JavaScript

Not to be confused withJava (programming language),Javanese script,orECMAScript.

JavaScript

Screenshot of JavaScript source code
Paradigm	Multi-paradigm:event-driven,functional,imperative,procedural,object-oriented
Designed by	Brendan EichofNetscapeinitially; others have also contributed to theECMAScriptstandard
First appeared	4 December 1995(1995-12-04)
Stable release	ECMAScript 2024 / June 2024(June 2024)
Preview release	ECMAScript 2025 / 27 March 2024(27 March 2024)
Typing discipline	Dynamic,weak,duck
Memory management	Garbage collection
Filename extensions	.js .cjs .mjs
Website	ecma-international.org/publications-and-standards/standards/ecma-262/
Majorimplementations
V8,JavaScriptCore,SpiderMonkey,Chakra
Influenced by
Java,Scheme,Self,AWK,HyperTalk
Influenced
ActionScript,ArkTS,AssemblyScript,CoffeeScript,Dart,Haxe,JS++,Opa,TypeScript
JavaScriptat Wikibooks

JavaScript(/ˈdʒɑːvəskrɪpt/), often abbreviated asJS,is aprogramming languageand core technology ofthe Web,alongsideHTMLandCSS.99% ofwebsitesuse JavaScript on theclientside forwebpagebehavior.

Web browsershave a dedicatedJavaScript enginethat executes the clientcode.These engines are also utilized in someserversand a variety ofapps.The most popularruntime systemfor non-browser usage isNode.js.

JavaScript is ahigh-level,oftenjust-in-time compiledlanguage that conforms to theECMAScriptstandard.It hasdynamic typing,prototype-basedobject-orientation,andfirst-class functions.It ismulti-paradigm,supportingevent-driven,functional,andimperativeprogramming styles.It hasapplication programming interfaces(APIs) for working with text, dates,regular expressions,standarddata structures,and theDocument Object Model(DOM).

The ECMAScript standard does not include anyinput/output(I/O), such asnetworking,storage,orgraphicsfacilities. In practice, the web browser or other runtime system provides JavaScript APIs for I/O.

AlthoughJavaand JavaScript are similar in name,syntax,and respectivestandard libraries,the two languages are distinct and differ greatly in design.

History

Creation at Netscape

The first popularweb browserwith agraphical user interface,Mosaic,was released in 1993. Accessible to non-technical people, it played a prominent role in the rapid growth of the earlyWorld Wide Web.The lead developers of Mosaic then founded theNetscapecorporation, which released a more polished browser,Netscape Navigator,in 1994. This quickly became the most-used.

During these formative years of the Web,web pagescould only be static, lacking the capability for dynamic behavior after the page was loaded in the browser. There was a desire in the flourishing web development scene to remove this limitation, so in 1995, Netscape decided to add aprogramming languageto Navigator. They pursued two routes to achieve this: collaborating withSun Microsystemsto embed theJavalanguage, while also hiringBrendan Eichto embed theSchemelanguage.

The goal was a "language for the masses","to help nonprogrammers create dynamic, interactiveWeb sites".Netscape management soon decided that the best option was for Eich to devise a new language, with syntax similar to Java and less like Scheme or other extantscripting languages.Although the new language and itsinterpreterimplementation were called LiveScript when first shipped as part of a Navigatorbetain September 1995, the name was changed to JavaScript for the official release in December.

The choice of the JavaScript name has caused confusion, implying that it is directly related to Java. At the time, thedot-com boomhad begun and Java was a popular new language, so Eich considered the JavaScript name a marketing ploy by Netscape.

Adoption by Microsoft

MicrosoftdebutedInternet Explorerin 1995, leading to abrowser warwith Netscape. On the JavaScript front, Microsoft created its owninterpretercalledJScript.

Microsoft first released JScript in 1996, alongside initial support forCSSand extensions toHTML.Each of theseimplementationswas noticeably different from their counterparts inNetscape Navigator.These differences made it difficult for developers to make their websites work well in both browsers, leading to widespread use of "best viewed in Netscape" and "best viewed in Internet Explorer" logos for several years.

The rise of JScript

Brendan Eichlater said of this period: "It's still kind of asidekicklanguage. It's considered slow or annoying. People dopop-upsor those scrolling messages in the oldstatus barat the bottom of your oldbrowser."

