Working with Stylesheets

HTML is a markup language used for formatting text and images. It cannot respond to users interaction. Scripting languages can be combined with HTML to produce dynamic changes in web pages, affect the operation of plugins and ActiveX components.

The history of scripting languages started with LiveScript developed by Netscape. LiveScript added the capability of Navigator and its web server line of products. Navigator2 added support for Java applets. In this newversion of Navigator, Netscape replaced LiveScript with JavaScript. The initial version of JavaScript was just a renamed version of LiveScript but subsequent versions added more capabilities. Netscape's Navigator, Microsoft's Internet Explorer and most other popular browsers support JavaScript.

JavaScript can be used to write scripting codes for both browsers as well as servers. Browser scripts are used to create dynamic web pages that are more interactive, more responsive. JavaScript codes are embedded in HTML files. Whenever a browser requests for a web page the HTML file along with script is transferred to the browser. The browser reads the HTML file and identifies the elements contained in the file. This process is known as parsing. When a script is encountered, the script is executed first before the file is read further.

Microsoft implemented its own version of JavaScript known as the Jscript. Jscript is more or less like Java Script except that it adds a few more additional capabilities. Jscript is compatible with JavaScript 1.2. Microsoft also included server side JavaScript support with its Internet Information Server(IIS).

Both Netscape's JavaScript and Microsoft's Jscript were submitted to the European Computer Manufacturer's Association (ECMA) for standardization. Standard ECMA262 was released in June 1997. The standard is a compilation of the best features of JavaScript and Jscript. An updated version of this standard was released in June 1998. Jscript achieved compliance with ECMAScript through its new version Jscript3.1. Navigator achieved ECMAScript compliance with JavaScriptl.3. Internet Explorer 5 introduced Jscript 5, which provides additional scripting capabilities such as try catch like in Java.

The World Wide Web Consortium came. up with an idea to standardize the basic objects that are available in the browsers when processing HTML documents. As a result of this a specification known as the Document Object Model (DOM) Level 1 came up. This model specifies a standard set of objects for representing HTML documents, a standard model of how these objects can be combined and a standard interface for accessing and manipulating them. The DOM is like an AN for HTML documents. However this API is not complete in the sense that it does not specify the events that occur when a user interacts with the HTML document.

Unlike programming languages like C, Java etc. which are complex, JavaScript provides simple syntax and can be learnt quickly and easily. JavaScript is an Objectbased language. It supports built in objects but no classes or inheritance. JavaScript supports two types of objects namely:

1 . Static objects: These objects combine general functions and data constructs. The properties of such objects are usually read only.
2. Dynamic objects: These objects support creation of instances. These objects resemble a template.

JavaScript features dynamic binding by which all objects are checked at run time. Also JavaScript is a loosely typed language. By loosely typed it means that the type of the variable declared need not be specified. A variable of one data type can contain a value of another data type elsewhere in the script.

JavaScript provides excellent event handling capabilities. For example when a form is submitted, JavaScript can be used to check whether the user has filled in every data before the form is sent back to the server. These type validations reduce the workload on the server and the number of times the data has to travel between the browser and the server. JavaScript has its own limitations such as access to database information, support e commerce etc.

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