JavaScript

Not to be confused with Java (programming language), Java (software platform), or Javanese script.

JavaScript

Paradigm	Multi-paradigm: scripting, object-oriented (prototype-based), imperative, functional
Designed by	Brendan Eich
Developer	Netscape Communications Corporation, Mozilla Foundation, Ecma International
First appeared	1995 (1995)
Stable release	ECMAScript 6 / June 17, 2015 (2015-06-17)
Typing discipline	dynamic, duck
Major implementations
KJS, Rhino, SpiderMonkey, V8, Carakan, Chakra
Influenced by
Lua, Scheme, Self, Java, C, Python, AWK, HyperTalk
Influenced
ActionScript, AtScript, CoffeeScript, Dart, JScript .NET, Objective-J, QML, TypeScript, LiveScript
JavaScript at Wikibooks

JavaScript

Filename extension	.js
Internet media type	application/javascript text/javascript (obsolete)
Uniform Type Identifier (UTI)	com.netscape.javascript-source
Type of format	Scripting language

Part of a series on
JavaScript
JavaScript syntax JavaScript library Unobtrusive JavaScript JavaScript engine
Lists of Frameworks and Libraries
Ajax frameworks JavaScript web application frameworks Comparison of JavaScript frameworks List of JavaScript libraries JavaScript unit testing frameworks
JavaScript Object Notation
See also ECMAScript

JavaScript (/ˈdʒɑːvɑːˌskrɪpt/), also known as ECMAScript (the untrademarked name used for the standard), is a dynamic programming language. It is most commonly used as part of web browsers, whose implementations allow client-side scripts to interact with the user, control the browser, communicate asynchronously, and alter the document content that is displayed. JavaScript (at least the strict subset asm.js) is also considered an "assembly language of the web" – a compile target of source-to-source compilers – for making client side web applications, using other programming languages, supported by all the major browsers without plug-ins. It is also used in server-side network programming with runtime environments such as Node.js, game development and the creation of desktop and mobile applications.

JavaScript is a prototype-based scripting language with dynamic typing and first-class functions. This mix of features makes it a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.

Despite some naming, syntactic, and standard library similarities, JavaScript and Java are otherwise unrelated and have very different semantics. The syntax of JavaScript is actually derived from C, while the semantics and design are influenced by the Self and Scheme programming languages.

JavaScript is also used in environments that aren't web-based, such as PDF documents, site-specific browsers, and desktop widgets. Newer and faster JavaScript virtual machines (VMs) and platforms built upon them have also increased the popularity of JavaScript for server-side web applications. On the client side, JavaScript has been traditionally implemented as an interpreted language, but more recent browsers perform just-in-time compilation.

JavaScript has been standardized in the ECMAScript language specification.

History

Beginnings at Netscape

JavaScript was originally developed by Brendan Eich, while he was working for Netscape Communications Corporation. Indeed, while competing with Microsoft for user adoption of web technologies and platforms, Netscape considered their client-server offering a distributed OS with a portable version of Sun Microsystems' Java providing an environment in which applets could be run. Because Java was a competitor of C++ and aimed at professional programmers, Netscape also wanted a lightweight interpreted language that would complement Java by appealing to nonprofessional programmers, like Microsoft's Visual Basic (see JavaScript and Java).

Although it was developed under the name Mocha, the language was officially called LiveScript when it first shipped in beta releases of Netscape Navigator 2.0 in September 1995, but it was renamed JavaScript 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 as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new web programming language.

There is a common misconception that the JavaScript language was influenced by an earlier web page scripting language developed by Nombas named C--, not to be confused with the later C-- created in 1997. Brendan Eich, however, had never heard of C-- before he created LiveScript. Nombas did pitch their embedded web page scripting to Netscape, though web page scripting was not a new concept, as shown by ViolaWWW. Nombas later switched to offering JavaScript instead of C-- in their ScriptEase product and was part of the TC39 group that standardized ECMAScript.

Server-side JavaScript

Netscape introduced an implementation of the language for server-side scripting with Netscape Enterprise Server in December, 1995, soon after releasing JavaScript for browsers. Since the mid-2000s, there has been a resurgence of server-side JavaScript implementations, such as Node.js.

