JavaScript

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JavaScript

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JavaScript is the name of Netscape Communications Corporation's implementation of ECMAScript, a scripting programming language based on the concept of prototypes. The language is best known for its use in websites, but is also used to enable scripting access to objects embedded in other applications.

Despite the name, JavaScript is only distantly related to the Java programming language, the main similarity being their common debt to the C programming language syntax. Semantically, JavaScript has far more in common with the Self programming language and ActionScript which is also an ECMAScript.

JavaScript is a registered trademark of Sun Microsystems, Inc., used under license for technology invented and implemented by Netscape.

History

JavaScript was originally developed by Brendan Eich of Netscape Communications Corporation under the name Mocha, then 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. When web developers talk about using JavaScript in Internet Explorer, they are actually using JScript. The choice of name proved to be a source of much confusion.

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.

Usage

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 most well known examples. These are examined first.

One major use of web-based 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 to perform tasks not possible in HTML alone. Some common examples of this usage follow.

Opening or popping up a new window with programmatic control over the size, position and 'look' of the new window (i.e. whether or not the menus, toolbars etc are visible. Usually JavaScript is used to ensure that they are not): If this degree of control is not required, JavaScript is not necessary. Simply adding the attribute target="xxx" to the link element in HTML will reliably produce a new window the same size as the current one, with menus etc displayed as per the user's preferences. Note that many browsers now include mechanisms that, by default, block all JavaScript pop-ups, displaying only a small message to say that they have done so.
Checking or validating web form input values to make sure that they will be accepted before they are submitted to the server: There is always a time delay, and a processing overhead on the server, when a form has to be submitted. Nonetheless input validation should be repeated at the server in case the JavaScript failed to run (see below).
Changing images as the mouse cursor moves over them: This effect is still enjoyed by many designers, often to draw the user's attention to important links displayed as graphical elements. This "rollover" effect can also be implemented with CSS in recent browsers. With a mind for visually impaired users, who may be using extreme text magnification that makes non-resizing graphics largely irrelevant, others are moving away from this approach.

The DOM interfaces in various browsers differ and don't always match the W3C DOM standards. Rather than write different variants of a JavaScript function for each of the many browsers in common use today, it is usually possible, by carefully following the W3C DOM Level 1 or 2 standards, to provide the required functionality in a standards-compliant way that most browsers will execute correctly. Care must always be taken to ensure that the web page degrades gracefully and so is still usable by any user who:

has JavaScript execution disabled - for example as a security precaution
has a browser that does not understand the JavaScript - for example on a PDA or mobile phone
is visually or otherwise disabled and may be using an unusual browser, a speech browser or may have selected extreme text magnification. For more information on this, see the Web Accessibility Initiative

Other examples of JavaScript interacting with a web page's DOM have been called DHTML and SPA.

A different example of the use of JavaScript in web pages is to make calls to web and web-service servers after the page has loaded, depending upon user actions. These calls can obtain new information, which further JavaScript can merge with the existing page's DOM so that it is displayed. This is the basis of Ajax programming.

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 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, allow scripting through JavaScript.

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.

Syntax

Main article: JavaScript syntax

Debugging

JavaScript debugging has some specifics in comparison with stand-alone applications. First of all JavaScript is an interpreted language so errors do not appear until the relevant program block is run. Second, 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. All this makes JavaScript errors difficult to find. However nowadays both Internet Explorer and the Gecko-based browsers come with a reasonably good JavaScript debugger. Gecko browsers use the Venkman debugger or the FireBug debugger for Mozilla Firefox, while for Internet Explorer there is the Microsoft Script Debugger. Microsoft also provides Microsoft Script Editor (MSE) as part of their Microsoft Office package (Microsoft Office 2002 or higher). There are also some free online checking tools such as JSLint.

Because JavaScript is interpreted, loosely-typed, and must run at the client-side in varying environments (host 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 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 enable a dignified die.

Security

As JavaScript is a rich interpreted language running arbitrary server-provided code on a client computer, it has suffered its fair share of security issues. Many attempts at bypassing logical barriers to steal confidential information make use of JavaScript to perform their deeds behind the scenes, using techniques such as cross-site scripting. Non-expert users usually can't notice what is going on because the malicious code has no visible effect on their browsing experience. Besides, it is not obvious to people who are not programmers that their Web browser includes a full-blown programming language interpreter, with the power and risks implied.

