History
Creation at Netscape
The firstAdoption by Microsoft
The rise of JScript
In November 1996,Growth and standardization
During the period ofReaching maturity
Ambitious work on the language continued for several years, culminating in an extensive collection of additions and refinements being formalized with the publication ofTrademark
"JavaScript" is aWebsite client-side usage
JavaScript is the dominantExamples of scripted behavior
* Loading newLibraries and frameworks
Over 80% of websites use a third-party JavaScriptOther usage
The use of JavaScript has expanded beyond itsFeatures
The following features are common to all conforming ECMAScript implementations unless explicitly specified otherwise.Imperative and structured
JavaScript supports much of theif
statements, while
loops, switch
statements, do while
loops, etc.). One partial exception is scoping: originally JavaScript only had function scoping with var
; then block scoping was added in ECMAScript 2015 with the keywords let
and const
In the C, C++, D, JavaScript
JavaScript (), often abbreviated as JS, is a programming language
A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Inpu ...
. Like C, JavaScript makes a distinction between Weakly typed
JavaScript is weakly 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 bject Object/code> where Object
is 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 defining toString
and valueOf
functions on the prototype
A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics
Semantics (from grc, σημαντικός ''sēmantikós'', "significant") ...
for 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.
Often also mentioned is + []
resulting in 0
(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 you wrap the expression in parentheses ( + [])
the curly brackets are interpreted as an empty object and the result of the expression is "[object Object]"
as expected.
Dynamic
; Typing:JavaScript is dynamically typed
In programming language
A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming to i ...
like most other scripting language
A scripting language or script language is a programming language
A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming language ...
s. A type
Type may refer to:
Science and technology Computing
* Typing, producing text via a keyboard, typewriter, etc.
* Data type, collection of values used for computations.
* File type
* TYPE (DOS command), a command to display contents of a file.
* Type ...
is associated with a value
Value or values may refer to:
* Value (ethics) it may be described as treating actions themselves as abstract objects, putting value to them
** Values (Western philosophy) expands the notion of value beyond that of ethics, but limited to Western s ...
rather than an expression. For example, a variable initially bound to a number may be reassigned to a string
String or strings may refer to:
*String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects
Arts, entertainment, and media Films
* Strings (1991 film), ''Strings'' (1991 fil ...
. JavaScript supports various ways to test the type of objects, including duck typing
Duck typing in computer programming is an application of the duck test—"If it walks like a duck and it quacks like a duck, then it must be a duck"—to determine whether an object (computer science), object can be used for a particular pu ...
.
; Run-time evaluation: JavaScript includes an eval
In some programming language
A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming ...

function that can execute statements provided as strings at run-time.
Object-orientation (prototype-based)
Prototypal inheritance in JavaScript is described by Douglas Crockford
Douglas Crockford is an United States, American computer programmer and entrepreneur who is involved in the development of the JavaScript language. He popularized the data format JSON (JavaScript Object Notation), and has developed various JavaScr ...
as:
In JavaScript, an object
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Entity, something that is tangible and within the grasp of the senses
** Object (abstract), an object which does not exist at any particular time or pl ...
is an associative array
In computer science
Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application.
Computer science is the study of Algo ...
, augmented with a prototype (see below); each key provides the name for an object property
Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of the property, an owner of property may have the right to , alter, , , , , , , , or ...
, and there are two syntactical ways to specify such a name: dot notation (obj.x = 10
) and bracket notation (obj x'nbsp;= 10
). A property may be added, rebound, or deleted at run-time. Most properties
Property (''latin: Res Privata'') in the abstract is what belongs to or with something, whether as an attribute or as a component of said thing. In the context of this article, it is one or more components (rather than attributes), whether phys ...
of an object (and any property that belongs to an object's prototype inheritance chain) can be enumerated using a for...in
loop.
; Prototypes: JavaScript uses prototypes
A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototype ...
where many other object-oriented languages use classes
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differently f ...
for inheritance
Inheritance is the practice of passing on private property
Private property is a legal designation for the ownership of property by non-governmental legal entities. Private property is distinguishable from public property
Public property i ...
. 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
Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to:
*Scient ...
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
Function or functionality may refer to:
Computing
* Function key
A function key is a key on a computer
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
is first-class
First class (or 1st class, Firstclass) generally implies a high level of service, importance or quality. Specific uses of the term include:
Books and Comics
* ''First Class'', List of Dandy comic strips, a comic strip in ''The Dandy'' (1983-1998 ...
; 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 scopeIn computer programming
Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: analysis, generatin ...
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 function
Anonymous may refer to:
* Anonymity
Anonymity describes situations where the acting person's identity is unknown. Some writers have argued that namelessness, though technically correct, does not capture what is more centrally at stake in context ...
s.
