JavaScript

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

JavaScript

Paradigm	Multi-paradigm: event-driven, functional, imperative, object-oriented (prototype-based)
Designed by	Brendan Eich
Developer	Netscape Communications Corporation, Mozilla Foundation, Ecma International
First appeared	December 4, 1995 (1995-12-04)
Stable release	ECMAScript 2019 / June 2019 (2019-06)
Preview release	ECMAScript 2020
Typing discipline	Dynamic, duck
Filename extensions	.js .mjs
Major implementations
V8, JavaScriptCore, SpiderMonkey, Chakra
Influenced by
AWK, C, HyperTalk, Java, Lua, Perl, Python, Scheme, Self
Influenced
ActionScript, AtScript, CoffeeScript, Dart, JScript .NET, LiveScript, Objective-J, Opa, QML, Raku, TypeScript
JavaScript at Wikibooks

Part of a series on
JavaScript
Language
History Features Syntax ECMAScript Transpilers
Libraries
JavaScript libraries Frameworks Ajax frameworks Unit testing frameworks
Implementations
JavaScript engine Server side
See also
HTML JSON YAML XMLHttpRequest ASM.JS WASM WebGL

JavaScript (/ˈdʒɑːvəˌskrɪpt/), often abbreviated as JS, is a programming language that conforms to the ECMAScript specification. JavaScript is high-level, often just-in-time compiled, and multi-paradigm. It 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. JavaScript enables interactive web pages and is an essential part of web applications. The vast majority of websites use it for client-side page behavior, 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 application programming interfaces (APIs) for working with text, arrays, dates, regular expressions, and the Document Object Model (DOM). However, the language itself does not include any input/output (I/O), such as networking, storage, or graphics facilities, as the host environment (usually a web browser) provides it.

Initially only used in web browsers, JavaScript engines are now embedded in other types of software, including server-side website deployments and non-browser applications.

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. The JSON serialization format, used to store data structures in files or transmit them across networks, is based on JavaScript.

History

Creation at Netscape

The Mosaic web browser was released in 1993. As the first browser with a graphical user interface accessible to non-technical people, it played a prominent role in expanding the growth of the nascent World Wide Web. The lead developer, Marc Andreessen, then founded the Netscape company, which released a more polished browser, Netscape Navigator, in 1994. Navigator quickly became the dominant web browser.

Netscape wanted web pages to become more dynamic, by adding an easy to learn scripting language that would cater to the burgeoning web development scene. Netscape management ended up pursuing two routes to achieve this goal: collaborating with Sun Microsystems to embed its Java programming language in Navigator, while also hiring Brendan Eich to embed the Scheme language.

In 1995, Netscape management decided to go with a new language devised by Eich, but they mandated that it must complement Java. Thus the syntax became similar to Java and less like Scheme or other extant scripting languages. Although the new language and its interpreter implementation were officially called LiveScript when first shipped as part of a Navigator release in September 1995, the name was changed to JavaScript three months later.

The choice of the JavaScript name has caused confusion, sometimes giving the impression that it is a spin-off of Java. Since Java was the hot new programming language at the time, this has been characterized as a marketing ploy by Netscape to give its new language cachet.

Adoption by Microsoft

Microsoft reverse-engineered Netscape's JavaScript interpreter and dubbed their implementation JScript. It was first released as part of Internet Explorer 3 in 1996, alongside initial support for CSS and various extensions to HTML. Each of these implementations was noticeably different from their counterpart in Netscape Navigator. 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.

JavaScript thus acquired a reputation of being a roadblock to a cross-platform and standards-driven Web. Some developers took on the difficult task of trying to make their sites work in both major browsers, but many could not afford the time. With the release of Internet Explorer 4, Microsoft introduced the concept of Dynamic HTML, but the differences in language implementations and the different and proprietary Document Object Models remained obstacles to widespread adoption of JavaScript on the Web of the 1990s.

Messy standardization process

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.

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.

Reaching maturity

From 2015 to 2019, a new version of the ECMAScript standard was published each year, bringing many additions and refinements to the language. Thus JavaScript can now be considered mature, which is a stark contrast from its formative years, when many professional developers dismissed it as "strictly for the amateurs".

The current JavaScript ecosystem has many libraries and frameworks, established programming practices, and increased usage of JavaScript outside of web browsers. Plus, with the rise of single-page applications and other JavaScript-heavy websites, a number of transpilers have been created to aid the development process.

