JavaScript (/ˈdʒɑːvəˌskrɪpt/),^[8] often abbreviated as JS, is a high-level, interpreted scripting language that conforms to the ECMAScript specification. JavaScript has curly-bracket syntax, dynamic typing, prototype-based object-orientation, and first-class functions.

Alongside HTML and CSS, JavaScript is one of the core technologies of the World Wide Web.^[9] JavaScript enables interactive web pages and is an essential part of web applications. The vast majority of websites use it,^[10] and major web browsers have a dedicated JavaScript engine to execute it.

As a multi-paradigm language, JavaScript supports event-driven, functional, and imperative (including object-oriented and prototype-based) programming styles. It has APIs for working with text, arrays, dates, regular expressions, and the DOM, but the language itself does not include any I/O, such as networking, storage, or graphics facilities. It relies upon the host environment in which it is embedded to provide these features.

Initially only implemented client-side in web browsers, JavaScript engines are now embedded in many other types of host software, including server-side in web servers and databases, and in non-web programs such as word processors and PDF software, and in runtime environments that make JavaScript available for writing mobile and desktop applications, including desktop widgets.

The terms Vanilla JavaScript and Vanilla JS refer to JavaScript not extended by any frameworks or additional libraries. Scripts written in Vanilla JS are plain JavaScript code.^[11][12]

Although there are similarities between JavaScript and Java, including language name, syntax, and respective standard libraries, the two languages are distinct and differ greatly in design. JavaScript was influenced by programming languages such as Self and Scheme.^[13] The JSON serialization format, used to store data structures in files or transmit them across networks, is based on JavaScript.^[14]

History

Beginnings at Netscape

In 1993, the National Center for Supercomputing Applications (NCSA), a unit of the University of Illinois at Urbana-Champaign, released NCSA Mosaic, the first popular graphical Web browser, which played an important part in expanding the growth of the nascent World Wide Web beyond the NeXTSTEP niche where the WorldWideWeb had formed three years earlier. In 1994, a company called Mosaic Communications was founded in Mountain View, California and employed many of the original NCSA Mosaic authors to create Mosaic Netscape. However, it intentionally shared no code with NCSA Mosaic. The internal codename for the company's browser was Mozilla, a portmanteau of "Mosaic and Godzilla".^[15] The first version of the Web browser, Mosaic Netscape 0.9, was released in late 1994. Within four months it had already taken three-quarters of the browser market and became the main web browser for the 1990s. To avoid trademark ownership problems with the NCSA, the browser was subsequently renamed Netscape Navigator in the same year, and the company took the name Netscape Communications. Netscape Communications realized that the Web needed to become more dynamic. Marc Andreessen, the founder of the company, believed that HTML needed a "glue language" that was easy to use by Web designers and part-time programmers to assemble components such as images and plugins, where the code could be written directly in the Web page markup.

In 1995, Netscape Communications recruited Brendan Eich with the goal of embedding the Scheme programming language into its Netscape Navigator.^[16] Before he could get started, Netscape Communications collaborated with Sun Microsystems to include Sun's more static programming language, Java, in Netscape Navigator so as to compete with Microsoft for user adoption of Web technologies and platforms.^[17] Netscape Communications then decided that the scripting language they wanted to create would complement Java and should have a similar syntax, which excluded adopting other languages such as Perl, Python, TCL, or Scheme. To defend the idea of JavaScript against competing proposals, the company needed a prototype. Eich wrote one in 10 days, in May 1995.^{[citation needed]}

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 Navigator 2.0 beta 3 in December.^[2][18] 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^[19] 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 JavaScript was influenced by an earlier Web page scripting language developed by Nombas named Cmm (not to be confused with the later C-- created in 1997).^[20][21] Brendan Eich, however, had never heard of Cmm 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 the ViolaWWW Web browser.^[22] Nombas later switched to offering JavaScript instead of Cmm in their ScriptEase product and was part of the TC39 group that standardized ECMAScript.^[23]

