is confusing or disorientating to any web user. For example, using script to alter or disable the normal functionality of the browser, such as by changing the way the back-button or the refresh event work, is usually best avoided. Equally, triggering events that the user may not be aware of reduces the user's sense of control as do unexpected scripted changes to the page content.

Often the process of making a complex web page as accessible as possible becomes a nontrivial problem where issues become matters of debate and opinion, and where compromises are necessary in the end. However, user agents and assistive technologies are constantly evolving and new guidelines and relevant information are continually being published on the web.


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.

Cross-Site Vulnerabilities

A common JavaScript-related security problem is cross-site scripting, or XSS, a violation of the same-origin policy. XSS vulnerabilities occur when an attacker is able to cause a target web site, such as an online banking website, to include a malicious script in the webpage presented to a victim. The script in this example can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim's authorization. A solution to XSS vulnerabilities is to use HTML escaping whenever displaying untrusted data.

Some browsers include partial protection against reflected XSS attacks, in which the attacker provides a URL including malicious script. However, even users of those browsers are vulnerable to other XSS attacks, such as those where the malicious code is stored in a database. Only correct design of Web applications on the server side can fully prevent XSS.

XSS vulnerabilities can also occur because of implementation mistakes by browser authors.

Another cross-site vulnerability is cross-site request forgery or CSRF. In CSRF, code on an attacker's site tricks the victim's browser into taking actions the user didn't intend at a target site (like transferring money at a bank). It works because, if the target site relies only on cookies to authenticate requests, then requests initiated by code on the attacker's site will carry the same legitimate login credentials as requests initiated by the user. In general, the solution to CSRF is to require an authentication value in a hidden form field, and not only in the cookies, to authenticate any request that might have lasting effects. Checking the HTTP Referrer header can also help.

"JavaScript hijacking" is a type of CSRF attack in which a <script> tag on an attacker's site exploits a page on the victim's site that returns private information such as JSON or JavaScript. Possible solutions include requiring an authentication token in the POST and GET parameters for any response that returns private JSON (even if it has no side effects); using POST and never GET for requests that return private JSON; and modifying the response so that it can't be used via a <script> tag (by, for example, wrapping the JSON in a JavaScript comment).

Misplaced Trust In The Client

Client-server applications, whether they involve JavaScript or not, must recognize that untrusted clients may be under the control of attackers. Thus any secret embedded in JavaScript could be extracted by a determined adversary, and the application author cannot assume that his JavaScript runs as intended, or at all. Some implications:

  • Web site authors cannot perfectly conceal how their JavaScript operates, because the code is sent to the client, and obfuscated code can be reverse-engineered.
  • JavaScript form validation only provides convenience for users, not security. If a site verifies that the user agreed to its terms of service, or filters invalid characters out of fields that should only contain numbers, it must do so on the server, not only the client.
  • Scripts can be selectively disabled, so JavaScript can't be relied on to prevent operations such as "save image".
  • It would be extremely bad practice to embed a password in JavaScript (where it can be extracted by an attacker), then have JavaScript verify a user's password and pass "password_ok=1" back to the server (since the "password_ok=1" response is easy to forge).

Sandbox Implementation Errors

Web browsers are capable of running JavaScript outside of the sandbox, with the privileges necessary to, for example, create or delete files. Of course, such privileges aren't meant to be granted to code from the web.

Incorrectly granting privileges to JavaScript from the web has played a role in vulnerabilities in both Internet Explorer and Firefox. In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.

Microsoft Windows allows JavaScript source files on a computer's hard drive to be launched as general-purpose, non-sandboxed programs. This makes JavaScript (like VBScript) a theoretically viable vector for a Trojan horse, although JavaScript Trojan horses are uncommon in practice.

Uses Outside Web Pages

In addition to web browsers and servers, JavaScript interpreters are embedded in a number of tools. Each of these applications provides its own object model which provides access to the host environment, with the core JavaScript language remaining mostly the same in each application.