Trademark

"JavaScript" is a trademark of Oracle Corporation in the United States. It is used under license for technology invented and implemented by Netscape Communications and other parties.

Website client-side usage

JavaScript is the dominant client-side scripting language of the Web, with 95% of websites using it for this purpose. Scripts are embedded in or included from HTML documents and interact with the DOM. All major web browsers have a built-in JavaScript engine that executes the code on the user's device.

Examples of scripted behavior

See also: Dynamic HTML and Ajax (programming)

• Loading new page content without reloading the page. For example, social media websites use Ajax so that users can post new messages without leaving the page.
• Animation of page elements, such as fading them in and out, resizing, and moving them.
• Interactive content, such as games 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 behavior for analytics, ad tracking, and personalization.

Libraries and frameworks

The majority of websites use a third-party JavaScript library or web application framework as part of their client-side page scripting.

jQuery is the most popular library, used by over 70% of websites.

The Angular framework was created by Google for its web services; it is now open source and used by other websites. Likewise, Facebook created the React framework for its website and later released it as open source; other sites, including Twitter, now use it.

Other usage

The use of JavaScript has expanded beyond its web browser roots. JavaScript engines are now embedded in a variety of other software systems, both for server-side website deployments and non-browser applications.

Initial attempts at promoting server-side JavaScript usage were Netscape Enterprise Server and Microsoft's Internet Information Services, but they were small niches. Server-side usage eventually started to grow in the late-2000s, with the creation of Node.js and other approaches.

Electron, Cordova, and other software frameworks have been used to create many applications with behavior implemented in JavaScript. Other non-browser applications include Adobe Acrobat support for scripting PDF documents and GNOME Shell extensions written in JavaScript.

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 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.

Weakly typed

JavaScript is weakly typed, which means certain types are implicitly cast depending on the operation used. JavaScript has received criticism for the way it implements these conversions as well as the inconsistency between them. For example, when adding a number to a string, the number will be cast to a string before performing concatenation, but when subtracting a number from a string, the string is cast to a number before performing subtraction.

JavaScript includes many other type quirks that have been subject to criticism:

left operand	operator	right operand	result
[](empty array)	+	[](empty array)	""(empty string)
{} (empty object)	+	[] (empty array)	0 (number)
[] (empty array)	+	{} (empty object)	{} (empty object)
false (boolean)	+	[] (empty array)	"false" (string)
"123"(string)	+	1 (number)	"1231" (string)
"123" (string)	-	1 (number)	122 (number)

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. JavaScript supports various ways to test the type of objects, including duck typing.

Run-time evaluation

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

Object-orientation (prototype-based)

Prototypal inheritance in JavaScript is described by Douglas Crockford as:

You make prototype objects, and then … make new instances. Objects are mutable in JavaScript, so we can augment the new instances, giving them new fields and methods. These can then act as prototypes for even newer objects. We don't need classes to make lots of similar objects… Objects inherit from objects. What could be more object oriented than that?

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. It is possible to simulate many class-based features with prototypes in JavaScript.

Functions as object constructors

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

Functions as methods

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

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

Object composition and inheritance

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

Miscellaneous

Run-time environment

JavaScript typically relies on a run-time environment (e.g., a Web browser) to provide objects and methods by which scripts can interact with the environment (e.g., a webpage DOM). It also relies on the run-time environment to provide the ability to include/import scripts (e.g., HTML <script> elements). This is not a language feature per se, but it is common in most JavaScript implementations. 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.

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.

Promises

JavaScript also supports promises which is its way of handling asynchronous operations. There's a built-in Promise object that gives access to a lot of functionalities for handling promises and defines how they should be handled. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future. Recently, combinator methods were introduced in the JavaScript specification which allows developers to combine multiple JavaScript promises and do operations on the basis of different scenarios. The methods introduced are: Promise.race, Promise.all, Promise.allSettled and Promise.any.

Vendor-specific extensions

Historically, some JavaScript engines supported these non-standard features:

• conditional catch clauses (like Java)
• array comprehensions and generator expressions (like Python)
• concise function expressions (function(args) expr; this experimental syntax predated arrow functions)
• ECMAScript for XML (E4X), an extension that adds native XML support to ECMAScript (unsupported in Firefox since version 21)

Syntax

Main article: JavaScript syntax

Simple examples

Variables in JavaScript can be defined using either the var, let or const keywords.