Server-side JavaScript

In December 1995, soon after releasing JavaScript for browsers, Netscape introduced an implementation of the language for server-side scripting with Netscape Enterprise Server.^[24]

Since 1996, the IIS web-server has supported Microsoft's implementation of server-side Javascript—JScript—in ASP and .NET pages.^[25]

Since the mid-2000s, additional server-side JavaScript implementations have been introduced, such as Node.js in 2009.^[26]

Adoption by Microsoft

Microsoft script technologies including VBScript and JScript were released in 1996. JScript, a reverse-engineered implementation of Netscape's JavaScript, was part of Internet Explorer 3. JScript was also available for server-side scripting in Internet Information Server. Internet Explorer 3 also included Microsoft's first support for CSS and various extensions to HTML, but in each case the implementation was noticeably different from that found in Netscape Navigator at the time.^[27][28] 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 characterized these early years of the browser wars.^[29] 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.^[27] 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.^[27]

Standardization

In November 1996, Netscape submitted JavaScript to ECMA International to carve out a standard specification, which other browser vendors could then implement based on the work done at Netscape. This led to the official release of the language specification ECMAScript published in the first edition of the ECMA-262 standard in June 1997, with JavaScript being the most well known of the implementations. ActionScript and JScript were other well-known implementations of ECMAScript.

The release of ECMAScript 2 in June 1998 continued the standards process cycle, conforming some modifications to the ISO/IEC 16262 international standard. ECMAScript 3 was released in December 1999 and is the modern-day baseline for JavaScript. The original ECMAScript 4 work led by Waldemar Horwat (then at Netscape, now at Google) started in 2000. Microsoft initially participated and implemented some proposals in their JScript .NET language.

Over time it was clear that Microsoft had no intention of cooperating or implementing proper JavaScript in Internet Explorer, even though they had no competing proposal and they had a partial (and diverged at this point) implementation on the .NET server side. So by 2003, the original ECMAScript 4 work was mothballed.

The next major event was in 2005, with two major happenings in JavaScript's history. First, Brendan Eich and Mozilla rejoined Ecma International as a not-for-profit member and work started on ECMAScript for XML (E4X), the ECMA-357 standard, which came from ex-Microsoft employees at BEA Systems (originally acquired as Crossgain). This led to working jointly with Macromedia (later acquired by Adobe Systems), who were implementing E4X in ActionScript 3 (ActionScript 3 was a fork of original ECMAScript 4).

So, along with Macromedia, work restarted on ECMAScript 4 with the goal of standardizing what was in ActionScript 3. To this end, Adobe Systems released the ActionScript Virtual Machine 2, code named Tamarin, as an open source project. But Tamarin and ActionScript 3 were too different from web JavaScript to converge, as was realized by the parties in 2007 and 2008.

Alas, there was still turmoil between the various players; Douglas Crockford—then at Yahoo!—joined forces with Microsoft in 2007 to oppose ECMAScript 4, which led to the ECMAScript 3.1 effort. The development of ECMAScript 4 was never completed, but that work influenced subsequent versions.^[30]

While all of this was happening, the open source and developer communities set to work to revolutionize what could be done with JavaScript. This community effort was sparked in 2005 when Jesse James Garrett released a white paper in which he coined the term Ajax, and described a set of technologies, of which JavaScript was the backbone, used to create web applications where data can be loaded in the background, avoiding the need for full page reloads and leading to more dynamic applications. This resulted in a renaissance period of JavaScript usage spearheaded by open source libraries and the communities that formed around them, with libraries such as Prototype, jQuery, Dojo Toolkit, MooTools, and others being released.

In July 2008, the disparate parties on either side came together in Oslo. This led to the eventual agreement in early 2009 to rename ECMAScript 3.1 to ECMAScript 5 and drive the language forward using an agenda that is known as Harmony. ECMAScript 5 was finally released in December 2009.

