Summary: The most common client side scripting language implemented in many user agents

What is JavaScript?

JavaScript was created by Netscape as a cross-platform, object oriented scripting language. This is most commonly used as client side scripting, but it is also possible to use it on the server. JavaScript is similar to Java in syntax, but in many ways it is different.

Is it similar to Java?

"JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces. Variables, parameters, and function return types are not explicitly typed.

"Java is a class-based programming language designed for fast execution and type safety. Type safety means, for instance, that you can't cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java's class-based model means that programs consist exclusively of classes and their methods. Java's class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript authoring." Core Java Script Guide 1.5#1012569

JavaScript Implementations

JavaScript was later adopted by Micosoft Internet Exploder, and renamed JScript to reflect it's similarity to, but differences from Netscape's JavaScript.

More recently, this has been standardized using the DOM (Document Object Model) and ECMAScript, formally known as ECMA-262

Netscape/Mozilla's JavaScript 1.5 is fully compliant with ECMA-262. Given Microsoft's history with standards compliance, JScript may not be fully compliant, but it is compatible enough for general use.

JavaScript is also considered a functional programming language_index.html like Scheme and OCaml because it has closures and supports higher-order functions.

JavaScript Media Type

There is no official IANA registered Media Type for JavaScript, however it is commonly served as @text/javascript@. Some user agents also support @application/x-javascript@, however Microsoft's IE does not ,so it is not commonly used. Some authors may use content negotiation to label script as @text/javascript@ in HTML docuements served as @text/html@, and @application/x-javascript@ in XHTML documents served as @application/xhtml+xml@ or other XML Media Type.

Including Script in ""(X)HTML"" Docuements

When authors include script inline within their documents, it is common to use SGML comments to hide the script from older user agents so it is not rendered on the screen. eg.

		 <script type="text/javascript"><!--
		 ...
		 //--></script>
	

However, in most situations, this seems quite redundant since the use of JavaScript in many documents has been done in such a way, that the document is not even viewable in anything by the latest browser's; or worse, only viewable in Internet Exploder.

Also, when used in real XHTML documents served as an XML media type, the use of the comment means that an XML parser will completely ignore the script. So, any XHTML document served as @text/html@ contining scripts like that, will break when served as XML. There is, however, a more correct way to hide scripts from both HTML and XML user agents.

		 <script type="text/javascript"><!--//--><![CDATA[//><!--
		 ...
		 //--><!]]></script>
	

This syntax is slightly more complicated, and I will not attempt to explain it, because I'l probably get it wrong. (Someone who does understand it properly, can add the description here)

There is also a similar method used for Style Sheets, but that is not approprate for a JavaScript document. Hixie (Ian Hickson) explains this further in his XHTML advocacy document.

Accessability Issues

Since JavaScripts inception, it has been widely used to implement dynamic effects and interaction to web pages. However, most authors fail to consider the accessibility issues caused as a result of using script and air jordan 11. The W3C's WAI Guidlines state that authors using additional technologies such as scripts, applets, etc… should ensure that the page degrades gracefully. See Guideline 6#gl-new-technologies for more information.

There are many resources available for learning scripting languages and how to make them accessible. Some of the best techniques have been submitted to A List Apart by many web developers.

Bookmarklets

More recently, JavaScript has started to be used for creating bookmarklets, or favelets. These are small scripts written using the javascript: URI Scheme and placed in a bookmark in the user agent. There are many available that perfom different tasks, such as disabling page colours, revealing password fields in forms, disabling stylesheets, etc... Peple like Tantek Çeilc (http://www.tantek.com) have written many of them which are available to use freely. There are many more. Just search for bookmarklets of favelts in your favourite search engine.

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