Adoption by Microsoft

Microsoft Windows script technologies including VBScript and JScript were released in 1996. JScript, a port of Netscape's JavaScript, was released on July 16, 1996 and was part of Internet Explorer 3, as well as being available server-side in Internet Information Server. IE3 also included Microsoft's first support for Cascading Style Sheets and various extensions to HTML, but in each case the implementation was noticeably different to that found in Netscape Navigator at the time. These differences made it difficult for designers and programmers to make a single website work well in both browsers leading to the use of 'best viewed in Netscape' and 'best viewed in Internet Explorer' logos that characterised these early years of the browser wars. JavaScript began to acquire a reputation for being one of the roadblocks to a cross-platform and standards-driven web. Some developers took on the difficult task of trying to make their sites work in both major browsers, but many could not afford the time. With the release of Internet Explorer 4, Microsoft introduced the concept of Dynamic HTML, but the differences in language implementations and the different and proprietary Document Object Models remained, and were obstacles to widespread take-up of JavaScript on the web.

Standardization

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. In June 1997, Ecma International published the first edition of the ECMA-262 specification. In June 1998, some modifications were made to adapt it to the ISO/IEC-16262 standard, and the second edition was released. The third edition of ECMA-262 was published on December 1999.

Development of the fourth edition of the ECMAScript standard was never completed. The fifth edition was released in December 2009. The current edition of the ECMAScript standard is 6, released in June 2015.

Later developments

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 consisted of 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 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.

With the rise of the single-page web app and JavaScript-heavy sites, it is increasingly being used as a compile target for source-to-source compilers from both dynamic languages and static languages. In particular, Emscripten and highly optimized JIT compilers, in tandem with asm.js that is friendly to AOT compilers like OdinMonkey, have enabled C and C++ programs to be compiled into JavaScript and execute at near-native speeds, causing JavaScript to be considered the "assembly language of the web", according to its creator and others.

Trademark

"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.

Features

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

Imperative and structured

JavaScript supports much of the structured programming syntax from C (e.g., if statements, while loops, switch statements,do while loops, etc.). One partial exception is scoping: JavaScript originally had only function scoping with var. ECMAScript 2015 adds a let keyword for block scoping, meaning JavaScript now has both function and block scoping. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, which allows the semicolons that would normally terminate statements to be omitted.

Dynamic

Dynamic typing

As in most scripting languages, types are associated with values, not with 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. They support two equivalent syntaxes: dot notation (obj.x = 10) and bracket notation (obj['x'] = 10). 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 for...in 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 .call() and .bind(). A nested function is a function defined within another function. It is created each time the outer function is invoked. In addition, each created function forms a lexical closure: the lexical scope of the outer function, including any constants, local variables and argument values, becomes part of the internal state of each inner function object, even after execution of the outer function concludes. JavaScript also supports anonymous functions.

Prototype-based object-oriented programming

Prototypes

JavaScript uses prototypes where many other object-oriented languages use 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 will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the Object prototype). ECMAScript 5 offers the Object.create method, allowing explicit creation of an instance without automatically inheriting from the Object prototype (older environments can assign the prototype to null). The constructor's prototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the function used as a constructor. JavaScript's built-in constructors, such as Array or Object, also have prototypes that can be modified. While it is possible to modify the Object prototype, it is generally considered bad practice because most objects in JavaScript will inherit methods and properties from the Object prototype and they may not expect the prototype to 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; 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.

Implicit and explicit delegation

JavaScript is a delegation language.

Functions as Roles (Traits and Mixins)

JavaScript natively supports various function-based implementations of Role patterns like Traits and Mixins. Such a function defines additional behavior by at least one method bound to the this keyword within its function body. A Role then has to be delegated explicitly via call or apply to objects that need to feature additional behavior that is not shared via the prototype chain.

Object Composition and Inheritance

Whereas explicit function-based delegation does cover composition in JavaScript, implicit delegation already happens every time the prototype chain is walked in order to, e.g., find a method that might be related to but is not directly owned by an object. Once the method is found it gets called within this object's context. Thus inheritance in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.

Miscellaneous

Run-time environment

JavaScript typically relies on a run-time environment (e.g., a Web browser) to provide objects and methods by which scripts can interact with the environment (e.g., a webpage DOM). It also relies on the run-time 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. Variadic functions can also be created by using the bind method.