// Declares a function-scoped variable named `x`, and implicitly assigns the
// special value `undefined` to it.
var x;

// More explicit version of the above.
var x2 = undefined;

// Declares a block-scoped variable named `y`, and implicitly sets it to
// `undefined`. The `let` keyword was introduced in ECMAScript 2015.
let y;

// More explicit version of the above.
let y2 = undefined;

// Declares a block-scoped, un-reassign-able variable named `z`, and sets it to
// `undefined`. The `const` keyword was also introduced in ECMAScript 2015, and
// must be explicitly assigned to.
const z = undefined;

// Declares a variable named `myNumber`, and assigns a number literal (the value
// `2`) to it.
let myNumber = 2;

// Reassigns `myNumber`, setting it to a string literal (the value `"foo"`).
// JavaScript is a dynamically-typed language, so this is legal.
myNumber = "foo";

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:

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

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

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

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

Security

See also: Browser security

JavaScript and the DOM provide the potential for malicious authors to deliver scripts to run on a client computer via the Web. Browser authors 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). 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.

See also: Content Security Policy

Cross-site vulnerabilities

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

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

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

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

Another cross-site vulnerability is cross-site request forgery (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.
• It is considered very bad practice to embed sensitive information such as passwords in JavaScript because it can be extracted by an attacker.

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. "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." 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.

Browser and plugin coding errors

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

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

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

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

Sandbox implementation errors

Web browsers are capable of running JavaScript outside the sandbox, with the privileges necessary to, for example, create or delete files. 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 and Firefox. In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.

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

Hardware vulnerabilities

In 2015, a JavaScript-based proof-of-concept implementation of a rowhammer attack was described in a paper by security researchers.

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

Version history

See also: ECMAScript § Versions

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.

Version	Release date	Equivalent to	Netscape Navigator	Mozilla Firefox	Internet Explorer	Opera	Safari	Google Chrome
Old version, no longer maintained: 1.0	March 1996		2.0		3.0
Old version, no longer maintained: 1.1	August 1996		3.0
Old version, no longer maintained: 1.2	June 1997		4.0-4.05			3
Old version, no longer maintained: 1.3	October 1998	ECMA-262 1st + 2nd edition	4.06-4.7x		4.0	5
Old version, no longer maintained: 1.4			Netscape Server			6
Old version, no longer maintained: 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 maintained: 1.6	November 2005	1.5 + array extras + array and string generics + E4X		1.5
Old version, no longer maintained: 1.7	October 2006	1.6 + Pythonic generators + iterators + let		2.0				28.0.1500.95
Old version, no longer maintained: 1.8	June 2008	1.7 + generator expressions + expression closures		3.0		11.50
Old version, no longer maintained: 1.8.1		1.8 + native JSON support + minor updates		3.5
Old version, no longer maintained: 1.8.2	June 22, 2009	1.8.1 + minor updates		3.6
Old version, no longer maintained: 1.8.5	July 27, 2010	1.8.2 + new features for ECMA-262 5th edition compliance (last explicit versioning of JavaScript)		4.0

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.

Script debuggers are integrated within many mainstream browsers such as Internet Explorer, Firefox, Safari, Google Chrome, Opera and Node.js.

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), 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, 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.

In addition to the native computer software, there are online JavaScript integrated development environment (IDEs), which have 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 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.

One performance benchmarking tool is benchmark.js. Another is jsben.ch, where code snippets can be tested against each other.

Related technologies

Java

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

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.

JSON

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

WebAssembly

Since 2017, web browsers have supported WebAssembly, a binary format that enables a JavaScript engine to execute performance-critical portions of web page scripts close to native speed. It is designed to be used alongside regular JavaScript code.

asm.js is a subset of JavaScript that served as the forerunner of WebAssembly.

Transpilers

JavaScript is the dominant client-side language of the Web, and many websites are script-heavy. Thus transpilers have been created to convert code written in other languages, which can aid the development process.

External links

Listen to this article (info/dl)

This audio file was created from a revision of the article "JavaScript" dated 2013-08-20, and does not reflect subsequent edits to the article. (Audio help)

More spoken articles

• JavaScript at Curlie
• Douglas Crockford's video lectures on JavaScript