In June 2011, ECMAScript 5.1 was released to fully align with the third edition of the ISO/IEC 16262 international standard. ECMAScript 2015 was released in June 2015. ECMAScript 2016 was released in June 2016. The current version is ECMAScript 2017, released in June 2017.^[3]

Later developments

JavaScript has become one of the most popular programming languages on the Web. However, many professional programmers initially denigrated the language due to the perceived target audience of Web authors and other such "amateurs".^[31] 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.^[32]

With the rise of single-page applications 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.

Trademark

"JavaScript" is a trademark of Oracle Corporation in the United States.^[33] It is used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.^[34]

Features

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

Universal support

All popular modern Web browsers support JavaScript with built-in interpreters.

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 added keywords let and const 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.^[35]

Dynamic

Typing

JavaScript is dynamically typed like most other scripting languages. A type is associated with a value rather than an expression. For example, a variable initially bound to a number may be reassigned to a string.^[36] JavaScript supports various ways to test the type of objects, including duck typing.^[37]

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.^[38] It is possible to simulate many class-based features with prototypes in JavaScript.^[39]

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).^[40] 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).^[41] 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.^[42]

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().^[43] 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.^[44] 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^[45] patterns like Traits^[46][47] and Mixins.^[48] 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. JavaScript calls a function associated with each new message, creating a call stack frame with the function's arguments and local variables. The call stack shrinks and grows based on the function's needs. When the call stack is empty upon function completion, 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.^[49]

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.^[50]

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,^[51] ActionScript, and Rhino)^[52]
• 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^[53])

Syntax

Simple examples

Variables in JavaScript can be defined using either the var,^[54] let^[55] or const^[56] keywords.

let x; // declares the variable x and assigns to it the special value "undefined" (not to be confused with an undefined value)
let y = 2; // declares the variable y and assigns to it the value 2
let z = "Hello, World!"; // declares the variable z and assigns to it a string containing "Hello, World!"

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

There is no built-in Input/output 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 in JavaScript:

console.log("Hello World!");

A simple recursive function:

function factorial(n) {
    if (n === 0)
        return 1; // 0! = 1

    return n * factorial(n - 1);
}

factorial(3); // returns 6

An anonymous function (or lambda):

function counter() {
    let count = 0;

    return function() {
        return ++count;
    };
}

let closure = counter();
closure(); // returns 1
closure(); // returns 2
closure(); // returns 3

This example shows that, in JavaScript, function closures capture their non-local variables by reference.

Arrow functions were first introduced in 6th Edition - ECMAScript 2015 . They shorten the syntax for writing functions in JavaScript. Arrow functions are anonymous in nature; a variable is needed to refer to them in order to invoke them after their creation.

Example of arrow function:

// Arrow functions let us omit the `function` keyword. Here `long_example`
// points to an anonymous function value.
const long_example = (input1, input2) => {
    console.log("Hello, World!");
    const output = input1 + input2;

    return output;
};

// Arrow functions also let us automatically return the expression to the right
// of the arrow (here `input + 5`), omitting braces and the `return` keyword.
const short_example = input => input + 5;

long_example(2, 3); // Prints "Hello, World!" and returns 5.
short_example(2);  // Returns 7.

In JavaScript, objects are created in the same way as functions; this is known as a function object.

Object example:

function Ball(r) {
    this.radius = r; // the radius variable is local to the ball object
    this.area = pi * r ** 2;
    this.show = function(){ // objects can contain functions
        drawCircle(r); // references a circle drawing function
    }
}

let myBall = new Ball(5); // creates a new instance of the ball object with radius 5
myBall.show(); // this instance of the ball object has the show function performed on it

Variadic function demonstration (arguments is a special variable):^[59]

function sum() {
    let x = 0;

    for (let i = 0; i < arguments.length; ++i)
        x += arguments[i];

    return x;
}

sum(1, 2); // returns 3
sum(1, 2, 3); // returns 6

Immediately-invoked function expressions are often used to create modules; before ECMAScript 2015 there was no built-in module construct in the language. Modules allow gathering properties and methods in a namespace and making some of them private:

let counter = (function() {
    let i = 0; // private property

    return {   // public methods
        get: function() {
            alert(i);
        },
        set: function(value) {
            i = value;
        },
        increment: function() {
            alert(++i);
        }
    };
})(); // module

counter.get();      // shows 0
counter.set(6);
counter.increment(); // shows 7
counter.increment(); // shows 8