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 JavaScript engines support these new features:

• property getter and setter functions (supported by WebKit, Gecko, Opera, ActionScript, and Rhino)
• conditional catch clauses
• iterator protocol (adopted from Python)
• shallow generators-coroutines (adopted from Python)
• array comprehensions and generator expressions (adopted from Python)
• proper block scope via the 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 ECMAScript (unsupported in Firefox since version 21)

Syntax

Main article: JavaScript syntax

Simple examples

Variables in JavaScript can be defined using the var keyword:

var x; // defines the variable x, the special value “undefined” (not to be confused with an undefined value) is assigned to it by default
var y = 2; // defines the variable y and assigns the value of 2 to it

Note the comments in the example above, both of which were preceded with two forward slashes.

There is no built-in I/O functionality in JavaScript; the run-time environment provides that. The ECMAScript specification in edition 5.1 mentions:

... indeed, there are no provisions in this specification for input of external data or output of computed results.

However, most runtime environments have a console object that can be used to print output. Here is a minimalist Hello World program:

console.log("Hello World!");

A simple recursive function:

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

Anonymous function (or lambda) syntax and closure example:

var displayClosure = function() {
    var count = 0;
    return function () {
        return ++count;
    };
}
var inc = displayClosure();
inc(); // returns 1
inc(); // returns 2
inc(); // returns 3

Variadic function demonstration (arguments is a special variable).

var sum = function() {
    var i, x = 0;
    for (i = 0; i < arguments.length; ++i) {
        x += arguments[i];
    }
    return x;
}
sum(1, 2, 3); // returns 6

Immediately-invoked function expressions allow functions to pass around variables under their own closures.

var v;
v = 1;
var getValue = (function(v) {
  return function() {return v;};
})(v);

v = 2;

getValue(); // 1

More advanced example

This sample code displays various JavaScript features.

/* Finds the lowest common multiple (LCM) of two numbers */
function LCMCalculator(x, y) { // constructor function
    var checkInt = function (x) { // inner function
        if (x % 1 !== 0) {
            throw new TypeError(x + " is not an integer"); // throw an exception
        }
        return x;
    };
    this.a = checkInt(x)
    //   semicolons   ^^^^  are optional, a newline is enough
    this.b = checkInt(y);
}
// The prototype of object instances created by a constructor is
// that constructor's "prototype" property.
LCMCalculator.prototype = { // object literal
    constructor: LCMCalculator, // when reassigning a prototype, set the constructor property appropriately
    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) {
            // swap variables
            t = b;
            b = a;
            a = t;
        }
        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.
        // Note that this leads to a wrong result if the LCMCalculator object members "a" and/or "b" are altered afterwards.)
        // Also, 'gcd' === "gcd", this['gcd'] === this.gcd
        this['gcd'] = function () {
            return a;
        };
        return a;
    },
    // Object property names can be specified by strings delimited by double (") or single (') quotes.
    lcm : 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;
    }
};

// Define generic output function; this implementation only works for Web browsers
function output(x) {
    document.body.appendChild(document.createTextNode(x));
    document.body.appendChild(document.createElement('br'));
}

// Note: Array's map() and forEach() are defined in JavaScript 1.6.
// 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) {
    output(obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());
});

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

Use in web pages

See also: Dynamic HTML and Ajax (programming)

The most common use of JavaScript is to add client-side behavior to HTML pages, a.k.a. Dynamic HTML (DHTML). Scripts 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:

• Loading new page content or submitting data to the server via AJAX without reloading the page (for example, a social network might allow the user to post status updates without leaving the page)
• Animation of page elements, fading them in and out, resizing them, moving them, etc.
• Interactive content, for example games, and playing audio and video
• Validating input values of a Web form to make sure that they are acceptable before being submitted to the server.
• Transmitting information about the user's reading habits and browsing activities to various websites. Web pages frequently do this for web analytics, ad tracking, personalization or other purposes.