Delegative
JavaScript supports implicit and explicit delegation
Schermerhorn, J., Davidson, P., Poole, D., Woods, P., Simon, A., & McBarron, E. (2017). ''Management'' (6th ed., pp. 282–286). Brisbane: John Wiley & Sons Australia. Delegation is one of the core concepts of management
Management (or managing ...
.
; Functions as roles (Traits and Mixins): JavaScript natively supports various function-based implementations of Role
A role (also rôle or social role) is a set of connected behaviors, rights, moral obligation, obligations, beliefs, and social norm, norms as conceptualized by people in a social situation. It is an expected or free or continuously changing behav ...
patterns like Traits
Trait may refer to:
* Phenotypic trait in biology, which involve genes and characteristics of organisms
* Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language)
* Trait the ...
and Mixin
In object-oriented programming languages, a mixin (or mix-in) is a class that contains methods for use by other classes without having to be the parent class of those other classes. How those other classes gain access to the mixin's methods depen ...
s. 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
Composition or Compositions may refer to:
Arts
* Composition (dance), practice and teaching of choreography
* Composition (music), an original piece of music and its creation
*Composition (visual arts)
The term composition means "putting togethe ...
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
Inheritance is the practice of passing on private property
Private property is a legal designation for the ownership of property by non-governmental legal entities. Private property is distinguishable from public property
Public property i ...
in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.
Miscellaneous
JS is a zero-index language.
; Run-time environment:JavaScript typically relies on a run-time environment (e.g., a web browser
A web browser (commonly referred to as a browser) is application software
Application software (app for short) is computing software designed to carry out a specific task other than one relating to the operation of the computer itself, typical ...
) to provide objects and methods by which scripts can interact with the environment (e.g., a web page DOM). These environments are single- threaded. JavaScript also relies on the run-time environment to provide the ability to include/import scripts (e.g., HTML
The HyperText Markup Language, or HTML is the standard markup language #REDIRECT Markup language
In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntacticall ...
elements). This is not a language feature per se, but it is common in most JavaScript implementations. JavaScript processes messages
A message is a discrete unit of communication
Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Organization, groups through the use of suffi ...
from a queue__NOTOC__
Queue () may refer to:
* Queue area, or queue, a line or area where people wait for goods or services
Arts, entertainment, and media
*''ACM Queue'', a computer magazine
* The Queue (publishing company), an American publishing company
* ...
one at a time. JavaScript calls a function
Function or functionality may refer to:
Computing
* Function key
A function key is a key on a computer
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
associated with each new message, creating a call stack
In computer science
Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application.
Computer science is the study of Algor ...
frame with the function's arguments
In logic
Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, label=none, lit=possessed of reason, intellectual, dialectical, argumentative, translit=logikḗ)Also related to (''logos''), "word, thought, idea, argument, ...
and local variable
In computer science, a local variable is a Variable (programming), variable that is given ''local scope (programming), scope''. Local variable references in the subroutine, function or block (programming), block in which it is declared override the ...
s. 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
Event may refer to:
Gatherings of people
* Ceremony
A ceremony (, ) is a unified ritual
A ritual is a sequence of activities involving gestures, words, actions, or objects, performed in a sequestered place and according to a set sequence. Rit ...
, 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
In computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm
of an algorithm (Euclid's algorithm) for calculating the greatest commo ...
is performed using events
Event may refer to:
Gatherings of people
* Ceremony, an event of ritual significance, performed on a special occasion
* Convention (meeting), a gathering of individuals engaged in some common interest
* Event management, the organization of event ...
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
/code> 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
JSON (JavaScript Object Notation, pronounced ; also ) is an open standard
An open standard is a standard
Standard may refer to:
Flags
* Colours, standards and guidons
* Standard (flag), a type of flag used for personal identification
Norm ...
data format.
; Regular expressions: JavaScript also supports regular expression
A regular expression (shortened as regex or regexp; also referred to as rational expression) is a sequence of that specifies a ''search ''. Usually such patterns are used by s for "find" or "find and replace" operations on , or for input val ...
s in a manner similar to Perl
Perl is a family of two high-level
High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for ...
, 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 supports promises and Async/await for handling asynchronous operations. A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. Recently, combinator methods were introduced in the JavaScript specification, which allows 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 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 similar to traditional synchronous, blocking code.
Vendor-specific extensions
Historically, some JavaScript engine
A JavaScript engine is a computer program
A computer program is a collection of instructions that can be executed by a computer to perform a specific task.
A computer program is usually written by a computer programmer in a programming lang ...
s 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 XMLECMAScript for XML (E4X) is the standard ISO/IEC 22537:200programming language
A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programm ...