Exporting and Importing modules in javascript^[60]

Export example:

/* mymodule.js */
// This function remains private, as it is not exported
let sum = (a, b) => {
    return a + b;
}

// Export variables
export let name = 'Alice';
export let age = 23;

// Export named functions
export function add(num1, num2){
    return num1 + num2;
}

// Export class
export class Multiplication {
    constructor(num1, num2) {
        this.num1 = num1;
        this.num2 = num2;
    }

    add() {
        return sum(this.num1, this.num2);
    }
}

Import example:

// Import one property
import { add } from './mymodule.js';

console.log(add(1, 2)); // 3

// Import multiple properties
import { name, age } from './mymodule.js';
console.log(name, age);
//> "Alice", 23

// Import all properties from a module
import * from './module.js'
console.log(name, age);
//> "Alice", 23
console.log(add(1,2));
//> 3

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
    let checkInt = function(x) { // inner function
        if (x % 1 !== 0)
            throw new TypeError(x + "is not an integer"); // var a =  mouseX

        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:
        let a = Math.abs(this.a), b = Math.abs(this.b), t;

        if (a < b) {
            // swap variables
            // t = b; b = a; a = t;
            [a, b] = [b, a]; // swap using destructuring assignment (ES6)
        }

        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 do not collide with object properties, e.g., |lcm| is not |this.lcm|.
        // not using |this.a*this.b| to avoid FP precision issues
        let 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((a, b) => a.lcm() - b.lcm()) // sort with this comparative function; => is a shorthand form of a function, called "arrow function"
    .forEach(printResult);

function printResult(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

As of May 2017 94.5% of 10 million most popular web pages used JavaScript.^[10] The most common use of JavaScript is to add client-side behavior to HTML pages, also known as 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.

JavaScript code can run locally in a user's browser (rather than on a remote server), increasing the application's overall responsiveness to user actions. JavaScript code can also 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, 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 ECMAScript 3. The Rhino engine, created primarily by Norris Boyd (formerly at Netscape, now at Google) is a JavaScript implementation in Java. Rhino, like SpiderMonkey, is ECMAScript 3 compliant.

A Web browser is the most common host environment for JavaScript. However, a Web browser does not have to execute JavaScript code. (For example, text-based browsers have no JavaScript engines; and users of other browsers may disable scripts through a preference or extension.)

A Web browser typically creates "host objects" to represent the 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.

JavaScript is the only language that the most popular browsers share support for and has inadvertently become a target language for frameworks in other languages.^[61] The increasing speed of JavaScript engines has made the language a feasible compilation target, despite the performance limitations inherent to its dynamic nature.

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>
  <head>
    <title>Example</title>
  </head>
  <body>
    <button id="hellobutton">Hello</button>
    <script>
        document.getElementById('hellobutton').onclick = function() {
            alert('Hello world!');                    // Show a dialog
            var myTextNode = document.createTextNode('Some new words.');
            document.body.appendChild(myTextNode);    // Append "Some new words" to the page
        };
    </script>
  </body>
</html>

Compatibility considerations

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 are officially defined by the W3C in a standardization effort separate from JavaScript. The implementation of these DOM interfaces differ between web browsers.

JavaScript authors can deal with these differences by writing standards-compliant code that can be executed correctly by most browsers. Failing that, they can write code that behaves differently in the absence of certain browser features.^[62] Authors may also find it practical to detect what browser is running, as two browsers may implement the same feature with differing behavior.^[63][64] Libraries and toolkits that take browser differences into account are also useful to programmers.