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 that 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 create "host objects" to represent the Document Object Model (DOM) in JavaScript. The web server is another common host environment. A JavaScript web server would typically expose host objects representing HTTP request and response objects, which a JavaScript program could then interrogate and 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 script

Below is a minimal example of a standards-conforming web page containing JavaScript (using HTML 5 syntax) and the DOM:

<!DOCTYPE html>

<meta charset="utf-8">
<title>Minimal Example</title>

<h1 id="header">This is JavaScript</h1>

<script>
    document.body.appendChild(document.createTextNode('Hello World!'));

    var h1 = document.getElementById('header'); // holds a reference to the <h1> tag
    h1 = document.getElementsByTagName('h1')[0]; // accessing the same <h1> element
</script>

<noscript>Your browser either does not support JavaScript, or has it turned off.</noscript>

Compatibility considerations

Main article: Web interoperability

Because JavaScript runs in widely varying environments, an important part of testing and debugging is to test and verify that the JavaScript works across multiple browsers.

The DOM interfaces for manipulating web pages are not part of the ECMAScript standard, or of JavaScript itself. Officially, the DOM interfaces 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 that 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 that 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 that 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 that degrade gracefully on user agents (browsers) that 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. An alternative approach that many find preferable is to first author content using basic technologies that work in all browsers, then enhance the content for users that have JavaScript enabled. This is known as progressive enhancement.

Accessibility

Main article: Web accessibility

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 or keyboard specific events because a user may be physically unable to use these input devices. For this reason, device-agnostic events such as onfocus and onchange are preferable to device-centric events such as onmouseover and onkeypress in most cases.

JavaScript should not be used in a way that is confusing or disorienting 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" or "refresh" buttons 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.

Security

See also: Browser 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.

There are subsets of general JavaScript — ADsafe, Secure ECMA Script (SES) — that provide greater level of security, especially on code created by third parties (such as advertisements).

Content Security Policy is the main intended method of ensuring that only trusted code is executed on a web page.

See also: Content Security Policy

Cross-site vulnerabilities

Main articles: Cross-site scripting and Cross-site request forgery

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 information

Misplaced trust in the client

Developers of client-server applications must recognize that untrusted clients may be under the control of attackers. The application author cannot assume that his JavaScript code will run as intended (or at all) because any secret embedded in the code could be extracted by a determined adversary. Some implications are:

• Web site authors cannot perfectly conceal how their JavaScript operates because the raw source code must be sent to the client. The code can be obfuscated, but obfuscation 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 right-clicking on an image to save it.
• It is extremely bad practice to embed sensitive information such as passwords in JavaScript because it can be extracted by an attacker.

Browser and plugin coding errors

JavaScript provides an interface to a wide range of browser capabilities, some of which may have flaws such as buffer overflows. These flaws can allow attackers to write scripts that would run any code they wish on the user's system. This code is not by any means limited to another JavaScript application. For example, a buffer overrun exploit can allow an attacker to gain access to the operating system's API with superuser privileges.

These flaws have affected major browsers including Firefox, Internet Explorer, and Safari.

Plugins, such as video players, Adobe Flash, and the wide range of ActiveX controls enabled by default in Microsoft Internet Explorer, may also have flaws exploitable via JavaScript (such flaws have been exploited in the past).

In Windows Vista, Microsoft has attempted to contain the risks of bugs such as buffer overflows by running the Internet Explorer process with limited privileges. Google Chrome similarly confines its page renderers to their own "sandbox".

Sandbox implementation errors

Web browsers are capable of running JavaScript outside 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 (see: Windows Script Host). 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 that provides access to the host environment. The core JavaScript language remains mostly the same in each application.

Embedded scripting language

• Google's Chrome extensions, Opera's extensions, Apple's Safari 5 extensions, Apple's Dashboard Widgets, Microsoft's Gadgets, Yahoo! Widgets, Google Desktop Gadgets, and Serence Klipfolio are implemented using JavaScript.
• The MongoDB database accepts queries written in JavaScript. MongoDB and NodeJS are the core components of MEAN: a solution stack for creating Web applications using just JavaScript.
• Adobe's Acrobat and Adobe Reader support JavaScript in PDF files.
• Tools in the Adobe Creative Suite, including Photoshop, Illustrator, Dreamweaver, and InDesign, allow scripting through JavaScript.
• OpenOffice.org, an office application suite, as well as its popular fork LibreOffice, allows JavaScript to be used as a scripting language.
• 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.
• Apple's Logic Pro X digital audio workstation (DAW) software can create custom MIDI effects plugins using JavaScript.
• ECMAScript was included in the VRML97 standard for scripting nodes of VRML scene description files.
• The open-source Re-Animator framework allows developing 2D sprite-based games using JavaScript and XML.
• The Unity game engine supports a modified version of JavaScript for scripting via Mono.
• DX Studio (3D engine) uses the SpiderMonkey implementation of JavaScript for game and simulation logic.
• Maxwell Render (rendering software) provides an ECMA standard based scripting engine for tasks automation.
• Google Apps Script in Google Spreadsheets and Google Sites allows users to create custom formulas, automate repetitive tasks and also interact with other Google products such as Gmail.
• Many IRC clients, like ChatZilla or XChat, use JavaScript for their scripting abilities.
• SpinetiX products use the SpiderMonkey JavaScript engine to allow scripting within SVG files to create digital signage projects.

