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It has been suggested that Client-side JavaScript be merged into this article or section. (Discuss) |
Not to be confused with Java (programming language).
| JavaScript | |
|---|---|
| Paradigm | multi-paradigm |
| Appeared in | 1995 |
| Designed by | Brendan Eich |
| Developer | Netscape Communications Corporation, Mozilla Foundation |
| Typing discipline | dynamic, weak, duck |
| Major implementations | SpiderMonkey, Rhino, KJS, JavaScriptCore |
| Dialects | JScript, JScript .NET |
| Influenced by | Self, C, Scheme, Perl, Python |
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 have a similar look to Java, but be easier for non-programmers to work with.TechVision: Innovators of the Net: Brendan Eich and JavaScript 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 Windows Vista Sidebar).
Despite the name, JavaScript is essentially unrelated to the Java programming language, though both have the common C syntax, 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 browser, which was dominant at the time. 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.Sun Trademarks. Sun Microsystems. Retrieved on 2007-11-08.
Contents |
JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, later LiveScript, and finally renamed to JavaScript. The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigator web 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 spinoff 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.Programming languages used on the Internet and the World Wide Web (WWW)O\'Reilly - Safari Books Online - 0596101996 - JavaScript: The Definitive Guide, 5th Edition
To avoid trademark issues, Microsoft named its dialect of the language JScript. 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 so similar that the terms "JavaScript" and "JScript" are often used interchangeably (including in this article).
Netscape submitted JavaScript to Ecma International for standardization resulting in the standardized version named ECMAScript.Press Release
JavaScript supports all the structured programming syntax in C, e.g. if statement, while loops, switch statement, etc. One exception is scoping: JavaScript supports function-level scoping, but not block-level scoping.
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.
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.
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.
this keyword is bound to that object for that invocation.
arguments object.
As of 2006, the latest version of the language is JavaScript 1.7. 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.
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:
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 Navigator web 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
Your browser either does not support JavaScript, or you have JavaScript turned off.
The DOM interfaces for manipulating Web pages are not part of the ECMAScript standard, or of JavaScript itself. Officially, they are 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 are not available.Peter-Paul Koch, Object detection 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.Peter-Paul Koch, Mission Impossible - mouse positionPeter-Paul Koch, Browser detect Programmers may also use libraries or toolkits which take browser differences into account.
Furthermore, scripts will not work for all users. For example, a user may:
To support these users, Web authors can try to create pages which degrade gracefully on user agents (browsers) which do not support the page\'s JavaScript.
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 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.
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.MozillaZine, Mozilla Cross-Site Scripting Vulnerability Reported and Fixed
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.
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 should not be trusted by the server. Some implications:
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 FirefoxMozilla Corporation, Buffer overflow in crypto.signText(), Internet ExplorerPaul Festa, CNet, Buffer-overflow bug in IE, and Safari.SecurityTracker.com, Apple Safari JavaScript Buffer Overflow Lets Remote Users Execute Arbitrary Code and HTTP Redirect Bug Lets Remote Users Access Files
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.SecurityFocus, Microsoft WebViewFolderIcon ActiveX Control Buffer Overflow VulnerabilityFusion Authority, Macromedia Flash ActiveX Buffer Overflow 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.Mike Friedman, Protected Mode in Vista IE7
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 ExplorerUS CERT, Vulnerability Note VU#713878: Microsoft Internet Explorer does not properly validate source of redirected frame and FirefoxMozilla Foundation, Mozilla Foundation Security Advisory 2005-41: Privilege escalation via DOM property overrides. In Windows XP Service Pack 2, Microsoft tightened the rules on what JScript would be run with high privileges by Internet Explorer.Microsoft Corporation, Changes to Functionality in Microsoft Windows XP Service Pack 2: Part 5: Enhanced Browsing Security
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.For one example of a rare JavaScript Trojan Horse, see Symantec Corporation, JS.Seeker.K (See Windows Script Host.)
Outside of 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.
javax.script package, based on Mozilla Rhino. Thus, Java applications can host scripts that access the application\'s variables and objects, much as Web browsers host scripts that access the browser\'s Document Object Model (DOM) for a webpage.http://java.sun.com/javase/6/webnotes/index.html#scripting javax.script release notesFlanagan 5th Edition, Pp 214 et seq
QtScript module to interpret JavaScript, analogous to javax.script.Trolltech ASA, QtScript Module
MacOS object for interaction with the operating system and third-party applications.AppleScript#Open_Scripting_Architecture
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 is useful to have access to a debugger for each of the browsers a web application is being targeted at.
Currently, Internet Explorer, Firefox, and Safari all have script debuggers available for them. Opera announced a debugging developer tool in a preview released in February 2007.
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 OfficeJScript development in Microsoft Office 11 (MS InfoPath 2003)), and finally the free Microsoft Script Debugger which is far more basic than the other two. 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 engineIntroducing Drosera - Surfin\' Safari powering Apple\'s Safari.
There are also some free tools such as JSLint, a code quality tool that will scan JavaScript code looking for problemsJSLint help page, as well as a non-free tool called JavaScript 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.
Each script block is parsed separately. On web pages where JavaScript in script blocks is mixed with HTML, syntax errors can be identified more readily by keeping discrete functions in separate script blocks, or (for preference), using many small linked .js files. This way, a syntax error will not cause parsing/compiling to fail for the whole page, which can help to allow a graceful exit.
There is not 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 is 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.The Little JavaScripter shows the relationship with Scheme in more detail.
Wikibooks has a book on the topic of
| ECMAScript |
|---|
Dialects
List of ECMAScript engines |
| People Brendan Eich · Ecma International |
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia