JavaScript explained


JavaScript is a scripting language most often used for client-side web development. Its proper name is ECMAScript, though "JavaScript" is much more commonly used. "JavaScript" is actually Netscape Communications Corporation's (and now the Mozilla Foundation's) implementation of the ECMAScript standard.

JavaScript 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.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.

Despite the name, JavaScript is unrelated to the Java programming language; though both have a common debt to C syntax. 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. JavaScript semantics is much more similar to the Self programming language.

"JavaScript" is a registered trademark of Sun Microsystems, Inc. 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 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, which is not the case; 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.

To avoid trademark issues, Microsoft named its implementation 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 JavaScript at the time. Microsoft submitted JScript to ECMA for standardization resulting in the standardized version named ECMAScript.

Language
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.

JavaScript is a prototype-based scripting language with a syntax loosely based on C. Like C, the language has no input or output constructs of its own. Where C relies on standard I/O libraries, a JavaScript engine relies on a host environment into which it is embedded. There are many such host environment applications, of which web technologies are the best-known examples. These are examined first.

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 (i.e. whether the menus, toolbars, etc. are visible).
* Validation of web form input values to make sure that they will be accepted before they are 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 runs on the client rather than the 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 attempt to 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 seeks to exploit JavaScript's strengths.

Compatibility considerations
DOM interfaces 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. 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.

Furthermore, scripts will not 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 cannot 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 do not 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 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.

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. JavaScript hijacking is an important example of XSRF.

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 should not be trusted. Some implications:

* Web site authors cannot 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 users agreed to the terms of service, or filters invalid characters out of fields that should 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 have 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 JavaScript 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.

Uses outside Web pages
Outside of the Web, JavaScript interpreters are embedded in a number of tools. Adobe Acrobat and Adobe Reader support JavaScript in PDF files. The Mozilla platform, which underlies several common web browsers, uses JavaScript to implement the user interface and transaction logic of its various products. JavaScript interpreters are also embedded in proprietary applications that lack scriptable interfaces. Dashboard Widgets in Apple's Mac OS X v10.4 and Yahoo! Widgets are implemented using JavaScript. Microsoft's Active Scripting technology supports JavaScript-compatible JScript as an operating system scripting language. JScript .NET is a CLI-compliant language that is similar to JScript, but has further object oriented programming features. Tools in the Adobe Creative Suite, including Photoshop, Illustrator, and InDesign, allow scripting through JavaScript. An ECMAScript implementation called Qt Script for Applications is available with the Qt application development toolkit. For best web hosting provider, you only need speed for bandwidth when comes any performance in discussion.

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.

The Java programming language, in version SE 6 (JDK 1.6), introduced the javax.script package, based on Mozilla Rhino. This allows Java applications to read, interpret and execute JavaScript scripts at run-time. The Java developer can make objects and variables that are part of the host application available to the JavaScript code using a Bindings object. These aspects of the running application can then be accessed and manipulated at run-time from JavaScript in a similar manner to the way that client-side scripts access the DOM of a displayed page in a web browser.

JavaScript OSA (JavaScript for OSA, or JSOSA), is a scripting language for Mac OS based on the Mozilla 1.5 JavaScript implementation, SpiderMonkey. It is a freeware component made available by Late Night Software. Interaction with the operating system and with third-party applications is scripted via a MacOS object. Otherwise, the language is virtually identical to the core Mozilla implementation. It was offered as an alternative to the more commonly used AppleScript language.

ActionScript, the programming language used in Macromedia Flash, is another implementation of the ECMAScript standard.

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.

Debugging
JavaScript debugging has some specifics in comparison with stand-alone applications. JavaScript programs usually rely on interaction with the loaded page's Document Object Model (DOM) so errors may be caused by wrong DOM usage in a technically correct script. This makes JavaScript errors difficult to find. However, currently both Internet Explorer and Gecko-based browsers come with a JavaScript debugger. Gecko browsers use the Venkman debugger or the Firebug tool for Mozilla Firefox. Opera announced a debugging developer tool in a preview released in February 2007. Client-side JScript in Internet Explorer can be debugged with Microsoft Script Debugger. Some client-side JavaScript errors might depend on specific client side conditions such as browser version, user input or other installed software. Microsoft Visual Studio can be used to debug server-side and command-line JScript. Microsoft also provides Microsoft Script Editor (MSE) as part of their Microsoft Office package for scripts embedded in Microsoft Office documents. There are also some free tools such as JSLint, which will scan JavaScript code looking for problems, as well as a professional JavaScript Debugger called SplineTech JavaScript HTML Debugger for Microsoft Windows. A debugger for the engine powering Apple's Safari, Webkit is also available under the name Drosera.

Since JavaScript is interpreted, loosely-typed, and, when run at the client-side, may be hosted in varying environments, applications, implementations and versions, the 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 nicely 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.

Related languages
There is no relationship between Java (see best java hosting) 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. Like C and C++, all Java variables and members are statically typed, whereas all JavaScript variables (as well as object properties and array elements) may hold a value 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.

ECMAScript was included in the VRML97 standard for scripting nodes of VRML scene description files.

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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