Embedded scripting language

  • Apple's Dashboard Widgets, Apple's Safari 5 extensions, Microsoft's Gadgets, Yahoo! Widgets, Google Desktop Gadgets, Serence Klipfolio are implemented using JavaScript.
  • Adobe's Acrobat and Adobe Reader (formerly Acrobat Reader) support JavaScript in PDF files.
  • Tools in the Adobe Creative Suite, including Photoshop, Illustrator, Dreamweaver and InDesign, allow scripting through JavaScript.
  • office application suite allows for JavaScript as one of its scripting languages.
  • The interactive music signal processing software Max/MSP released by Cycling '74, offers a JavaScript model of its environment for use by developers. It allows much more precise control than the default GUI-centric programming model.
  • ECMAScript was included in the VRML97 standard for scripting nodes of VRML scene description files.
  • Some high-end Philips universal remote panels, including TSU9600 and TSU9400, can be scripted using a JavaScript-based tool called ProntoScript.
  • Sphere is an open source and cross platform computer program designed primarily to make role-playing games that use JavaScript as a scripting language.
  • The open-source Re-Animator framework allows developing 2D sprite-based games using JavaScript and XML.
  • Methabot is a web crawler that uses JavaScript as scripting language for custom filetype parsers and data extraction using E4X.
  • The game engine Unity supports three scripting languages: JavaScript, C#, and Boo.
  • DX Studio (3D engine) uses the SpiderMonkey implementation of JavaScript for game and simulation logic.
  • Maxwell Render provides an ECMA standard based scripting engine for tasks automation.
  • Google Docs Spreadsheet has a script editor which allows users to create custom formulas, automate repetitive tasks and also interact with other Google products such as Gmail.

Scripting engine

  • Microsoft's Active Scripting technology supports JScript as a scripting language. This is often considered compatible with JavaScript, but Microsoft lists many JScript features that are not compliant with ECMA standards.
  • The Java programming language, in version SE 6 (JDK 1.6), introduced the javax.script package, including a JavaScript implementation based on Mozilla Rhino. Thus, Java applications can host scripts that access the application's variables and objects, much like web browsers host scripts that access the browser's Document Object Model (DOM) for a webpage.
  • The Qt C++ toolkit includes a QtScript module to interpret JavaScript, analogous to javax.script.
  • Late Night Software's JavaScript OSA (aka JavaScript for OSA, or JSOSA), is a freeware alternative to AppleScript for Mac OS X. It is based on the Mozilla 1.5 JavaScript implementation, with the addition of a MacOS object for interaction with the operating system and third-party applications.

Development Tools

Within JavaScript, access to a debugger becomes invaluable when developing large, non-trivial programs. Because there can be implementation differences between the various browsers (particularly within the Document Object Model) it is useful to have access to a debugger for each of the browsers that a web application targets.

Script debuggers are available for Internet Explorer, Firefox, Safari, Google Chrome, and Opera.

Three debuggers are available for Internet Explorer: Microsoft Visual Studio is the richest of the three, closely followed by Microsoft Script Editor (a component of Microsoft Office), and finally the free Microsoft Script Debugger which is far more basic than the other two. The free Microsoft Visual Web Developer Express provides a limited version of the JavaScript debugging functionality in Microsoft Visual Studio.

Web applications within Firefox can be debugged using the Firebug add-on, or the older Venkman debugger. Firefox also has a simpler built-in Error Console, which logs and evaluates JavaScript. It also logs CSS errors and warnings.

Opera includes a set of tools called DragonFly.

WebKit's Web Inspector includes a JavaScript debugger in Apple's Safari.

Some debugging aids are themselves written in JavaScript and built to run on the Web. An example is the program JSLint, developed by Douglas Crockford, currently senior JavaScript architect at Yahoo! who has written extensively on the language. JSLint scans JavaScript code for conformance to a set of standards and guidelines. Web development bookmarklets and Firebug Lite provide variations on the idea of the cross-browser JavaScript console.

MiniME is an open source JavaScript minifier, obfuscator and code checking tool for the .NET platform.

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