JavaScript is a scripting language most often used for client-side web development. It was the originating dialect of the ECMAScript standard. It is a dynamic, weakly typed, prototype-based language with first-class functions. JavaScript was influenced by many languages and was designed to look like Java, but be easier for non-programmers to work with. The language is best known for its use in websites (as client-side JavaScript) but is also used to enable scripting access to objects embedded in other applications (for example Microsoft Gadgets in the Windows Sidebar).
JavaScript, despite the name, is essentially unrelated to the Java programming language though both have the common Csyntax, and JavaScript copies many Java names and naming conventions. The language was renamed from LiveScript in a co-marketing deal between Netscape and Sun in exchange for Netscape bundling Sun's Java runtime with their then-dominant browser. The key design principles within JavaScript are inherited from the Self programming language.
"JavaScript" is a trademark of Sun Microsystems. It was used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.
History and naming
JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, later LiveScript, and finally renamed to JavaScript. The language was originally implemented in Common Lisp (and retains many Lisp-like qualities). The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigatorweb browser. JavaScript was first introduced and deployed in the Netscape browser version 2.0B3 in December of 1995. The naming has caused confusion, giving the impression that the language is a spin-off of Java, and 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.
Microsoft named its dialect of the language JScript to avoid trademark issues. JScript was first supported in Internet Explorer version 3.0, released in August 1996 and included Y2K compliant date functions, unlike those based on java.util.Date in JavaScript at the time. The dialects are perceived to be so similar that the terms "JavaScript" and "JScript" are often used interchangeably (including in this article). Microsoft, however, notes dozens of ways in which JScript is not ECMA compliant.
Netscape submitted JavaScript to Ecma International for standardization resulting in the standardized version named ECMAScript.
Features
Structured programming
JavaScript supports all the structured programming syntax in C, for example if statements, while loops, switch statements, etc. One partial exception is scoping: C-style block-level scoping isn't supported. However, JavaScript 1.7 supports block-level scoping with the let keyword.
Dynamic programming
dynamic typing: As in most scripting languages, types are associated with values, not 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.
; objects as associative arrays: JavaScript is heavily object-based. Objects are associative arrays, such that object property names are associative array keys. obj.x = 10 and obj["x"] = 10 are equivalent, the dot notation being merely syntactic sugar. Properties and their values can be added, changed, or deleted at run-time. The properties of an object can also be enumerated via a for...in loop.
interpreted: Conforming JavaScript engines must be able to interpret (as opposed to compile) source code. This allows JavaScript to include an eval function.
Function-level programming
first-class functions: Functions are first-class; they're objects themselves. As such, they've properties and can be passed around and interacted with like any other object.
; inner functions and closures: Inner functions (functions defined within other functions) are created each time the outer function is invoked, and variables of the outer functions for that invocation continue to exist as long as the inner functions still exist, even after that invocation is finished (for example if the inner function was returned, it still has access to the outer function's variables) — this is the mechanism behind closures within JavaScript.
Prototype-based
prototypes: JavaScript uses prototypes instead of classes for defining object properties, including methods, and 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 creates a new object and calls that function with its local this keyword bound to that object for that invocation. The function's prototype property determines the new object's prototype.
functions as methods: Unlike many object-oriented languages, there's no distinction between a function definition and a method definition. Rather, the distinction occurs during function calling; a function can be called as a method. When a function is invoked as a method of an object, the function's local this keyword is bound to that object for that invocation.
Others
run-time environment: JavaScript typically relies on a run-time environment (for example in a web browser) to provide objects and methods by which scripts can interact with "the outside world". (This isn't a language feature per se, but it's common in most JavaScript implementations.)
; variadic functions: An indefinite number of parameters can be passed to a function. The function can both access them through formal parameters and the local arguments object.
regular expressions: JavaScript also supports regular expressions in a manner similar to Perl, which provide a concise and powerful syntax for text manipulation that's more sophisticated than the built-in string functions.
Syntax
As of 2008, the latest version of the language is JavaScript 1.8. The previous version 1.6 corresponded to ECMA-262 Edition 3 like JavaScript 1.5, except for Array extras, and Array and String generics. ECMAScript, in simple terms, is a standardized version of JavaScript. The ECMA-357 standard specifies E4X, a language extension dealing with XML.
Use in web pages
The primary use of JavaScript is to write functions that are embedded in or included from HTML pages and interact with the Document Object Model (DOM) of the page. Some simple examples of this usage are:
Opening or popping up a new window with programmatic control over the size, position and 'look' of the new window (for example whether the menus, toolbars, etc. are visible).
