JavaScript is a prototype-based
scripting languageA scripting language, script language, or extension language is a programming language that allows control of one or more applications. "Scripts" are distinct from the core code of the application, as they are usually written in a different language and are often created or at least modified by the...
that is dynamic,
weakly typedIn computer science, weak typing is a property attributed to the type systems of some programming languages. It is the opposite of strong typing, and consequently the term weak typing has as many different meanings as strong typing does.One of the more common definitions states that weakly typed...
and has first-class functions. It is a multi-paradigm language, supporting
object-orientedObject-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
,
imperativeIn computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
, and
functionalIn computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
programming styles.
JavaScript was formalized in the
ECMAScriptECMAScript is the scripting language standardized by Ecma International in the ECMA-262 specification and ISO/IEC 16262. The language is widely used for client-side scripting on the web, in the form of several well-known dialects such as JavaScript, JScript, and ActionScript.- History :JavaScript...
language standard and is primarily used in the form of
client-side JavaScriptClient-side JavaScript is JavaScript that runs on the client-side. While JavaScript was originally created to run this way, the term was coined because the language is no longer limited to just client-side, since server-side JavaScript is now available.-Environment:The most common Internet media...
, implemented as part of a
Web browserA web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
in order to provide enhanced
user interfaceThe user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the...
s and dynamic
websiteA website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...
s. This enables
programmaticComputer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
access to computational objects within a host environment.
JavaScript's use in
applicationsApplication software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
outside Web pages — for example in
PDFPortable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....
documents,
site-specific browserA site-specific browser is a software application that is dedicated to accessing pages from a single source on a computer network such as the Internet or a private intranet. SSBs typically simplify the more complex functions of a web browser by excluding the menus, toolbars and browser chrome...
s, and desktop widgets — is also significant. Newer and faster JavaScript
VMsA virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
and frameworks built upon them (notably
Node.jsNode.js is a software system designed for writing highly-scalable internet applications, notably web servers.Programs are written in JavaScript, using event-driven, asynchronous I/O to minimize overhead and maximize scalability....
) have also increased the popularity of JavaScript for server-side web applications.
JavaScript uses syntax influenced by that of
CC is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
. JavaScript copies many names and naming conventions from
JavaJava is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, but the two languages are otherwise unrelated and have very different semantics. The key design principles within JavaScript are taken from the Self and Scheme programming languages.
History
JavaScript was originally developed by
Brendan EichBrendan Eich is a computer programmer and creator of the JavaScript scripting language. He is the chief technology officer at the Mozilla Corporation.-Education:...
of Netscape under the name
Mocha, which was later renamed to
LiveScript, and finally to JavaScript mainly because it was more influenced by the Java programming language.
LiveScript was the official name for the language when it first shipped in beta releases of Netscape Navigator 2.0 in September 1995, but it was renamed JavaScript in a joint announcement with Sun Microsystems on December 4, 1995, when it was deployed in the Netscape browser version 2.0B3.
The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its
Netscape NavigatorNetscape Navigator was a proprietary web browser that was popular in the 1990s. It was the flagship product of the Netscape Communications Corporation and the dominant web browser in terms of usage share, although by 2002 its usage had almost disappeared...
web browser. The final choice of name caused confusion, giving the impression that the language was a spin-off of the Java programming language, and the choice has been characterized by many as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new web-programming language.
It has also been claimed that the language's name is the result of a co-marketing deal between
NetscapeNetscape Communications is a US computer services company, best known for Netscape Navigator, its web browser. When it was an independent company, its headquarters were in Mountain View, California...
and
SunSun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
, in exchange for Netscape bundling Sun's Java runtime with its then-dominant browser.
JavaScript very quickly gained widespread success as a client-side scripting language for web pages. As a consequence,
MicrosoftMicrosoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
named its implementation
JScriptJScript is a scripting language based on the ECMAScript standard that is used in Microsoft's Internet Explorer.JScript is implemented as a Windows Script engine. This means that it can be "plugged in" to any application that supports Windows Script, such as Internet Explorer, Active Server Pages,...
to avoid trademark issues. JScript added new date methods to fix the
Y2KThe Year 2000 problem was a problem for both digital and non-digital documentation and data storage situations which resulted from the practice of abbreviating a four-digit year to two digits.In computer programs, the practice of representing the year with two...
-problematic methods in JavaScript, which were based on Java's
java.util.Date class. JScript was included in
Internet ExplorerWindows Internet Explorer is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year...
3.0, released in August 1996.
In November 1996, Netscape announced that it had submitted JavaScript to
Ecma InternationalEcma International is an international, private non-profit standards organization for information and communication systems. It acquired its name in 1994, when the European Computer Manufacturers Association changed its name to reflect the organization's global reach and activities...
for consideration as an industry standard, and subsequent work resulted in the standardized version named
ECMAScriptECMAScript is the scripting language standardized by Ecma International in the ECMA-262 specification and ISO/IEC 16262. The language is widely used for client-side scripting on the web, in the form of several well-known dialects such as JavaScript, JScript, and ActionScript.- History :JavaScript...
.
JavaScript has become one of the most popular programming languages on the web. Initially, however, many professional programmers denigrated the language because its target audience was web authors and other such "amateurs", among other reasons. The advent of
AjaxAjax is a group of interrelated web development methods used on the client-side to create asynchronous web applications...
returned JavaScript to the spotlight and brought more professional programming attention. The result was a proliferation of comprehensive
frameworks and libraries, improved JavaScript programming practices, and increased usage of JavaScript outside of web browsers, as seen by the proliferation of
server-side JavaScriptServer-side JavaScript refers to JavaScript that runs on the server-side. This term was coined because the language is predominantly used on the client-side, i.e. client-side JavaScript ....
platforms.