In November 1996,Netscapesubmitted JavaScript toEcma International,as the starting point for a standard specification that all browser vendors could conform to. This led to the official release of the firstECMAScriptlanguage specification in June 1997.

The standards process continued for a few years, with the release of ECMAScript 2 in June 1998 and ECMAScript 3 in December 1999. Work on ECMAScript 4 began in 2000.

However, the effort to fully standardize the language was undermined byMicrosoftgaining an increasingly dominant position in the browser market. By the early 2000s,Internet Explorer's market share reached 95%.This meant thatJScriptbecame the de facto standard forclient-side scriptingon the Web.

Microsoft initially participated in the standards process and implemented some proposals in its JScript language, but eventually it stopped collaborating on ECMA work. Thus ECMAScript 4 was mothballed.

Growth and standardization

During the period ofInternet Explorerdominance in the early 2000s, client-side scripting was stagnant. This started to change in 2004, when the successor of Netscape,Mozilla,released theFirefoxbrowser. Firefox was well received by many, taking significant market share from Internet Explorer.

In 2005, Mozilla joined ECMA International, and work started on theECMAScript for XML(E4X) standard. This led to Mozilla working jointly withMacromedia(later acquired byAdobe Systems), who were implementing E4X in their ActionScript 3 language, which was based on an ECMAScript 4 draft. The goal became standardizing ActionScript 3 as the new ECMAScript 4. To this end, Adobe Systems released theTamarinimplementation as anopen sourceproject. However, Tamarin and ActionScript 3 were too different from established client-side scripting, and without cooperation fromMicrosoft,ECMAScript 4 never reached fruition.

Meanwhile, very important developments were occurring in open-source communities not affiliated with ECMA work. In 2005,Jesse James Garrettreleased a white paper in which he coined the termAjaxand described a set of technologies, of which JavaScript was the backbone, to createweb applicationswhere data can be loaded in the background, avoiding the need for full page reloads. This sparked a renaissance period of JavaScript, spearheaded by open-source libraries and the communities that formed around them. Many new libraries were created, includingjQuery,Prototype,Dojo Toolkit,andMooTools.

Googledebuted itsChromebrowser in 2008, with theV8JavaScript engine that was faster than its competition.The key innovation wasjust-in-time compilation(JIT),so other browser vendors needed to overhaul their engines for JIT.

In July 2008, these disparate parties came together for a conference inOslo.This led to the eventual agreement in early 2009 to combine all relevant work and drive the language forward. The result was the ECMAScript 5 standard, released in December 2009.

Reaching maturity

Ambitious work on the language continued for several years, culminating in an extensive collection of additions and refinements being formalized with the publication ofECMAScript 6in 2015.

The creation ofNode.jsin 2009 byRyan Dahlsparked a significant increase in the usage of JavaScript outside of web browsers. Node combines theV8engine, anevent loop,andI/OAPIs,thereby providing a stand-alone JavaScript runtime system.As of 2018, Node had been used by millions of developers,andnpmhad the most modules of anypackage managerin the world.

The ECMAScript draft specification is currently maintained openly onGitHub,and editions are produced via regular annual snapshots.Potential revisions to the language are vetted through a comprehensive proposal process.Now, instead of edition numbers, developers check the status of upcoming features individually.

The current JavaScript ecosystem has manylibrariesandframeworks,established programming practices, and substantial usage of JavaScript outside of web browsers.Plus, with the rise ofsingle-page applicationsand other JavaScript-heavy websites, severaltranspilershave been created to aid the development process.

Trademark

"JavaScript" is atrademarkofOracle Corporationin the United States.The trademark was originally issued toSun Microsystemson 6 May 1997, and was transferred to Oracle when they acquired Sun in 2009.

Website client-side usage

JavaScript is the dominantclient-sidescripting languageof the Web, with 99% of allwebsitesusing it for this purpose.Scripts are embedded in or included fromHTMLdocuments and interact with theDOM.

All majorweb browsershave a built-inJavaScript enginethat executes thecodeon the user's device.

Examples of scripted behavior

• Loading newweb pagecontent without reloading the page, viaAjaxor aWebSocket.For example,usersofsocial mediacan send and receive messages without leaving the current page.
• Web page animations, such as fading objects in and out, resizing, and moving them.
• Playingbrowser games.
• Controlling theplaybackofstreaming media.
• Generatingpop-up adsor alert boxes.
• Validatinginput values of aweb formbefore the data is sent to aweb server.
• Logging data about the user's behavior then sending it to a server. The website owner can use this data foranalytics,ad tracking,andpersonalization.
• Redirecting a user to another page.
• Storing and retrieving data on the user's device, via thestorageorIndexedDBstandards.

Libraries and frameworks

Over 80% of websites use a third-party JavaScriptlibraryorweb frameworkas part of their client-side scripting.

jQueryis by far the most-used.Other notable ones includeAngular,Bootstrap,Lodash,Modernizr,React,Underscore,andVue.Multiple options can be used in conjunction, such as jQuery and Bootstrap.

However, the term "Vanilla JS" was coined for websites not using any libraries or frameworks at all, instead relying entirely on standard JavaScript functionality.

Other usage

The use of JavaScript has expanded beyond itsweb browserroots.JavaScript enginesare now embedded in a variety of other software systems, both forserver-sidewebsite deployments and non-browserapplications.

Initial attempts at promoting server-side JavaScript usage wereNetscape Enterprise ServerandMicrosoft'sInternet Information Services,but they were small niches.Server-side usage eventually started to grow in the late 2000s, with the creation ofNode.jsandother approaches.

Electron,Cordova,React Native,and otherapplication frameworkshave been used to create many applications with behavior implemented in JavaScript. Other non-browser applications includeAdobe Acrobatsupport for scriptingPDFdocumentsandGNOME Shellextensions written in JavaScript.

JavaScript has been used in someembedded systems,usually by leveraging Node.js.

Execution

JavaScript engine

This section is an excerpt fromJavaScript engine.[edit]

AJavaScript engineis asoftware componentthat executes JavaScriptcode.The first JavaScriptengineswere mereinterpreters,but all relevant modern engines usejust-in-time compilationfor improved performance.

JavaScript engines are typically developed byweb browservendors, and every major browser has one. In a browser, the JavaScript engine runs in concert with therendering enginevia theDocument Object ModelandWeb IDLbindings.However, the use of JavaScript engines is not limited to browsers; for example, theV8 engineis a core component of theNode.jsruntime system.

SinceECMAScriptis the standardized specification of JavaScript, ECMAScript engine is another name for theseimplementations.With the advent ofWebAssembly,some engines can also execute this code in the samesandboxas regular JavaScript code.

Runtime system

A JavaScript engine must be embedded within aruntime system(such as aweb browseror a standalone system) to enable scripts to interact with the broader environment. The runtime system includes the necessary APIs forinput/outputoperations, such asnetworking,storage,andgraphics,and provides the ability to import scripts.

JavaScript is a single-threadedlanguage. The runtime processesmessagesfrom aqueueone at a time, and it calls afunctionassociated with each new message, creating acall stackframe with the function'sargumentsandlocal 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 theevent loop,described as "run to completion" because each message is fully processed before the next message is considered. However, the language'sconcurrency modeldescribes the event loop asnon-blocking:program I/O is performed usingeventsandcallback functions.This means, for example, that JavaScript can process a mouse click while waiting for a database query to return information.

The notable standalone runtimes areNode.js,Deno,andBun.

Features

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

Imperative and structured

Main article:Structured programming

JavaScript supports much of thestructured programmingsyntax fromC(e.g.,ifstatements,whileloops,switchstatements,do whileloops, etc.). One partial exception isscoping:originally JavaScript only hadfunction scopingwithvar;block scopingwas added in ECMAScript 2015 with the keywordsletandconst.Like C, JavaScript makes a distinction betweenexpressionsandstatements.One syntactic difference from C isautomatic semicolon insertion,which allow semicolons (which terminate statements) to be omitted.

Weakly typed

Main article:Weakly typed

JavaScript isweakly typed,which means certain types are implicitly cast depending on the operation used.

• The binary+operator casts both operands to a string unless both operands are numbers. This is because the addition operator doubles as a concatenation operator
• The binary-operator always casts both operands to a number
• Both unary operators (+,-) always cast the operand to a number

Values are cast to strings like the following:

• Strings are left as-is
• Numbers are converted to their string representation
• Arrays have their elements cast to strings after which they are joined by commas (,)
• Other objects are converted to the string[object Object]whereObjectis the name of the constructor of the object

Values are cast to numbers by casting to strings and then casting the strings to numbers. These processes can be modified by definingtoStringandvalueOffunctions on theprototypefor string and number casting respectively.