Similar security issues have been noted with Java, but they are considered a lesser threat because the Java virtual machine provides a well-defined sandboxing model and few Web sites today (2006) require Java, whereas many use JavaScript.

Related languages

There is no real 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. 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.

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.

Due to the success of JavaScript, Microsoft developed a compatible language, which it called JScript. JScript was first supported in Internet Explorer version 3.0, released in August, 1996; when web developers talk about the use of JavaScript in the IE browser, they actually mean JScript.

The need for common specifications for these two languages was the basis of the ECMA 262 standard for ECMAScript (see external links below), three editions of which have been published since the work started in November 1996. The object model of browser-based JavaScript, the Document Object Model (DOM), is actually not part of the ECMAScript standard. It is defined in a set of separate standards, developed by the W3C and is applicable to the access and manipulation of HTML and XML documents in many computer languages and platforms.

ActionScript, the programming language used in Macromedia Flash, is based on the ECMAScript standard, so it closely resembles JavaScript in syntax.

JSON, or JavaScript Object Notation, is a general-purpose data interchange format that is defined as a subset of JavaScript.

JavaScript OSA (JavaScript for OSA, or JSOSA), is a Macintosh scripting language 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.

Of only historical interest now, 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. The Little Javascripter shows the relationship with Scheme in more detail.

References

Nigel McFarlane: Rapid Application Development with Mozilla, Prentice Hall Professional Technical References, ISBN 0131423436
David Flanagan, Paula Ferguson: JavaScript: The Definitive Guide, O'Reilly & Associates, ISBN 0596000480
Danny Goodman, Scott Markel: JavaScript and DHTML Cookbook, O'Reilly & Associates, ISBN 0596004672
Danny Goodman, Brendan Eich: JavaScript Bible, Wiley, John & Sons, ISBN 0764533428
Andrew H. Watt, Jinjer L. Simon, Jonathan Watt: Teach Yourself JavaScript in 21 Days, Pearson Education, ISBN 0672322978
Thomas A. Powell, Fritz Schneider: JavaScript: The Complete Reference, McGraw-Hill Companies, ISBN 0072191279
Scott Duffy: How to do Everything with JavaScript, Osborne, ISBN 0072228873
Andy Harris, Andrew Harris: JavaScript Programming, Premier Press, ISBN 0761534105
Joe Burns, Andree S. Growney, Andree Growney: JavaScript Goodies, Pearson Education, ISBN 0789726122
Gary B. Shelly, Thomas J. Cashman, William J. Dorin, Jeffrey Quasney: JavaScript: Complete Concepts and Techniques, Course Technology, ISBN 0789562332
Nick Heinle, Richard Koman: Designing with JavaScript, O'Reilly & Associates, ISBN 1565923006
Sham Bhangal, Tomasz Jankowski: Foundation Web Design: Essential HTML, JavaScript, CSS, PhotoShop, Fireworks, and Flash, APress L. P., ISBN 1590591526
Emily Vander Veer: JavaScript For Dummies, 4th Edition, Wiley, ISBN 0764576593

Databases and libraries

AJILE - Advanced JavaScript Importing & Loading Extension
DHTML goodies
Dojo A Javascript library.
Dynamic Drive
jQuery A compact Javascript library.
JSAN - the JavaScript Archive Network
The Prototype Javascript Framework
X — a cross-browser Javascript library with many demos.

Common problems

History

Innovators of the Net: Brendan Eich and JavaScript (Marc Andreesen, Netscape TechVision, 24 June 1998)
Brendan Eich and JavaScript (about.com)
Brendan's Roadmap Updates: JavaScript 1, 2, and in between - the author's blog entry

History

Usage

Syntax

Debugging

Security

Related languages

See also

References

External links

Reference material

Authoring guides

Tutorials

Databases and libraries

Common problems

History

See also

Gourt Home::Gourt :: JavaScript

History

Usage

Syntax

Debugging

Security

Related languages

See also

References

External links

Reference material

Authoring guides

Tutorials

Databases and libraries

Common problems

History

See also