In January 2009, the
CommonJSCommonJS is a project with the goal of specifying an ecosystem for JavaScript outside the browser . The project was started by Kevin Dangoor in January 2009 and initially named ServerJS....
project was founded with the goal of specifying a common standard library mainly for JavaScript development outside the browser.
Trademark
"JavaScript" is a
trademarkA trademark, trade mark, or trade-mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or...
of
Oracle CorporationOracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...
. It is used under license for technology invented and implemented by Netscape Communications and current entities such as the
Mozilla FoundationThe Mozilla Foundation is a non-profit organization that exists to support and provide leadership for the open source Mozilla project. The organization sets the policies that govern development, operates key infrastructure and controls trademarks and other intellectual property...
.
Features
The following features are common to all conforming ECMAScript implementations, unless explicitly specified otherwise.
Imperative and structured
JavaScript supports much of the
structured programmingStructured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...
syntax from C (e.g.,
if statements,
while loops,
switch statements, etc.). One partial exception is scoping: C-style block-level scoping is not supported (instead, JavaScript has function-level scoping). JavaScript 1.7, however, supports block-level scoping with the
let keyword. Like C, JavaScript makes a distinction between
expressionsAn expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
and
statementsIn computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program written in such a language is formed by a sequence of one or more statements. A statement will have internal components .Many languages In computer programming...
. One syntactic difference from C is automatic semicolon insertion, in which the semicolons that terminate statements can be omitted.
Dynamic
dynamic typing: As in most scripting languages,
typesA type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...
are associated with
valuesIn computer science, a value is an expression which cannot be evaluated any further . The members of a type are the values of that type. For example, the expression "1 + 2" is not a value as it can be reduced to the expression "3"...
, not with
variablesIn computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
. For example, a variable
x could be bound to a number, then later rebound to a
stringIn formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
. JavaScript supports various ways to test the type of an object, including
duck typingIn computer programming with object-oriented programming languages, duck typing is a style of dynamic typing in which an object's current set of methods and properties determines the valid semantics, rather than its inheritance from a particular class or implementation of a specific interface...
.
object based: JavaScript is almost entirely
object-basedIn general, object-based indicates that something such as a theory, language, or model is based on the concept of object.In computer science, the term object-based has two different senses:...
. JavaScript
objectsIn computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
are
associative arrayIn computer science, an associative array is an abstract data type composed of a collection of pairs, such that each possible key appears at most once in the collection....
s, augmented with prototypes (see below). Object property names are string keys:
obj.x = 10 and
obj['x'] = 10 are equivalent, the dot notation being
syntactic sugarSyntactic sugar is a computer science term that refers to syntax within a programming language that is designed to make things easier to read or to express....
. Properties and their values can be added, changed, or deleted at run-time. Most properties of an object (and those on its prototype inheritance chain) can be enumerated using a
for...in loop. JavaScript has a small number of built-in objects such as
Function and
Date.
run-time evaluation: JavaScript includes an
evalIn some programming languages, eval is a function which evaluates a string as though it were an expression and returns a result; in others, it executes multiple lines of code as though they had been included instead of the line including the eval...
function that can execute statements provided as strings at run-time.
Functional
first-class functions:
FunctionsIn computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
are
first-classIn computer science, a programming language is said to have first-class functions if it treats functions as first-class objects. Specifically, this means that the language supports passing functions as arguments to other functions, returning them as the values from other functions, and assigning...
; they are objects themselves. As such, they have properties and methods, such as
length and
call; and they can be assigned to variables, passed as arguments,
returned by other functions, and manipulated like any other object. Any reference to a function allows it to be invoked using the
operator.
nested functions: "Inner" or "nested" functions are functions defined within another function. They are created each time the outer function is invoked. In addition to that, the
scopeIn computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
of the outer function, including any constants, local variables and argument values, become part of the internal state of each inner function object, even after execution of the outer function concludes.
closures: JavaScript allows nested functions to be created, with the lexical scope in force at their definition, and has a
operator to invoke them now or later. This combination of code that can be executed outside the scope in which it is defined, with its own scope to use during that execution, is called a
closureIn computer science, a closure is a function together with a referencing environment for the non-local variables of that function. A closure allows a function to access variables outside its typical scope. Such a function is said to be "closed over" its free variables...
in computer science.
Prototype-based
prototypes: JavaScript uses
prototypesPrototype-based programming is a style of object-oriented programming in which classes are not present, and behavior reuse is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as classless, prototype-oriented or instance-based programming...
instead of
classesIn object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
for
inheritanceIn object-oriented programming , inheritance is a way to reuse code of existing objects, establish a subtype from an existing object, or both, depending upon programming language support...
. 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 creates a new object and calls that function with its local
this keyword bound to that object for that invocation. The constructor's
prototype property determines the object used for the new object's internal prototype. JavaScript's built-in constructors, such as
Array, also have prototypes that can be modified.
functions as methods: Unlike many object-oriented languages, there is no distinction between a function definition and a
methodIn object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
definition. Rather, the distinction occurs during function calling; a function can be called as a method. 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.
Miscellaneous
run-time environment: JavaScript typically relies on a run-time environment (e.g. in a
web browserA web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
) to provide objects and methods by which scripts can interact with "the outside world". In fact, it relies on the environment to provide the ability to include/import scripts (e.g.
HTMLHyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....