JavaScript has received criticism for the way it implements these conversions as the complexity of the rules can be mistaken for inconsistency.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 type conversions

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

Often also mentioned is{} + []resulting in0(number). This is misleading: the{}is interpreted as an empty code block instead of an empty object, and the empty array is cast to a number by the remaining unary+operator. If the expression is wrapped in parentheses -({} + [])– the curly brackets are interpreted as an empty object and the result of the expression is"[object Object]"as expected.

Dynamic

Main article:Dynamic Programming

Typing

Main article:Dynamic typing

JavaScript isdynamically typedlike most otherscripting languages.Atypeis associated with avaluerather than an expression. For example, avariableinitially bound to a number may be reassigned to astring.JavaScript supports various ways to test the type of objects, includingduck typing.

Run-time evaluation

Main article:eval

JavaScript includes anevalfunction that can execute statements provided as strings at run-time.

Object-orientation (prototype-based)

Prototypal inheritance in JavaScript is described byDouglas Crockfordas:

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, anobjectis anassociative array,augmented with a prototype (see below); each key provides the name for an objectproperty,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. Mostpropertiesof an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using afor...inloop.

Prototypes

Main article:Prototype-based programming

JavaScript usesprototypeswhere many other object-oriented languages useclassesforinheritance.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 withnewwill create an instance of a prototype, inheriting properties and methods from the constructor (including properties from theObjectprototype).ECMAScript 5 offers theObject.createmethod, allowing explicit creation of an instance without automatically inheriting from theObjectprototype (older environments can assign the prototype tonull).The constructor'sprototypeproperty 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 asArrayorObject,also have prototypes that can be modified. While it is possible to modify theObjectprototype, it is generally considered bad practice because most objects in JavaScript will inherit methods and properties from theObjectprototype, and they may not expect the prototype to be modified.

Functions as methods

Main article:Method (computer science)

Unlike in many object-oriented languages, in JavaScript there is no distinction between a function definition and amethoddefinition. Rather, the distinction occurs during function calling. When a function is called as a method of an object, the function's localthiskeyword is bound to that object for that invocation.

Functional

Main article:Functional programming

JavaScriptfunctionsarefirst-class;a function is considered to be an object.As such, a function may have properties and methods, such as.call()and.bind().

Lexical closure

Main article:Closure (computer programming)

Anestedfunction is a function defined within another function. It is created each time the outer function is invoked.

In addition, each nested function forms alexical closure:thelexical scopeof 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.

Anonymous function

Main article:Anonymous function

JavaScript also supportsanonymous functions.

Delegative

Main article:Delegation (object-oriented programming)

JavaScript supports implicit and explicitdelegation.

Functions as roles (Traits and Mixins)

Main articles:Role-oriented programming,Traits (computer science),andMixin

JavaScript natively supports various function-based implementations ofRolepatterns likeTraitsandMixins.Such a function defines additional behavior by at least one method bound to thethiskeyword within itsfunctionbody. A Role then has to be delegated explicitly viacallorapplyto 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 covercompositionin 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. Thusinheritancein JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.

Miscellaneous

Zero-based numbering

JavaScript is azero-indexlanguage.

Variadic functions

Main article:Variadic function

An indefinite number of parameters can be passed to a function. The function can access them throughformal parametersand also through the localargumentsobject.Variadic functionscan also be created by using thebindmethod.

Array and object literals

Main articles:Associative arraysandObject literal

Like in many scripting languages, arrays and objects (associative arraysin other languages) can each be created with a succinct shortcut syntax. In fact, theseliteralsform the basis of theJSONdata format.

Regular expressions

Main article:Regular expression

In a manner similar toPerl,JavaScript also supportsregular expressions,which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.

Promises and Async/await

JavaScript supportspromisesandAsync/awaitfor handling asynchronous operations.

Promises

Main article:Futures and promises

A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. Recently, the JavaScript specification introduced combinator methods, which allow developers to combine multiple JavaScript promises and do operations based on different scenarios. The methods introduced are: Promise.race, Promise.all, Promise.allSettled and Promise.any.

Async/await

Main article:Async/await

Async/await allows an asynchronous, non-blocking function to be structured in a way similar to an ordinary synchronous function. Asynchronous, non-blocking code can be written, with minimal overhead, structured similarly to traditional synchronous, blocking code.