Scripting engine

• Microsoft's Active Scripting technology supports JScript as a scripting language.
• The Java programming language introduced the javax.script package in version 6 that includes 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 a webpage's Document Object Model (DOM).
• The Qt C++ toolkit includes a QtScript module to interpret JavaScript, analogous to Java's javax.script package.
• JSDB (JavaScript for Databases) is an open-source JavaScript shell for Windows, Mac OS X, Linux, and Unix, which extends the Mozilla JavaScript engine with file, database, email, and network objects.
• jslibs is an open-source JavaScript shell for Windows and Linux that extends the Mozilla JavaScript engine. It has the ability to call functions in commonly used libraries like NSPR, SQLite, libTomCrypt, OpenGL, OpenAL, and librsvg.
• OS X Yosemite introduced JavaScript for Automation (JXA), which is built upon JavaScriptCore and the Open Scripting Architecture. It features an Objective-C bridge which enables entire Cocoa applications to be programmed in JavaScript.
• Late Night Software's JavaScript OSA (a.k.a. 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.

Application platform

• ActionScript, the programming language used in Adobe Flash, is another implementation of the ECMAScript standard.
• Adobe Integrated Runtime is a JavaScript runtime that allows developers to create desktop applications.
• CA, Inc.'s AutoShell cross-application scripting environment is built on the SpiderMonkey JavaScript engine. It contains preprocessor-like extensions for command definition, as well as custom classes for various system-related tasks like file I/O, operation system command invocation and redirection, and COM scripting.
• GNOME Shell, the shell for the GNOME 3 desktop environment, made JavaScript its default programming language in 2013.
• The Mozilla platform, which underlies Firefox, Thunderbird, and some other web browsers, uses JavaScript to implement the graphical user interface (GUI) of its various products.
• myNFC is a JavaScript based framework that allows developers to create applications for smart phones.
• Qt Quick's markup language (available since Qt 4.7) uses JavaScript for its application logic. Its declarative syntax is also similar to JavaScript.
• TypeScript is a programming language based on JavaScript that adds support for optional type annotations and some other language extensions such as classes, interfaces and modules. A TS-script compiles into plain JavaScript and can be executed in any JS host supporting ECMAScript 3 or higher. The compiler is itself written in TypeScript.
• Ubuntu Touch provides a JavaScript API for its unified usability interface.
• webOS uses the WebKit implementation of JavaScript in its SDK to allow developers to create stand-alone applications solely in JavaScript.
• WinJS provides a special Windows Library for JavaScript functionality in Windows 8 that enables the development of Modern style (formerly Metro style) applications in HTML5 and JavaScript.

Microcontrollers

As the specifications of Microcontrollers have increased over the last few years, it has become possible to use JavaScript to control hardware in embedded devices. There are currently two main implementations:

• Espruino is a JavaScript interpreter for low power microcontrollers
• Tessel is a microcontroller board with built-in WiFi

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, Opera and Node.js

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 that 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. Internet Explorer has included developer tools since version 8 (reached by pressing the F12 key).

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, which is used in Safari. A modified version called Blink DevTools is used in Google Chrome.

Node.js has node-inspector, an interactive debugger that integrates with the Blink DevTools, available in Google Chrome.

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 who has written extensively on the language. JSLint scans JavaScript code for conformance to a set of standards and guidelines.

Version history

See also: ECMAScript § Versions and ECMAScript § Version correspondence

The following table is based on information from multiple sources.

