JavaScript

JavaScript
Paradigm Multi-paradigm: scripting, object-oriented (prototype-based), imperative, functional[1]
Designed by Brendan Eich
Developer Netscape Communications Corporation, Mozilla Foundation, Ecma International
First appeared May 23, 1995 (1995-05-23)
Stable release ECMAScript 6[2] / June 17, 2015 (2015-06-17)
Typing discipline dynamic, duck
Major implementations
Chakra, SpiderMonkey, V8
Influenced by
Lua, Scheme, Perl, Self, Java, C, Python, AWK, HyperTalk
Influenced
ActionScript, AtScript, CoffeeScript, Dart, JScript .NET, LiveScript, Objective-J, Opa, Perl 6, QML, TypeScript
JavaScript
Filename extension .js
Internet media type
  • application/javascript
  • text/javascript (obsolete)[3]
Uniform Type Identifier (UTI) com.netscape.javascript-source[4]
Type of format Scripting language

JavaScript (/ˈdʒɑːvəˌskrɪpt/[5]) is a high-level, dynamic, untyped, and interpreted programming language.[6] It has been standardized in the ECMAScript language specification.[7] Alongside HTML and CSS, it is one of the three core technologies of World Wide Web content production; the majority of websites employ it and it is supported by all modern Web browsers without plug-ins.[6] JavaScript is prototype-based with first-class functions, making it a multi-paradigm language, supporting object-oriented,[8] imperative, and functional programming styles.[6] It has an API for working with text, arrays, dates and regular expressions, but does not include any I/O, such as networking, storage, or graphics facilities, relying for these upon the host environment in which it is embedded.[7]

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.[9]

JavaScript is also used in environments that are not 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. It is also used in game development, the creation of desktop and mobile applications, and server-side network programming with runtime environments such as Node.js.

History

Beginnings at Netscape

JavaScript was originally developed in 10 days in May 1995 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's 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).[10]

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[11] when it was deployed in the Netscape browser version 2.0B3.[12]

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.[13][14]

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).[15][16][17] Brendan Eich, however, had never heard of C-- before he created LiveScript.[18] Nombas did pitch their embedded Web page scripting to Netscape, though Web page scripting was not a new concept, as shown by ViolaWWW.[19] Nombas later switched to offering JavaScript instead of C-- in their ScriptEase product and was part of the TC39 group that standardized ECMAScript.[20][21]

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.[22][23] Since the mid-2000s, there has been a resurgence of server-side JavaScript implementations, such as Node.js.[24][25]

Adoption by Microsoft

Microsoft Windows script technologies including VBScript and JScript were released in 1996. JScript, a reverse-engineered implementation 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.[26][27] 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.[28] 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.[26] 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.[26]

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.[29]

Development of the fourth edition of the ECMAScript standard was never completed.[30] The fifth edition was released in December 2009. The current edition of the ECMAScript standard is 6, released in June 2015.[31]

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.[32] 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.[33]

With the rise of the single-page application 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 ahead-of-time compilers (AOT) 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",[34] according to its creator and others.

Trademark

"JavaScript" is a trademark of Oracle Corporation.[35] It is used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.[36]

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.[37]

Dynamic

Typing
As with most scripting languages, JavaScript is dynamically typed; a type is associated with each value, rather than just with each expression. For example, a variable that is at one time bound to a number may later be re-bound to a string.[38] JavaScript supports various ways to test the type of an object, including duck typing.[39]
Run-time evaluation
JavaScript includes an eval function that can execute statements provided as strings at run-time.

Prototype-based (Object-oriented)

JavaScript is almost entirely object-based. In JavaScript, an object is an associative array, augmented with a prototype (see below); each string key provides the name for an object property, and there are two syntactical ways to specify such a name: dot notation (obj.x = 10) and bracket notation (obj['x'] = 10). A property may be added, rebound, or deleted at run-time. Most properties of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using a for...in loop.

JavaScript has a small number of built-in objects, including Function and Date.

Prototypes
JavaScript uses prototypes where many other object-oriented languages use classes for inheritance.[40] It is possible to simulate many class-based features with prototypes in JavaScript.[41]
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).[42] 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).[43] 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.[44]
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.

Functional

A function is first-class; a function is considered to be an object. As such, a function may have properties and methods, such as .call() and .bind().[45] A nested function is a function defined within another function. It is created each time the outer function is invoked. In addition, each nested function forms a lexical closure: The lexical scope of the outer function (including any constant, local variable, or argument value) becomes part of the internal state of each inner function object, even after execution of the outer function concludes.[46] JavaScript also supports anonymous functions.