Vendor-specific extensions

Historically, someJavaScript enginessupported these non-standard features:

• conditionalcatchclauses (like Java)
• array comprehensionsand 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

Variablesin JavaScript can be defined using either thevar,letorconstkeywords. Variables defined without keywords will be defined at the global scope.

// Declares a function-scoped variable named `x`, and implicitly assigns the
// special value `undefined` to it. Variables without value are automatically
// set to undefined.
// var is generally considered bad practice and let and const are usually preferred.
varx;

// Variables can be manually set to `undefined` like so
letx2=undefined;

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

// Declares a block-scoped, un-reassignable variable named `z`, and sets it to
// a string literal. The `const` keyword was also introduced in ECMAScript 2015,
// and must be explicitly assigned to.

// The keyword `const` means constant, hence the variable cannot be reassigned
// as the value is `constant`.
constz="this value cannot be reassigned!";

// Declares a global-scoped variable and assigns 3. This is generally considered
// bad practice, and will not work if strict mode is on.
t=3;

// Declares a variable named `myNumber`, and assigns a number literal (the value
// `2`) to it.
letmyNumber=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 thecommentsin the examples above, all of which were preceded with twoforward slashes.

There is no built-inInput/outputfunctionality in JavaScript, instead it is provided by the run-time environment. The ECMAScript specification in edition 5.1 mentions that "there are no provisions in this specification for input of external data or output of computed results". However, most runtime environments have aconsoleobject that can be used to print output.Here is a minimalist"Hello, World!" programin JavaScript in a runtime environment with a console object:

console.log("Hello, World!");

In HTML documents, a program like this is required for an output:

// Text nodes can be made using the "write" method.
// This is frowned upon, as it can overwrite the document if the document is fully loaded.
document.write('foo');

// Elements can be made too. First, they have to be created in the DOM.
constmyElem=document.createElement('span');

// Attributes like classes and the id can be set as well
myElem.classList.add('foo');
myElem.id='bar';

// After setting this, the tag will look like this: `<span class= "foo" id= "bar" data-attr= "baz" ></span>`
myElem.setAttribute('data-attr','baz');// Which could also be written as `myElem.dataset.attr = 'baz'`

// Finally append it as a child element to the <body> in the HTML
document.body.appendChild(myElem);

// Elements can be imperatively grabbed with querySelector for one element, or querySelectorAll for multiple elements that can be looped with forEach
document.querySelector('.class');// Selects the first element with the "class" class
document.querySelector('#id');// Selects the first element with an `id` of "id"
document.querySelector('[data-other]');// Selects the first element with the "data-other" attribute
document.querySelectorAll('.multiple');// Returns an Array-like NodeList of all elements with the "multiple" class

A simplerecursivefunction to calculate thefactorialof anatural number:

functionfactorial(n){
// Checking the argument for legitimacy. Factorial is defined for positive integers.
if(isNaN(n)){
console.error("Non-numerical argument not allowed.");
returnNaN;// The special value: Not a Number
}
if(n===0)
return1;// 0! = 1
if(n<0)
returnundefined;// Factorial of negative numbers is not defined.
if(n%1){
console.warn(`${n}will be rounded to the closest integer. For non-integers consider using gamma function instead.`);
n=Math.round(n);
}
// The above checks need not be repeated in the recursion, hence defining the actual recursive part separately below.

// The following line is a function expression to recursively compute the factorial. It uses the arrow syntax introduced in ES6.
constrecursivelyCompute=a=>a>1?a*recursivelyCompute(a-1):1;// Note the use of the ternary operator `?`.
returnrecursivelyCompute(n);
}

factorial(3);// Returns 6

Ananonymous function(or lambda):

constcounter=function(){
letcount=0;
returnfunction(){
return++count;
}
};

constx=counter();
x();// Returns 1
x();// Returns 2
x();// Returns 3

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

Arrow functions were first introduced in6th Edition – ECMAScript 2015.They shorten the syntax for writing functions in JavaScript. Arrow functions are anonymous, so a variable is needed to refer to them in order to invoke them after their creation, unless surrounded by parenthesis and executed immediately.