Version	Release date	Equivalent to	Netscape Navigator	Mozilla Firefox	Internet Explorer	Opera	Safari	Google Chrome
Old version, no longer supported: 1.0	March 1996		2.0		3.0
Old version, no longer supported: 1.1	August 1996		3.0
Old version, no longer supported: 1.2	June 1997		4.0-4.05			3
Old version, no longer supported: 1.3	October 1998	ECMA-262 1st + 2nd edition	4.06-4.7x		4.0	5
Old version, no longer supported: 1.4			Netscape Server			6
Old version, no longer supported: 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 5.8)	7.0	3.0-5	1.0-10.0.666
Old version, no longer supported: 1.6	November 2005	1.5 + array extras + array and string generics + E4X		1.5
Old version, no longer supported: 1.7	October 2006	1.6 + Pythonic generators + iterators + let		2.0				28.0.1500.95
Old version, no longer supported: 1.8	June 2008	1.7 + generator expressions + expression closures		3.0		11.50
Old version, no longer supported: 1.8.1		1.8 + native JSON support + minor updates		3.5
Old version, no longer supported: 1.8.2	June 22, 2009	1.8.1 + minor updates		3.6

Related languages and features

JSON, or JavaScript Object Notation, is a general-purpose data interchange format that is defined as a subset of JavaScript's object literal syntax.

jQuery is a popular JavaScript library designed to simplify DOM-oriented client-side HTML scripting along with offering cross-browser compatibility because various browsers respond differently to certain vanilla JavaScript code.

Underscore.js is a utility JavaScript library for data manipulation that is used in both client-side and server-side network applications.

Mozilla browsers currently support LiveConnect, a feature that allows JavaScript and Java to intercommunicate on the Web. However, Mozilla-specific support for LiveConnect is scheduled to be phased out in the future in favor of passing on the LiveConnect handling via NPAPI to the Java 1.6+ plug-in (not yet supported on the Mac as of March 2010). Most browser inspection tools, such as Firebug in Firefox, include JavaScript interpreters that can act on the visible page's DOM.

asm.js is a subset of JavaScript that can be run in any JavaScript engine or run faster in an ahead-of-time (AOT) compiling engine.

Use as an intermediate language

As JavaScript is the most widely supported client-side language that can run within a web browser, it has become an intermediate language for other languages to target. This has included both newly created languages and ports of existing languages. Some of these include:

• Objective-J, a superset of JavaScript that compiles to standard JavaScript. It adds traditional inheritance and Smalltalk/Objective-C style dynamic dispatch and optional pseudo-static typing to JavaScript.
• Processing.js, a JavaScript port of Processing, a programming language designed to write visualizations, images, and interactive content. It allows web browsers to display animations, visual applications, games and other graphical rich content without the need for a Java applet or Flash plugin.
• CoffeeScript, an alternate syntax for JavaScript intended to be more concise and readable. It adds features like array comprehensions (also available in JavaScript since version 1.7) and pattern matching. Like Objective-J, it compiles to JavaScript. Ruby and Python have been cited as influential on CoffeeScript syntax.
• Google Web Toolkit translates a subset of Java to JavaScript.
• Scala, an object-oriented and functional programming language, has an experimental Scala-to-JavaScript compiler.
• Pyjamas, a port of Google Web Toolkit to Python (translates a subset of Python to JavaScript)
• Dart, an open-source programming language developed by Google, can be compiled to JavaScript.
• Whalesong, a Racket-to-JavaScript compiler.
• Emscripten, a LLVM-backend for porting native libraries to JavaScript.
• Fantom a programming language that runs on JVM, .NET and JavaScript.
• TypeScript, a free and open-source programming language developed by Microsoft. It is a superset of JavaScript, and essentially adds optional static typing and class-based object-oriented programming to the language.
• Haxe, an open-source high-level multiplatform programming language and compiler that can produce applications and source code for many different platforms including JavaScript.
• ClojureScript, a compiler for Clojure that targets JavaScript. It is designed to emit JavaScript code that is compatible with the advanced compilation mode of the Google Closure optimizing compiler.
• Kotlin, a statically-typed language that also compiles to Java byte code.