(E4X), an extension that adds native XML support to ECMAScript (unsupported in Firefox since version 21)
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. Variables without value are automatically
// set to undefined.
var x;
// Variables can be manually set to `undefined` like so
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;
// 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`.
const z = "this value cannot be reassigned!";
// 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
In computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm
of an algorithm (Euclid's algorithm) for calculating the greatest commo ...
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
''Hello'' is a salutation or greeting habits, greeting in the English language. It is first attested in writing from 1826.
Early uses
''Hello'', with that spelling, was used in publications in the US as early as the 18 October 1826 edition of ...
in JavaScript:
console.log("Hello World!");
A simple recursive
Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics
Linguistics is the science, scientific study of language. It e ...
function:
function factorial(n)
factorial(3); // returns 6
An anonymous function
Anonymous may refer to:
* Anonymity
Anonymity describes situations where the acting person's identity is unknown. Some writers have argued that namelessness, though technically correct, does not capture what is more centrally at stake in context ...
(or lambda):
function counter()
let x = counter();
x(); // returns 1
x(); // returns 2
x(); // 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, so 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) => ;
// If there are no braces, the arrow function simply returns the expression
// So here it's (input1 + input2)
const short_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 only has one parameter, the parentheses can be removed.
const no_parentheses = input => input + 2;
no_parentheses(3); // Returns 5
In JavaScript, objects are created in the same way as functions; this is known as a function object
In computer programming
Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: analysis, generat ...
.
Object example:
function Ball(r)
let myBall = new Ball(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
Variadic function
In mathematics
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
demonstration (arguments
is a special variable):
function sum()
sum(1, 2); // returns 3
sum(1, 2, 3); // returns 6
Immediately-invoked function expression
An immediately invoked function expression (or IIFE, pronounced "iffy", International Phonetic Alphabet, IPA /ˈɪf.i/) is a Programming idiom, programming language idiom which produces a scope (computer science), lexical scope using function scopin ...
s are often used to create closures. Closures allow gathering properties and methods in a namespace and making some of them private:
let counter = (function() )(); // 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) =>
// Export variables
export let name = 'Alice';
export let age = 23;
// Export named functions
export function add(num1, num2)
// Export class
export class Multiplication
Import example:
// Import one property
import from './mymodule.js';
console.log(add(1, 2)); // 3
// Import multiple properties
import 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)
// The prototype of object instances created by a constructor is
// that constructor's "prototype" property.
LCMCalculator.prototype = ;
// Define generic output function; this implementation only works for Web browsers
function output(x)
// 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.html"_;"title="5,_55.html"_;"title="____[25,_55">____[25,_55
____[21,_56">5,_55.html"_;"title="____[25,_55">____[25,_55
____[21,_56
____[22,_58.html" ;"title="5,_55">____[25,_55
____[21,_56.html" ;"title="5,_55.html" ;"title=" [25, 55"> [25, 55
[21, 56">5,_55.html" ;"title=" [25, 55"> [25, 55
[21, 56
[22, 58">5,_55">____[25,_55
____[21,_56.html" ;"title="5,_55.html" ;"title=" [25, 55"> [25, 55
[21, 56">5,_55.html" ;"title=" [25, 55"> [25, 55
[21, 56
[22, 58
[28, 56]
].map(function(pair) ).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)
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
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
A sandbox is a sandpit
A sandpit (most Commonwealth countries) or sandbox (US and Canada) is a low, wide container or shallow depression filled with soft (beach) sand
Sand is a granular material composed of finely divided rock ...
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 policyIn computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. ...
: 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). Closure Toolkit is another project for safe embedding and isolation of third-party JavaScript and HTML.
Content Security Policy
Content Security Policy (CSP) is a computer security standard introduced to prevent cross-site scripting (XSS), clickjacking and other code injection attacks resulting from execution of malicious content in the trusted web page context. It is a ...
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
Cross-site scripting (XSS) is a type of security vulnerability
Vulnerability refers to the inability (of a system or a unit) to withstand the effects of a hostile environment. A window of vulnerability (WOV) is a time frame within which defensi ...
(XSS), a violation of the same-origin policyIn computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. ...
. XSS vulnerabilities occur when an attacker can 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
Cross-site request forgery, also known as one-click attack or session riding and abbreviated as CSRF (sometimes pronounced ''sea-surf'') or XSRF, is a type of malicious exploit of a website
A website (also written as web site) is a collection ...
(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
tag on an attacker's site exploits a page on the victim's site that returns private information such as JSON
JSON (JavaScript Object Notation, pronounced ; also ) is an open standard
An open standard is a standard
Standard may refer to:
Flags
* Colours, standards and guidons
* Standard (flag), a type of flag used for personal identification
Norm ...
or JavaScript. Possible solutions include:
* requiring an authentication token in the POST
Post or POST commonly refers to:
*Mail
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since ...
