JavaScript (JS) is an interpreted computer programming language. As part of web browsers, implementations allow client-side scripts to interact with the user, control the browser, communicate asynchronously, and alter the document content that is displayed. It has also become common in server-side programming, game development and the creation of desktop applications.
JavaScript is a prototype-based scripting language with dynamic typing and has first-class functions. Its syntax was influenced by C. JavaScript copies many names and naming conventions from Java, 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. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.
The application of JavaScript to uses outside of web pages for example, in PDF documents, site-specific browsers, and desktop widgets is also significant. Newer and faster JavaScript VMs and frameworks built upon them (notably Node.js) have also increased the popularity of JavaScript for server-side web applications.
JavaScript was formalized in the ECMAScript language standard and is primarily used as part of a web browser (client-side JavaScript). This enables programmatic access to computational objects within a host environment.
Birth at Netscape
JavaScript was originally developed by Brendan Eich. While battling with Microsoft over the Internet, Netscape considered their client-server offering a distributed OS, running a portable version of Sun Microsystems' Java. Because Java was a competitor of C++ and aimed at professional programmers, Netscape also wanted a lightweight interpreted language that would complement Java by appealing to nonprofessional programmers, like Microsoft's Visual Basic (see JavaScript and Java)
Although it was developed under the name Mocha, the language was officially called LiveScript when it first shipped in beta releases of Netscape Navigator 2.0 in September 1995, but it was renamed JavaScript 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 Navigator 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.
Server-side JavaScript
Netscape introduced an implementation of the language for server-side scripting (SSJS) with Netscape Enterprise Server, first released in December, 1994 (soon after releasing JavaScript for browsers). Since the mid-2000s, there has been a proliferation of server-side JavaScript implementations. Node.js is one recent notable example of server-side JavaScript being used in real-world applications.
Adoption by Microsoft
JavaScript very quickly gained widespread success as a client-side scripting language for web pages. Microsoft introduced JavaScript support in its own web browser, Internet Explorer, in version 3.0, released in August 1996.[not in citation given] Microsoft's webserver, Internet Information Server, introduced support for server-side scripting in JavaScript with release 3.0 (1996). Microsoft started to promote webpage scripting using the umbrella term Dynamic HTML. Microsoft's JavaScript implementation was later renamed JScript to avoid trademark issues. JScript added new date methods to fix the Y2K-problematic methods in JavaScript, which were based on Java's java.util.Date class.
Standardization
In November 1996, Netscape announced that it had submitted JavaScript to Ecma International for consideration as an industry standard, and subsequent work resulted in the standardized version named ECMAScript. In June 1997, Ecma International published the first edition of the ECMA-262 specification. A year later, in June 1998, some modifications were made to adapt it to the ISO/IEC-16262 standard, and the second edition was released. The third edition of ECMA-262 (published on December 1999) is the version most browsers currently use.
A fourth edition of the ECMAScript standard was not released and does not exist. Fifth edition of the Ecmascript standard was released in December 2009. The current edition of the ECMAScript standard is 5.1, released in June 2011.
Later developments
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 consisted of web authors and other such "amateurs", among other reasons. The advent of Ajax 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 JavaScript platforms.
In January 2009, the CommonJS project was founded with the goal of specifying a common standard library mainly for JavaScript development outside the browser.
Trademark
Today, "JavaScript" is a trademark of Oracle Corporation. It is used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.
Features
The following features are common to all conforming ECMAScript implementations, unless explicitly specified otherwise.
Imperative and Structured
JavaScript supports much of the structured programming syntax from C (e.g., if statements, while loops, switch statements, etc.). One partial exception is scoping: C-style block scoping is not supported. Instead, JavaScript has function scoping (although, block scoping using the let keyword was added in JavaScript 1.7). Like C, JavaScript makes a distinction between expressions and statements. 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, types are associated with values, not with variables. For example, a variable x could be bound to a number, then later rebound to a string. JavaScript supports various ways to test the type of an object, including duck typing.
Object based
JavaScript is almost entirely object-based. JavaScript objects are associative arrays, augmented with prototypes (see below). Object property names are string keys. They support two equivalent syntaxes: dot notation (obj.x = 10) and bracket notation (obj['x'] = 10). 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 eval function that can execute statements provided as strings at run-time.
Functional
First-class functions
Functions are first-class; they are objects themselves. As such, they 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 created function forms a lexical closure: the lexical scope of the outer function, including any constants, local variables and argument values, becomes part of the internal state of each inner function object, even after execution of the outer function concludes.
Prototype-based
Prototypes
JavaScript uses prototypes where many other object oriented languages use classes for inheritance. It is possible to simulate many class-based features with prototypes in JavaScript.
Functions as object constructors
Functions double as object constructors along with their typical role. Prefixing a function call with new will create an instance of a prototype, inheriting properties and methods from the constructor (including properties from the Object prototype). ECMAScript 5 offers the Object.create method, allowing explicit creation of an instance without automatically inheriting from the Object prototype (older environments can assign the prototype to null). The constructor's prototype property determines the object used for the new object's internal prototype. New methods can be added by modifying the prototype of the object used as a constructor. JavaScript's built-in constructors, such as Array or Object, also have prototypes that can be modified. While it is possible to modify the Object prototype, it is generally considered bad practice because most objects in Javascript will inherit methods and properties from the Object prototype and they may not expect the prototype to be modified.
Functions as methods
Unlike many object-oriented languages, there is no distinction between a function definition and a method definition. Rather, the distinction occurs during function calling; when a function is called as a method of an object, the function's local this keyword is bound to that object for that invocation.
Miscellaneous
Run-time environment
JavaScript typically relies on a run-time environment (e.g. a web browser) to provide objects and methods by which scripts can interact with the environment (e.g. a webpage DOM). It also relies on the run-time environment to provide the ability to include/import scripts. This is not a language feature per se, but it is common in most JavaScript implementations.
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 apply method.
Array and object literals
Like many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, these literals form the basis of the JSON data format.
Regular expressions
JavaScript also supports regular expressions in a manner similar to Perl, which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.
Vendor-specific extensions
JavaScript is officially managed by Mozilla Foundation, and new language features are added periodically. However, only some JavaScript engines support these new features:
* property getter and setter functions (supported by WebKit, Opera, ActionScript, and Rhino)
* conditional catch clauses
* iterator protocol (adopted from Python)
* shallow generators-coroutines (adopted from Python)
* array comprehensions and generator expressions (adopted from Python)
* proper block scope via the let keyword
* array and object destructuring (limited form of pattern matching)
* concise function expressions (function(args) expr)
* ECMAScript for XML (E4X), an extension that adds native XML support to ECMAScript