JavaScript and Java

A common misconception is that JavaScript is similar or closely related to Java. It is true that both have a C-like syntax (the C language being their most immediate common ancestor language). They also are both typically sandboxed (when used inside a browser), and JavaScript was designed with Java's syntax and standard library in mind. In particular, all Java keywords were reserved in original JavaScript, JavaScript's standard library follows Java's naming conventions, and JavaScript's Math and Date objects are based on classes from Java 1.0, but the similarities end there.

The differences between the two languages are more prominent than their similarities. Java has static typing, while JavaScript's typing is dynamic. Java is loaded from compiled bytecode, while JavaScript is loaded as human-readable source code. Java's objects are class-based, while JavaScript's are prototype-based. Finally, Java did not support functional programming until Java 8, while JavaScript does, as it contains many features based on Scheme.

Further reading

• Bhangal, Sham; Jankowski, Tomasz (2003). Foundation Web Design: Essential HTML, JavaScript, CSS, PhotoShop, Fireworks, and Flash. APress L. P. ISBN 1-59059-152-6. 
• Burns, Joe; Growney, Andree S. (2001). JavaScript Goodies. Pearson Education. ISBN 0-7897-2612-2. 
• Duffy, Scott (2003). How to do Everything with JavaScript. Osborne. ISBN 0-07-222887-3. 
• Zakas, Nicholas C. (2012). Professional JavaScript for Web Developers (3rd ed.). Wrox. ISBN 978-1-1180-2669-4. 
• Flanagan, David; Ferguson, Paula (2006). JavaScript: The Definitive Guide (5th ed.). O'Reilly & Associates. ISBN 0-596-10199-6. 
• Goodman, Danny; Eich, Brendan (2001). JavaScript Bible. John Wiley & Sons. ISBN 0-7645-3342-8. 
• Goodman, Danny; Markel, Scott (2003). JavaScript and DHTML Cookbook. O'Reilly & Associates. ISBN 0-596-00467-2. 
• Harris, Andy (2001). JavaScript Programming for the Absolute Beginner. Premier Press. ISBN 0-7615-3410-5. 
• Haverbeke, Marijn (2011). Eloquent JavaScript. No Starch Press. ISBN 978-1593272821. 
• Heinle, Nick; Koman, Richard (1997). Designing with JavaScript. O'Reilly & Associates. ISBN 1-56592-300-6. 
• Husted, Robert; Kuslich, JJ (1999). Server-Side JavaScript: Developing Integrated Web Applications (1st ed.). Addison-Wesley. ISBN 020143329X. 
• McDuffie, Tina Spain (2003). JavaScript Concepts & Techniques: Programming Interactive Web Sites. Franklin, Beedle & Associates. ISBN 1-887902-69-4. 
• McFarlane, Nigel (2003). Rapid Application Development with Mozilla. Prentice Hall Professional Technical References. ISBN 0-13-142343-6. 
• Powell, Thomas A.; Schneider, Fritz (2001). JavaScript: The Complete Reference. McGraw-Hill Companies. ISBN 0-07-219127-9. 
• Shelly, Gary B.; Cashman, Thomas J.; Dorin, William J.; Quasney, Jeffrey J. (2000). JavaScript: Complete Concepts and Techniques. Cambridge: Course Technology. ISBN 0-7895-6233-2. 
• Vander Veer, Emily A. (2004). JavaScript For Dummies (4th ed.). Wiley Pub. ISBN 0-7645-7659-3. 
• Watt, Andrew H.; Watt, Jonathan A.; Simon, Jinjer L. (2002). Teach Yourself JavaScript in 21 Days. Pearson Education. ISBN 0-672-32297-8. 

External links

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• Codeacademy's JavaScript Track
• Douglas Crockford's video lectures on JavaScript
• Douglas Crockford's A Survey of the JavaScript Programming Language
• Eloquent JavaScript by Marijn Haverbeke—a free, Creative Commons–licensed eBook
• JavaScript at DMOZ
• JavaScript—Opera Developer Community
• JS Beginner's tutorial
• List of languages that compile to JS
• Mozilla Developer Network
  • List of JavaScript releases: versions 1.5+
  • Mozilla's Official Documentation on JavaScript
  • Overview over new features in JavaScript
  • Re-Introduction to JavaScript
  • References for Core JavaScript versions: 1.5+