Validation of web form input values to make sure that that'll be accepted before they're submitted to the server.
Changing images as the mouse cursor moves over them: This effect is often used to draw the user's attention to important links displayed as graphical elements.
Because JavaScript code can run locally in a user's browser (rather than on a remote server), it can respond to user actions quickly, making an application feel more responsive. Furthermore, JavaScript code can detect user actions which HTML alone cannot, such as individual keystrokes. Applications such as Gmail take advantage of this: much of the user-interface logic is written in JavaScript, and JavaScript dispatches requests for information (such as the content of an e-mail message) to the server. The wider trend of Ajax programming similarly exploits this strength.
A JavaScript engine (also known as JavaScript interpreter or JavaScript implementation) is an interpreter that interprets JavaScript source code and executes the script accordingly. The first ever JavaScript engine was created by Brendan Eich at Netscape Communications Corporation, for the Netscape Navigatorweb browser. The engine, code-named SpiderMonkey, is implemented in C. It has since been updated (in JavaScript 1.5) to conform to ECMA-262 Edition 3. The Rhino engine, created primarily by Norris Boyd (also at Netscape) is a JavaScript implementation in Java. Like SpiderMonkey, Rhino is ECMA-262 Edition 3 compliant.
By far, the most common host environment for JavaScript is a web browser. Web browsers typically use the public API to create "host objects" responsible for reflecting the DOM into JavaScript. The web server is another common application of the engine. A JavaScript webserver would expose host objects representing a HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages.
A minimal example of a web page containing JavaScript (using HTML 4.01 syntax) would be: simple page
Compatibility considerations
The DOM interfaces for manipulating web pages are not part of the ECMAScript standard, or of JavaScript itself. Officially, they're defined by a separate standardization effort by the W3C; in practice, browser implementations differ from the standards and from each other, and not all browsers execute JavaScript.
To deal with these differences, JavaScript authors can attempt to write standards-compliant code which will also be executed correctly by most browsers; failing that, they can write code that checks for the presence of certain browser features and behaves differently if they're not available. In some cases, two browsers may both implement a feature but with different behavior, and authors may find it practical to detect what browser is running and change their script's behavior to match. Programmers may also use libraries or toolkits which take browser differences into account.
Furthermore, scripts won't work for all users. For example, a user may:
use an old or rare browser with incomplete or unusual DOM support,
use a PDA or mobile phone browser which can't execute JavaScript,
have JavaScript execution disabled as a security precaution,
or be visually or otherwise disabled and use a speech browser
To support these users, web authors can try to create pages which degrade gracefully on user agents (browsers) which don't support the page's JavaScript.
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 contain this risk using two restrictions. First, scripts run in a sandbox 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 web site don't 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.
Cross-site vulnerabilities
A common JavaScript-related security problem is cross-site scripting, or XSS, a violation of the same origin policy. XSS vulnerabilities occur when an attacker is able to cause a trusted web site, such as an online banking website, to include a malicious script in the webpage presented to a victim. In that example, the script can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim's authorization.
XSS vulnerabilities can also occur because of implementation mistakes by browser authors.
XSS is related to, but not the same as, cross-site request forgery or XSRF. In XSRF, one website causes a victim's browser to generate fraudulent requests to another site, with the victim's legitimate HTTP cookies attached to the request.
Misunderstanding the client-server boundary
Client-server applications, whether they involve JavaScript or not, must assume that untrusted clients may be under the control of attackers. Thus any secret embedded in JavaScript could be extracted by a determined adversary, and the output of JavaScript operations shouldn't be trusted by the server. Some implications:
Web site authors can't perfectly conceal how their JavaScript operates, because the code is sent to the client, and obfuscated code 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.
It would be extremely bad practice to embed a password in JavaScript (where it can be extracted by an attacker), then have JavaScript verify a user's password and pass "password_ok=1" back to the server (since the "password_ok=1" response is easy to forge).
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 which would run any code they wish on the user's system.
These flaws have affected major browsers including Firefox, Internet Explorer, and Safari.
Plugins, such as video players, Macromedia Flash, and the wide range of ActiveX controls enabled by default in Microsoft Internet Explorer, may also have flaws exploitable via JavaScript,
and 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.
Sandbox implementation errors
Web browsers are capable of running JavaScript outside of the sandbox, with the privileges necessary to, for example, create or delete files. Of course, such privileges aren't meant 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 tightened the rules on what JScript would be run with high privileges by Internet Explorer.