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 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 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 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 overflow
In information security
Information security, sometimes shortened to infosec, is the practice of protecting information by mitigating information risks. It is part of Risk management information systems, information risk management. It typically i ...
s. 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
In computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
with superuser privileges.
These flaws have affected major browsers including Firefox, Internet Explorer, and Safari.
Plugins, such as video players, Adobe Flash
Adobe Flash is a multimedia software platform
A computing platform or digital platform is the environment in which a piece of software
Software is a collection of Instruction (computer science), instructions and data (computing), data that ...
, and the wide range of ActiveX
ActiveX is a deprecated software framework created by Microsoft
Microsoft Corporation is an American multinational corporation, multinational technology company with headquarters in Redmond, Washington. It develops, manufactures, licenses, s ...
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
Google Chrome is a cross-platform
In computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both ...
similarly confines its page renderers to their own "sandbox
A sandbox is a sandpit
A sandpit (most Commonwealth countries) or sandbox (US and Canada) is a low, wide container or shallow depression filled with soft (beach) sand
Sand is a granular material composed of finely divided rock ...
".
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
Microsoft Windows, commonly referred to as Windows, is a group of several proprietary
{{Short pages monitor Both indeed have a C-like syntax (the C language being their most immediate common ancestor language). They are also 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 and objects are based on classes from Java 1.0.
Java
Java ( id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 147.7 million people, Java is the world's List of ...
and JavaScript both first appeared in 1995, but Java was developed by James Gosling
James Arthur Gosling, often referred to as "Dr. Java," Order of Canada, OC (born May 19, 1955) is a Canadian computer scientist, best known as the founder and lead designer behind the Java (programming language), Java programming language.
Gosli ...
of Sun Microsystems, and JavaScript by Brendan Eich
Brendan Eich (; born July 4, 1961) is an American technologist and creator of the JavaScript
JavaScript (), often abbreviated JS, is a programming language
A programming language is a formal language comprising a Instruction set archit ...
of Netscape Communications.
The differences between the two languages are more prominent than their similarities. Java has static typing
In programming language
A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming to ...
, while JavaScript's typing is dynamic
Dynamics (from Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece
Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million ...
. 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
JSON (JavaScript Object Notation, pronounced ; also ) is an open standard
An open standard is a standard
Standard may refer to:
Flags
* Colours, standards and guidons
* Standard (flag), a type of flag used for personal identification
Norm ...
, 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
WebAssembly (sometimes abbreviated Wasm) defines a portable
Portable may refer to:
General
* Portable building, a manufactured structure that is built off site and moved in upon completion of site and utility work
* Portable classroom, a tempora ...
, a binary format that enables a JavaScript engine
A JavaScript engine is a computer program
A computer program is a collection of instructions that can be executed by a computer to perform a specific task.
A computer program is usually written by a computer programmer in a programming lang ...
to execute performance-critical portions of web page
A web page (or webpage) is a hypertext
Hypertext is text displayed on a or other with references () to other text that the reader can immediately access. Hypertext documents are interconnected by hyperlinks, which are typically acti ...
scripts close to native speed. WebAssembly code runs in the same sandbox
A sandbox is a sandpit
A sandpit (most Commonwealth countries) or sandbox (US and Canada) is a low, wide container or shallow depression filled with soft (beach) sand
Sand is a granular material composed of finely divided rock ...
as 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 transpiler
A source-to-source translator, source-to-source compiler (S2S compiler), transcompiler, or transpiler is a type of translator (computing), translator that takes the source code of a program written in a programming language as its input and prod ...
s have been created to convert code written in other languages, which can aid the development process.
References
Further reading
* Flanagan, David. ''JavaScript: The Definitive Guide''. 7th edition. Sebastopol, California: O'Reilly, 2020.
* Haverbeke, Marijn. ''Eloquent JavaScript''. 3rd edition. No Starch Press, 2018. 472 pages. .''(download)''
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* Zakas, Nicholas. ''Principles of Object-Oriented JavaScript'', 1st edition. No Starch Press, 2014. 120 pages. .
External links
*
* Douglas Crockford
Douglas Crockford is an United States, American computer programmer and entrepreneur who is involved in the development of the JavaScript language. He popularized the data format JSON (JavaScript Object Notation), and has developed various JavaScr ...
'
video lectures on JavaScript
{{Authority control
American inventions
Articles with example JavaScript code
Cross-platform software
Dynamically typed programming languages
Functional languages
Object-based programming languages
High-level programming languages
Programming languages created in 1995
Programming languages with an ISO standard
Prototype-based programming languages
Scripting languages
Web programming