Delegative

JavaScript supports implicit and explicit delegation.

Functions as Roles (Traits and Mixins)
JavaScript natively supports various function-based implementations of Role[47] patterns like Traits[48][49] and Mixins.[50] 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.
JavaScript processes messages from a queue one at a time. Upon loading a new message, JavaScript calls a function associated with that message, which creates a call stack frame (the function's arguments and local variables). The call stack shrinks and grows based on the function's needs. Upon function completion, when the stack is empty, JavaScript proceeds to the next message in the queue. This is called the event loop, described as "run to completion" because each message is fully processed before the next message is considered. However, the language's concurrency model describes the event loop as non-blocking: program input/output is performed using events and callback functions. This means, for instance, that JavaScript can process a mouse click while waiting for a database query to return information.[51]
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.[52]

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:

Syntax

Main article: JavaScript syntax

Simple examples

Variables in JavaScript can be defined using the var keyword:[56]

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:[57]

… 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[58] 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).[59]

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

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:

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.[61] 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>
<html>
<meta charset="utf-8">
<title>Minimal Example</title>
<body>
<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>
</body>
</html>

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.[62] 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.[63][64] 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:

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.

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).[65][66] Caja is another project for safe embedding and isolation of third-party JavaScript and HTML.

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

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.[67]

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:

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:

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,[69] Internet Explorer,[70] and Safari.[71]

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).[72][73]

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.[74] 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[75] and Firefox.[76] In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.[77]

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.[78]

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

Scripting engine

Application platform

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.[94]

Script debuggers are integrated within Internet Explorer, Firefox, Safari, Google Chrome, Opera and Node.js[95][96][97]

In addition to the native Internet Explorer Developer Tools, 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),[98] 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).

In comparison to Internet Explorer, Firefox has a more comprehensive set of developer tools, which include a debugger as well. Old versions of Firefox without these tools used a Firefox addon called Firebug, or the older Venkman debugger. Also, WebKit's Web Inspector includes a JavaScript debugger,[99] 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. Last but not least, Opera includes a set of tools called Dragonfly.[100]

In addition to the native computer software, there are online JavaScript IDEs, 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. Many libraries for JavaScript, such as three.js, provide links to demonstration code that can be edited by users. They are also used as a pedagogical tool by institutions such as Khan Academy[101] to allow students to experience writing code in an environment where they can see the output of their programs, without needing any setup beyond a Web browser.

Version history

JavaScript was initially developed in 1996 for use in the Netscape Navigator browser. In the same year Microsoft released an implementation for Internet Explorer. This implementation was called JScript due to trademark issues. In 1997 the first standardized version of the language was released under the name ECMAScript.

The following table is based on information from multiple sources.[102][103][104]

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[105]
Old version, no longer supported: 1.3 October 1998 ECMA-262 1st + 2nd edition 4.06-4.7x 4.0 5[106]
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. Like much of JavaScript (regexps and anonymous functions as 1st class elements, closures, flexible classes, 'use strict'), JSON, except for replacing Perl's key-value operator '=>' by an RFC 822[107] inspired ':', is syntactically pure Perl.

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).[108] 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.[109]

JSFuck is an esoteric programming language. Programs are written using only six different characters, but are still valid JavaScript code.

p5.js[110] is an object oriented JavaScript library designed for artists and designers. It is based on the ideas of the Processing project but is for the web.

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:

As JavaScript has unusual limitations – such as no separate integer type, using floating point – languages that compile to JavaScript commonly have slightly different behavior than in other environments.

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,[116] 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 has done so from the beginning, being influenced by Scheme.

