History
Creation at Netscape
The first popularAdoption by Microsoft
The rise of JScript
In November 1996, Netscape submitted JavaScript toGrowth 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 of ECMAScript 6 in 2015. The creation of Node.js in 2009 by Ryan Dahl sparked a significant increase in the usage of JavaScript outside of web browsers. Node combines the V8 engine, an event loop, and I/O APIs, thereby providing a stand-alone JavaScript runtime system.Sams Teach Yourself Node.js in 24 HoursTrademark
"JavaScript" is a trademark of Oracle Corporation in the United States. The trademark was originally issued toWebsite client-side usage
JavaScript is the dominant client-sideExamples of scripted behavior
* Loading newLibraries and frameworks
Over 80% of websites use a third-party JavaScriptOther usage
The use of JavaScript has expanded beyond itsExecution
JavaScript engine
Runtime system
A JavaScript engine must be embedded within a runtime system (such as aFeatures
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: originally JavaScript only had function scoping with var; block scoping was added in ECMAScript 2015 with the keywords let and const. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, which allow semicolons (which terminate statements) to be omitted.
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. However, + always casts to Number ( binary64) while - preserves BigInt (,)
* 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, design, electronics, and Software prototyping, software programming. A prototype ...
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 + 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
Typing
JavaScript is dynamically typed like most other scripting language
In computing, a script is a relatively short and simple set of instructions that typically automation, automate an otherwise manual process. The act of writing a script is called scripting. A scripting language or script language is a programming ...
s. 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:
In JavaScript, an object is an associative array, 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 consume, alter, share, re ...
, 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.
Prototypes
JavaScript uses prototypes where many other object-oriented languages use classes for inheritance
Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Offi ...
, but it's still possible to simulate most class-based features with the prototype system. Additionally, ECMAScript version 6 (released June 2015) introduced the keywords class, extends and super, which serve as syntactic sugar to abstract the underlying prototypal inheritance system with a more conventional interface. Constructors are declared by specifying a method named constructor, and all classes are automatically subclasses of the base class Object, similarly to Java.
class Person
class Student extends Person
const bob = new Student("Robert", 12345);
console.log(bob.name); // Robert
Though the underlying object mechanism is still based on prototypes, the newer syntax is similar to other object oriented languages. Private variables are declared by prefixing the field name with a number sign
The symbol is known as the number sign, hash, (or in North America) the pound sign. The symbol has historically been used for a wide range of purposes including the designation of an ordinal number and as a Typographic ligature, ligatured abbre ...
(#), and polymorphism is not directly supported, although it can be emulated by manually calling different functions depending on the number and type of arguments provided.
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.// This code is completely equivalent to the previous snippet
function Person(name)
function Student(name, id)
var bob = new Student("Robert", 12345);
console.log(bob.name); // Robert JavaScript's built-in classes, such as Array and Object, also have prototypes that can be modified. However, it's generally considered bad practice to modify built-in objects, because third-party code may use or inherit methods and properties from these objects, and may not expect the prototype to be modified.
Functions as methods
Unlike in many object-oriented languages, in JavaScript 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
JavaScript functions are first-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
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.
Anonymous function
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
Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Offi ...
in JavaScript is covered by a delegation automatism that is bound to the prototype property of constructor functions.
Miscellaneous
Zero-based numbering
JavaScript is a zero-index language.
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 in many scripting languages, arrays and objects (associative arrays
In computer science, an associative array, key-value store, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In math ...
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 or ) is an open standard file format and electronic data interchange, data interchange format that uses Human-readable medium and data, human-readable text to store and transmit data objects consi ...
data format.
Regular expressions
JavaScript supports regular expressions for text searches and manipulation.
= Promises
=
A built-in Promise object provides functionality for handling promises and associating handlers with an asynchronous action's eventual result. JavaScript supplies 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
=
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, some JavaScript engines supported these non-standard features:
* 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
Variables in JavaScript can be defined using either the var, let or const keywords. Variables defined without keywords will be defined at the global scope.
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, unless surrounded by parenthesis and executed immediately.
Here is an example of JavaScript syntax.
// 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.
var x;
// Variables can be manually set to `undefined` like so
let 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 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.
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 examples above, all of which were preceded with two forward slashes.
More examples can be found at the Wikibooks page on JavaScript syntax examples.
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 wide, shallow playground construction to hold sand, often made of wood or plastic.
Sandbox or sand box may also refer to:
Arts, entertainment, and media
* Sandbox (band), a Canadian rock music group
* Sandbox (Gu ...
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 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 Policy is the main intended method of ensuring that only trusted code is executed on a Web page.
Cross-site scripting
A common JavaScript-related security problem is cross-site scripting (XSS), a violation of the same-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 is HTML sanitization.
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.
Cross-site request forgery
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 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 or ) is an open standard file format and electronic data interchange, data interchange format that uses Human-readable medium and data, human-readable text to store and transmit data objects consi ...
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 author of an application should not 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 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.
* Prototype pollution is a runtime vulnerability in which attackers can overwrite arbitrary properties in an object's prototype.
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 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
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
A sandbox is a sandpit, a wide, shallow playground construction to hold sand, often made of wood or plastic.
Sandbox or sand box may also refer to:
Arts, entertainment, and media
* Sandbox (band), a Canadian rock music group
* Sandbox (Gu ...
".
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
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
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 is called "ASLR⊕Cache" or AnC.
In 2018, the paper that announced the Spectre attacks 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-in web development tools, including a JavaScript debugger.
* Static program analysis tools, such as ESLint and JSLint, scan JavaScript code for conformance to a set of standards and guidelines.
* Some browsers have built-in profilers. Stand-alone profiling libraries have also been created, such as benchmark.js and jsbench.
* Many text editors have syntax highlighting support for JavaScript code.
Related technologies
Java
A common misconception is that JavaScript is directly related to Java
Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
. Both indeed have a C-like syntax (the C language being their most immediate common ancestor language). They are also typically sandboxed, 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.
Both languages 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
JSON (JavaScript Object Notation, pronounced or ) is an open standard file format and electronic data interchange, data interchange format that uses Human-readable medium and data, human-readable text to store and transmit data objects consi ...
is 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, so transpilers have been created to convert code written in other languages, which can aid the development process.
TypeScript and CoffeeScript are two notable languages that transpile to JavaScript.
WebAssembly
WebAssembly is a newer language with a bytecode format designed to complement JavaScript, especially the performance-critical portions of web page
A web page (or webpage) is a World Wide Web, Web document that is accessed in a web browser. A website typically consists of many web pages hyperlink, linked together under a common domain name. The term "web page" is therefore a metaphor of pap ...
scripts. All of the major JavaScript engines support WebAssembly, which runs in the same sandbox
A sandbox is a sandpit, a wide, shallow playground construction to hold sand, often made of wood or plastic.
Sandbox or sand box may also refer to:
Arts, entertainment, and media
* Sandbox (band), a Canadian rock music group
* Sandbox (Gu ...
as regular JavaScript code.
asm.js is a subset of JavaScript that served as the forerunner of WebAssembly.
References
Further reading
*
*
*
External links
The Modern JavaScript Tutorial
A community maintained continuously updated collection of tutorials on the entirety of the language.
*
{{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
Programming languages