Some versions of Microsoft Windows allow JavaScript stored on a computer's hard drive to run as a general-purpose, non-sandboxed program. This makes JavaScript (like VBScript) a theoretically viable vector for a Trojan horse, although JavaScript Trojan horses are uncommon in practice. (See Windows Script Host.)
Uses outside web pages
Outside the web, JavaScript interpreters are embedded in a number of tools. Each of these applications provides its own object model which provides access to the host environment, with the core JavaScript language remaining mostly the same in each application.
ActionScript, the programming language used in Adobe Flash, is another implementation of the ECMAScript standard.
The Mozilla platform, which underlies Firefox and some other web browsers, uses JavaScript to implement the graphical user interface (GUI) of its various products.
Adobe's Acrobat and Adobe Reader (formerly Acrobat Reader) support JavaScript in PDF files.
Microsoft's Active Scripting technology supports the JavaScript-compatible JScript as an operating system scripting language.
The Java programming language, in version SE 6 (JDK 1.6), introduced the javax.script package, based on Mozilla Rhino. Thus, Java applications can host scripts that access the application's variables and objects, much like web browsers host scripts that access the browser's Document Object Model (DOM) for a webpage.
Newer versions of the Qt C++ toolkit include a QtScript module to interpret JavaScript, analogous to javax.script.
The interactive music signal processing software Max/MSP released by Cycling '74, offers a JavaScript model of its environment for use by developers. It allows much more precise control than the default GUI-centric programming model.
Late Night Software's JavaScript OSA (aka JavaScript for OSA, or JSOSA), is a freeware alternative to AppleScript for Mac OS X. It is based on the Mozilla 1.5 JavaScript implementation, with the addition of a MacOS object for interaction with the operating system and third-party applications.
ECMAScript was included in the VRML97 standard for scripting nodes of VRML scene description files.
Some high-end Philips universal remote panels, including TSU9600 and TSU9400, can be scripted using JavaScript.
Sphere is an open source and cross platform computer program designed primarly to make role-playing games that uses JavaScript as scripting language.
Adobe Integrated Runtime is a JavaScript runtime that allows developers to create desktop applications.
Debugging
Within JavaScript, access to a debugger becomes invaluable when developing large, non-trivial programs. Because there can be implementation differences between the various browsers (particularly within the Document Object Model) it's useful to have access to a debugger for each of the browsers a web application is being targeted at.
Currently, Internet Explorer, Firefox, Safari, and Opera all have script debuggers available for them.
Internet Explorer has three debuggers available for it: Microsoft Visual Studio is the richest of the three, closely followed by Microsoft Script Editor (a component of Microsoft Office), and finally the free Microsoft Script Debugger which is far more basic than the other two. The free Microsoft Visual Web Developer Express provides a limited version of the JavaScript debugging functionality in Microsoft Visual Studio.
Web applications within Firefox can be debugged using the Firebug plug-in, or the older Venkman debugger, which also works with the Mozilla browser. Drosera is a debugger for the WebKit engine on Macintosh and Windows powering Apple's Safari.
There are also some free tools such as JSLint, a code quality tool that will scan JavaScript code looking for problems, as well as a non-free tool called SplineTech JavaScript HTML Debugger.
Since JavaScript is interpreted, loosely-typed, and may be hosted in varying environments, each with their own compatibility differences, a programmer has to take extra care to make sure the code executes as expected in as wide a range of circumstances as possible, and that functionality degrades gracefully when it does not.
Related languages
There isn't a particularly close genealogical relationship between Java and JavaScript; their similarities are mostly in basic syntax because both are ultimately derived from C. Their semantics are quite different and their object models are unrelated and largely incompatible. In Java, as in C and C++, all data is statically typed, whereas JavaScript variables, properties, and array elements may hold values of any type.
The standardization effort for JavaScript also needed to avoid trademark issues, so the ECMA 262 standard calls the language ECMAScript, three editions of which have been published since the work started in November 1996.
Microsoft's own VBScript, like JavaScript, can be run client-side in web pages. VBScript has syntax derived from Visual Basic and will only run if the web pages are viewed in Internet Explorer.
JSON, or JavaScript Object Notation, is a general-purpose data interchange format that's defined as a subset of JavaScript.
JavaScript is also considered a functional programming language like Scheme and OCaml because it has closures and supports higher-order functions.
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