References

  1. ↑ Flanagan 2011, pp. 1–2.
  2. ↑ "Standard ECMA-262". Ecma International. 2015-06-17.
  3. ↑ "RFC 4329". Apps.ietf.org. Archived from the original on 2014-03-16. Retrieved 16 February 2016.
  4. ↑ "System-Declared Uniform Type Identifiers". Mac OS X Reference Library. Apple Inc. Retrieved 2010-03-05.
  5. ↑ "JavaScript". Collins English Dictionary – Complete & Unabridged 2012 Digital Edition. William Collins Sons & Co. 2012. Retrieved 21 August 2015.
  6. 1 2 3 Flanagan 2011, p. 1.
  7. 1 2 Flanagan 2011, p. 2.
  8. ↑ "ECMAScript Language Specification" (PDF).
  9. ↑ "ECMAScript Language Overview" (PDF). 2007-10-23. p. 4. Retrieved 2009-05-03.
  10. ↑ Severance, Charles (February 2012). "JavaScript: Designing a Language in 10 Days". Computer (IEEE Computer Society) 45 (2): 7–8. doi:10.1109/MC.2012.57. Retrieved 23 March 2013.
  11. ↑ Press release announcing JavaScript, "Netscape and Sun announce JavaScript", PR Newswire, December 4, 1995
  12. ↑ "TechVision: Innovators of the Net: Brendan Eich and JavaScript". web.archive.org. Archived from the original on 2008-02-08.
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  33. ↑ Kris Kowal (1 December 2009). "CommonJS effort sets JavaScript on path for World domination". Ars Technica. Condé Nast Publications. Retrieved 18 April 2010.
  34. ↑ Hanselman, Scott (July 19, 2011). "JavaScript is Assembly Language for the Web: Part 2 – Madness or just Insanity?".
  35. ↑ "USPTO Copyright entry #75026640". USPTO.
  36. ↑ "Sun Trademarks". Sun Microsystems. Archived from the original on 28 May 2010. Retrieved 2007-11-08.
  37. ↑ Flanagan 2006, p. 16.
  38. ↑ JavaScript data types and data structures
  39. ↑ Flanagan 2006, pp. 176–178.
  40. ↑ "Inheritance and the prototype chain". Mozilla Developer Network. Mozilla. Retrieved 6 April 2013.
  41. ↑ Herman, David (2013). Effective JavaScript. Addison-Wesley. p. 83. ISBN 978-0-321-81218-6.
  42. ↑ Haverbeke, Marjin (2011). Eloquent JavaScript. No Starch Press. pp. 95–97. ISBN 978-1-59327-282-1.
  43. ↑ Katz, Yehuda. "Understanding "Prototypes" in JavaScript". Retrieved 6 April 2013.
  44. ↑ Herman, David (2013). Effective JavaScript. Addison-Wesley. pp. 125–127. ISBN 978-0-321-81218-6.
  45. ↑ "Properties of the Function Object". Es5.github.com. Retrieved 2013-05-26.
  46. ↑ Flanagan 2006, p. 141.
  47. ↑ The many talents of JavaScript for generalizing Role-Oriented Programming approaches like Traits and Mixins, April 11, 2014.
  48. ↑ Traits for JavaScript, 2010.
  49. ↑ CocktailJS – ANNOTATIONS. TRAITS. TALENTS., April 2013.
  50. ↑ Angus Croll, A fresh look at JavaScript Mixins, published May 31, 2011.
  51. ↑ "Concurrency model and Event Loop". Mozilla Developer Network. Retrieved 2015-08-28.
  52. ↑ Haverbeke, Marijn (2011). Eloquent JavaScript. No Starch Press. pp. 139–149. ISBN 978-1-59327-282-1.
  53. ↑ Robert Nyman, Getters And Setters With JavaScript – Code Samples And Demos, published 29 May 2009, accessed 2 January 2010.
  54. ↑ John Resig, JavaScript Getters and Setters, 18 July 2007, accessed 2 January 2010
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  57. ↑ "ECMAScript Language Specification – ECMA-262 Edition 5.1". Ecma International. Retrieved 22 December 2012.
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  60. ↑ "JavaScript tracking – Piwik". Piwik. Retrieved 31 March 2012.
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  62. ↑ Peter-Paul Koch, Object detection
  63. ↑ Peter-Paul Koch, Mission Impossible – mouse position
  64. ↑ Peter-Paul Koch, Browser detect
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  66. ↑ "Secure ECMA Script (SES)". Code.google.com. Retrieved 2013-05-26.
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  73. ↑ Fusion Authority, Macromedia Flash ActiveX Buffer Overflow
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  75. ↑ US CERT, Vulnerability Note VU#713878: Microsoft Internet Explorer does not properly validate source of redirected frame
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  77. ↑ Microsoft Corporation, Changes to Functionality in Microsoft Windows XP Service Pack 2: Part 5: Enhanced Browsing Security
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  86. ↑ "RPG Maker MV | RPG Maker | Make Your Own Games!". Retrieved 28 August 2015.
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  90. ↑ Nokia Corporation, QtScript Module
  91. ↑ Open Scripting Architecture
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  107. ↑ "RFC 822".
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  110. ↑ "p5.js".
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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 978-1-118-02669-4. 
  • Flanagan, David (2006). JavaScript: The Definitive Guide (5th ed.). O'Reilly & Associates. ISBN 0-596-10199-6. 
  • Flanagan, David (2011). JavaScript: The Definitive Guide (6th ed.). O'Reilly & Associates. ISBN 978-0-596-80552-4. 
  • 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 978-1-59327-282-1. 
  • 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 0-201-43329-X. 
  • 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. 

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