Example of arrow function:

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

returnoutput;
};

// If there are no braces, the arrow function simply returns the expression
// So here it's (input1 + input2)
constshort_example=(input1,input2)=>input1+input2;

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

// If an arrow function has only one parameter, the parentheses can be removed.
constno_parentheses=input=>input+2;

no_parentheses(3);// Returns 5

// An arrow function, like other function definitions, can be executed in the same statement as they are created.
// This is useful when writing libraries to avoid filling the global scope, and for closures.
letthree=((a,b)=>a+b)(1,2);

constgenerate_multiplier_function=a=>(b=>isNaN(b)||!b?a:a*=b);
constfive_multiples=generate_multiplier_function(5);// The supplied argument "seeds" the expression and is retained by a.
five_multiples(1);// Returns 5
five_multiples(3);// Returns 15
five_multiples(4);// Returns 60

In JavaScript,objectscan be created asinstancesof aclass.

Object class example:

classBall{

constructor(radius){
this.radius=radius;
this.area=Math.PI*(radius**2);
}

// Classes (and thus objects) can contain functions known as methods
show(){
console.log(this.radius);
}
};

constmyBall=newBall(5);// Creates a new instance of the ball object with radius 5
myBall.radius++;// Object properties can usually be modified from the outside
myBall.show();// Using the inherited "show" function logs "6"

In JavaScript,objectscan be instantiated directly from a function.

Object functional example:

functionBall(radius){

constarea=Math.PI*(radius**2);
constobj={radius,area};

// Objects are mutable, and functions can be added as properties.
obj.show=()=>console.log(obj.radius);
returnobj;
};

constmyBall=Ball(5);// Creates a new ball object with radius 5. No "new" keyword needed.
myBall.radius++;// The instance property can be modified.
myBall.show();// Using the "show" function logs "6" - the new instance value.

Variadic functiondemonstration (argumentsis a specialvariable):

functionsum(){
letx=0;

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

returnx;
}

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

// As of ES6, using the rest operator.
functionsum(...args){
returnargs.reduce((a,b)=>a+b);
}

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

Immediately-invoked function expressionsare often used to create closures. Closures allow gathering properties and methods in a namespace and making some of them private:

letcounter=(function(){
leti=0;// Private property

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

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

Generatorobjects (in the form of generator functions) provide a function which can be called, exited, and re-entered while maintaining internal context (statefulness).

function*rawCounter(){
yield1;
yield2;
}

function*dynamicCounter(){
letcount=0;
while(true){
// It is not recommended to utilize while true loops in most cases.
yield++count;
}
}

// Instances
constcounter1=rawCounter();
constcounter2=dynamicCounter();

// Implementation
counter1.next();// {value: 1, done: false}
counter1.next();// {value: 2, done: false}
counter1.next();// {value: undefined, done: true}

counter2.next();// {value: 1, done: false}
counter2.next();// {value: 2, done: false}
counter2.next();// {value: 3, done: false}
//...infinitely

JavaScript can export and import from modules:

Export example:

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

// Export variables
exportletname='Alice';
exportletage=23;

// Export named functions
exportfunctionadd(num1,num2){
returnnum1+num2;
}

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

add(){
returnsum(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 */
functionLCMCalculator(x,y){// constructor function
if(isNaN(x*y))thrownewTypeError("Non-numeric arguments not allowed.");
constcheckInt=function(x){// inner function
if(x%1!==0)
thrownewTypeError(x+"is not an integer");

returnx;
};

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:
leta=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" or "b" are altered afterwards.)
// Also, 'gcd' === "gcd", this['gcd'] === this.gcd
this['gcd']=function(){
returna;
};

returna;
},

// 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
letlcm=this.a/this.gcd()*this.b;

// Only need to calculate lcm once, so "redefine" this method.
this.lcm=function(){
returnlcm;
};

returnlcm;
},

// Methods can also be declared using ES6 syntax
toString(){
// Using both ES6 template literals and the (+) operator to concatenate values
return`LCMCalculator: a =${this.a},b = `+this.b;
}
};

// Define generic output function; this implementation only works for Web browsers
functionoutput(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
returnnewLCMCalculator(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);

functionprintResult(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 theDOMprovide 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 asandboxin which they can only perform Web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by thesame-origin policy:scripts from one website 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).Closure Toolkit is another project for safe embedding and isolation of third-party JavaScript and HTML.