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. Some sites use the HTML <noscript> tag, which contains alt content if JS is disabled. An alternative approach that many^[quantify] 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. ^[65] 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 minimize 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 ECMAScript (SES)—that provide greater levels of security, especially on code created by third parties (such as advertisements).^[66][67] 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 (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.^[68]

Another cross-site vulnerability is cross-site request forgery (CSRF). In CSRF, code on an attacker's site tricks the victim's browser into taking actions the user did not intend at a target site (like transferring money at a bank). When target sites rely solely on cookies for request authentication, requests originating from code on the attacker's site can carry the same valid login credentials of the initiating 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 their 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 cannot be relied on to prevent operations such as right-clicking on an image to save it.^[69]
• It is considered very bad practice to embed sensitive information such as passwords in JavaScript because it can be extracted by an attacker.^[70]

Misplaced trust in developers

Package management systems such as npm and Bower are popular with JavaScript developers. Such systems allow a developer to easily manage their program's dependencies upon other developer's program libraries. Developers trust that the maintainers of the libraries will keep them secure and up to date, but that is not always the case. A vulnerability has emerged because of this blind trust. Relied-upon libraries can have new releases that cause bugs or vulnerabilities to appear in all programs that rely upon the libraries. Inversely, a library can go unpatched with known vulnerabilities out in the wild. In a study done looking over a sample of 133k websites, researchers found 37% of the websites included a library with at-least one known vulnerability.^[71] "The median lag between the oldest library version used on each website and the newest available version of that library is 1,177 days in ALEXA, and development of some libraries still in active use ceased years ago."^[71] Another possibility is that the maintainer of a library may remove the library entirely. This occurred in March 2016 when Azer Koçulu removed his repository from npm. This caused all tens of thousands of programs and websites depending upon his libraries to break.^[72][73]

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,^[74] Internet Explorer,^[75] and Safari.^[76]

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).^[77][78]

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.^[79] 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. Such privileges are not intended 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^[80] and Firefox.^[81] In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.^[82]

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.^{[83][failed verification]}

Hardware vulnerabilities

In 2015, a JavaScript-based proof-of-concept implementation of a rowhammer attack was described in a paper by security researchers.^{[84][85][86][87]}

In 2017, a JavaScript-based attack via browser was demonstrated that could bypass ASLR. It's called "ASLR⊕Cache" or AnC.^[88][89]

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.
• The Clusterpoint database accept queries written in JS/SQL, which is a combination of SQL and JavaScript. Clusterpoint has built-in computing engine that allows execution of JavaScript code right inside the distributed database.
• Adobe's Acrobat and Adobe Reader support JavaScript in PDF files.^[90]
• Tools in the Adobe Creative Suite, including Photoshop, Illustrator, After Effects, Dreamweaver, and InDesign, allow scripting through JavaScript.
• LibreOffice, an office application suite, allows JavaScript to be used as a scripting language.
• The visual programming language Max, released by Cycling '74, offers a JavaScript model of its environment for use by developers. It allows users to reduce visual clutter by using an object for a task rather than many.
• Apple's Logic Pro X digital audio workstation (DAW) software can create custom MIDI effects plugins using JavaScript.^[91]
• The Unity game engine supported a modified version of JavaScript for scripting via Mono until 2017.^[92]
• DX Studio (3D engine) uses the SpiderMonkey implementation of JavaScript for game and simulation logic.^[93]
• Maxwell Render (rendering software) provides an ECMA standard based scripting engine for tasks automation.^[94]
• 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.^[95]
• Many IRC clients, like ChatZilla or XChat, use JavaScript for their scripting abilities.^[96][97]
• RPG Maker MV uses JavaScript as its scripting language.^[98]
• The text editor UltraEdit uses JavaScript 1.7 as internal scripting language, introduced with version 13 in 2007.

Scripting engine

• Microsoft's Active Scripting technology supports JScript as a scripting language.^[99]
• Java 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).^[100][101]
• The Qt C++ toolkit includes a QtScript module to interpret JavaScript, analogous to Java's javax.script package.^[102]
• OS X Yosemite introduced JavaScript for Automation (JXA), which is built upon JavaScriptCore and the Open Scripting Architecture. It features an Objective-C bridge that enables entire Cocoa applications to be programmed in JavaScript.
• Late Night Software's JavaScript OSA (also known as JavaScript for OSA, or JSOSA) is a freeware alternative to AppleScript for OS X. It is based on the Mozilla JavaScript 1.5 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 AIR (Adobe Integrated Runtime) is a JavaScript runtime that allows developers to create desktop applications.
• Electron is an open-source framework developed by GitHub.
• CA Technologies 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.
• Apache Cordova is a mobile application development framework
• Cocos2d is an open source software framework. It can be used to build games, apps and other cross platform GUI based interactive programs
• Chromium Embedded Framework (CEF) is an open source framework for embedding a web browser engine based on the Chromium core
• RhoMobile Suite is a set of development tools for creating data-centric, cross-platform, native mobile consumer and enterprise applications.
• NW.js call all Node.js modules directly from DOM and enable a new way of writing applications with all Web technologies.^[103]
• GNOME Shell, the shell for the GNOME 3 desktop environment,^[104] made JavaScript its default programming language in 2013.^[105]
• The Mozilla application framework (XPFE) platform, which underlies Firefox, Thunderbird, and some other Web browsers, uses JavaScript to implement the graphical user interface (GUI) of its various products.
• Qt Quick's markup language (available since Qt 4.7) uses JavaScript for its application logic. Its declarative syntax is also similar to JavaScript.
• Ubuntu Touch provides a JavaScript API for its unified usability interface.
• Open webOS is the next generation of web-centric platforms built to run on a wide range of form factors.^[106]
• enyo JS is a framework to develop apps for all major platforms, from phones and tablets to PCs and TVs^[107]
• 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.
• NativeScript is an open-source framework to develop apps on the Apple iOS and Android platforms.
• Weex is a framework for building Mobile cross-platform UI, created by China Tech giant Alibaba^[108]
• XULRunner is packaged version of the Mozilla platform to enable standalone desktop application development

Development tools

Within JavaScript, access to a debugger becomes invaluable when developing large, non-trivial programs. There can be implementation differences between the various browsers (particularly within the DOM), so it is useful to have access to a debugger for each of the browsers that a Web application targets.^[109]

Script debuggers are integrated within many mainstream browsers such as Internet Explorer, Firefox, Safari, Google Chrome, Opera and Node.js.^{[110][111][112]}

In addition to the native Internet Explorer Developer Tools, three other debuggers are available for Internet Explorer: Microsoft Visual Studio has the most features of the three, closely followed by Microsoft Script Editor (a component of Microsoft Office),^[113] and finally the free Microsoft Script Debugger. The free Microsoft Visual Web Developer Express provides a limited version of the JavaScript debugging functionality in Microsoft Visual Studio.

In comparison to Internet Explorer, Firefox has a more comprehensive set of developer tools, which includes a debugger as well. Old versions of Firefox without these tools used a Firefox addon called Firebug, or the older Venkman debugger. WebKit's Web Inspector includes a JavaScript debugger,^[114] 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. Opera includes a set of tools called Dragonfly.^[115]

In addition to the native computer software, there are online JavaScript integrated development environment (IDEs), which has debugging aids that 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. Demonstration codes are also used as a pedagogical tool by institutions such as Khan Academy^[116] 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.

Benchmark tools for developers

JavaScript's increased usage in web development warrants further considerations about performance. Frontend code has inherited many responsibilities previously handled by the backend. Mobile devices in particular may encounter problems rendering poorly optimized frontend code.

A library for doing benchmarks is benchmark.js.^[117] A benchmarking library that supports high-resolution timers and returns statistically significant results.^{[citation needed]}

Another tool is jsben.ch.^[118] An online JavaScript benchmarking tool, where code snippets can be tested against each other.

Version history

JavaScript was initially developed in 1996 for use in the Netscape Navigator Web 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 in the first edition of the ECMA-262 standard.