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

Cross-site scripting

Main article:Cross-site scripting

A common JavaScript-related security problem iscross-site scripting(XSS), a violation of thesame-origin policy.XSS vulnerabilities occur when an attacker can cause a target Website, 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. One important solution to XSS vulnerabilities isHTML sanitization.

Some browsers include partial protection againstreflectedXSS 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.

Cross-site request forgery

Main article:Cross-site request forgery

Another cross-site vulnerability iscross-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 asJSONor JavaScript. Possible solutions include:

• requiring an authentication token in thePOSTandGETparameters 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:

• Website authors cannot perfectly conceal how their JavaScript operates because the raw source code must be sent to the client. The code can beobfuscated,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.
• Prototype pollutionis a runtime vulnerability in which attackers can overwrite arbitrary properties in an object's prototype.

Misplaced trust in developers

Package management systems such asnpmand Bower are popular with JavaScript developers. Such systems allow a developer to easily manage their program's dependencies upon other developers' 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 133,000 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 tens of thousands of programs and websites depending upon his libraries to break.

Browser and plugin coding errors

Further information:Buffer overflow

JavaScript provides an interface to a wide range of browser capabilities, some of which may have flaws such asbuffer 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'sAPIwith 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 ofActiveXcontrols 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 Chromesimilarly 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 Explorerand Firefox.In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.

Microsoft Windowsallows 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 (likeVBScript) a theoretically viable vector for aTrojan horse,although JavaScript Trojan horses are uncommon in practice.

Hardware vulnerabilities

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

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

In 2018, the paper that announced theSpectreattacks against Speculative Execution in Intel and other processors included a JavaScript implementation.

Development tools

Important tools have evolved with the language.

• Every major web browser has built-inweb development tools,including a JavaScriptdebugger.
• Static program analysistools, such asESLintandJSLint,scan JavaScript code for conformance to a set of standards and guidelines.
• Some browsers have built-inprofilers.Stand-alone profiling libraries have also been created, such as benchmark.js and jsbench.
• Manytext editorshave syntax highlighting support for JavaScript code.

Related technologies

Java

A common misconception is that JavaScript is directly related toJava.Both indeed have a C-like syntax (the C language being their most immediate common ancestor language). They are also typicallysandboxed,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'sMathandDateobjects are based on classes from Java 1.0.

Both languages first appeared in 1995, but Java was developed byJames Goslingof Sun Microsystems and JavaScript byBrendan Eichof Netscape Communications.

The differences between the two languages are more prominent than their similarities. Java hasstatic typing,while JavaScript's typing isdynamic.Java is loaded fromcompiledbytecode, while JavaScript is loaded as human-readable source code. Java's objects areclass-based,while JavaScript's areprototype-based.Finally, Java did not support functional programming until Java 8, while JavaScript has done so from the beginning, being influenced byScheme.

JSON

JSONis a data format derived from JavaScript; hence the name JavaScript Object Notation. It is a widely used format supported by many other programming languages.

Transpilers

Many websites are JavaScript-heavy, sotranspilershave been created to convert code written in other languages, which can aid the development process.

TypeScriptandCoffeeScriptare two notable languages that transpile to JavaScript.

WebAssembly

WebAssemblyis a newer language with abytecodeformat designed to complement JavaScript, especially the performance-critical portions ofweb pagescripts. All of the majorJavaScript enginessupport WebAssembly,which runs in the samesandboxas regular JavaScript code.

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

Sources

• Dere, Mohan (21 December 2017)."How to integrate create-react-app with all the libraries you need to make a great app".freeCodeCamp.Retrieved14 June2018.
• Panchal, Krunal (26 April 2022)."Angular vs React Detailed Comparison".Groovy Web.Retrieved5 June2023.

Further reading

See also:ECMAScript Specification Documents

• Flanagan, David.JavaScript: The Definitive Guide.7th edition. Sebastopol, California: O'Reilly, 2020.ISBN978-1-491-95202-3.
• Haverbeke, Marijn.Eloquent JavaScript.3rd edition. No Starch Press, 2018. 472 pages.ISBN978-1593279509.(download)
• Zakas, Nicholas.Principles of Object-Oriented JavaScript,1st edition. No Starch Press, 2014. 120 pages.ISBN978-1593275402.

External links

• The Modern JavaScript Tutorial.A community maintained continuously updated collection of tutorials on the entirety of the language.
• "JavaScript: The First 20 Years".Retrieved6 February2022.