The explicit versioning and opt-in of language features was Mozilla-specific and has been removed in later Firefox versions (at least by Firefox 59). Firefox 4 was the last version which referred to an explicit JavaScript version (1.8.5). With new editions of the ECMA-262 standard, JavaScript language features are now often mentioned with their initial definition in the ECMA-262 editions.

The following table of explicitly versioned JavaScript versions is based on information from multiple sources.^{[119][120][121]}

Related languages and technologies

• 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 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.
• Angular and AngularJS are web application frameworks to use for developing single-page applications and also cross-platform mobile apps.
• React is an open source JavaScript library providing views that are rendered using components specified as custom HTML tags.
• Vue.js is an open source JavaScript framework that features an incrementally adoptable architecture focusing on declarative rendering and component composition.
• Mozilla browsers currently support LiveConnect, a feature that allows JavaScript and Java to intercommunicate on the Web. However, Mozilla-specific support for LiveConnect was 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).^[123] 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.^[124]
• JSFuck is an esoteric programming language. Programs are written using only six different characters, but are still valid JavaScript code.
• p5.js^[125] 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.
• jsben.ch^[118] is an online JavaScript benchmarking tool, where different code snippets can be tested against each other.

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 (also called transpilers) to target. This has included both newly created languages and ports of existing languages. Some of these include:

• ClojureScript,^[126] a dialect of Clojure that targets JavaScript. Its compiler is designed to emit JavaScript code that is compatible with the advanced compilation mode of the Google Closure optimizing compiler.
• 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)^[127] and pattern matching. Like Objective-J, it compiles to JavaScript. Ruby and Python have been cited as influential on CoffeeScript syntax.
• Dart, an all-purpose, open source language that compiles to JavaScript.
• Elm, a pure functional language for web apps. Unlike handwritten JavaScript, Elm-generated JavaScript has zero runtime exceptions, a time-traveling debugger, and enforced semantic versioning.
• Emscripten, a LLVM-backend for porting native libraries to JavaScript, known as asm.js^[128]
• Fantom, a programming language that runs on JVM, .NET and JavaScript.
• Free Pascal,^[129] a compiler for Pascal that targets JavaScript.
• Google Web Toolkit, a toolkit that translates a subset of Java to JavaScript.
• Haxe, an open-source high-level multiplatform programming language and compiler that can produce applications and source code for many different platforms including JavaScript.
• OberonScript, a full implementation of the Oberon programming language that compiles to high-level JavaScript.^[130]
• 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 the Processing 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.
• Pyjs, a port of Google Web Toolkit to Python that translates a subset of Python to JavaScript.
• Scala, an object-oriented and functional programming language, has a Scala-to-JavaScript compiler.^[131]
• SqueakJS,^[132] a virtual machine and DOM environment for the open-source Squeak implementation of the Smalltalk programming language.
• TypeScript, a free and open-source programming language developed by Microsoft. It is a superset of JavaScript, and essentially 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.
• Whalesong,^[133] a Racket-to-JavaScript compiler.

As JavaScript has unusual limitations – such as no explicit integer type, only double-precision binary floating point – languages that compile to JavaScript and do not take care to use the integer-converting shift and bitwise logical operators may 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,^[134] but the similarities end there.

Java and JavaScript both first appeared in 1995, but Java was developed by James Gosling of Sun Microsystems, and JavaScript by Brendan Eich of Netscape Communications.

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.

WebAssembly

Starting in 2017, web browsers began supporting WebAssembly, a technology standardized by the W3C. The WebAssembly standard specifies a binary format, which can be produced by a compiler toolchain such as LLVM, to execute in the browser at near native speed. WebAssembly allows programming languages such as C, C++, C# and Java to be used as well as JavaScript to author client-side code for the World Wide Web.^[135]

See also

• WebAssembly

References

Further reading

External links

• Douglas Crockford's video lectures on JavaScript
• Douglas Crockford's A Survey of the JavaScript Programming Language
• JavaScript at Curlie
• List of languages that compile to JS