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JavaScript: The Definitive GuideJavaScript: The Definitive GuideSearch this book

1.6. Client-Side JavaScript Features

Another possible use of JavaScript is for writing programs to perform arbitrary computations. You can write simple scripts, for example, that compute Fibonacci numbers, or search for primes. In the context of the Web and web browsers, however, a more interesting application of the language might be a program that computed the sales tax on an online order, based on information supplied by the user in an HTML form. As mentioned earlier, the real power of JavaScript lies in the browser and document-based objects that the language supports. To give you an idea of JavaScript's potential, the following sections list and explain the important capabilities of client-side JavaScript and the objects it supports.

1.6.1. Control Document Appearance and Content

The JavaScript Document object, through its write( ) method, which we have already seen, allows you to write arbitrary HTML into a document as the document is being parsed by the browser. For example, you can include the current date and time in a document or display different content on different platforms.

You can also use the Document object to generate documents entirely from scratch. Properties of the Document object allow you to specify colors for the document background, the text, and the hypertext links within it. This amounts to the ability to generate dynamic and conditional HTML documents, a technique that works particularly well in multiframe documents. Indeed, in some cases dynamic generation of frame content allows a JavaScript program to replace a traditional server-side script entirely.

Internet Explorer 4 and Netscape 4 support proprietary techniques for producing Dynamic HTML effects that allow document content to be dynamically generated, moved, and altered. IE 4 also supports a complete DOM that gives JavaScript access to every single HTML element within a document. And IE 5.5 and Netscape 6 support the W3C DOM standard (or at least key portions of it), which defines a standard, portable way to access all of the elements and text within an HTML document and to position them and modify their appearance by manipulating their Cascading Style Sheets (CSS) style attributes. In these browsers, client-side JavaScript has complete power over document content, which opens an unlimited world of scripting possibilities.

1.6.3. Interact with HTML Forms

Another important aspect of client-side JavaScript is its ability to interact with HTML forms. This capability is provided by the Form object and the form element objects it can contain: Button, Checkbox, Hidden, Password, Radio, Reset, Select, Submit, Text, and Textarea objects. These element objects allow you to read and write the values of the input elements in the forms in a document. For example, an online catalog might use an HTML form to allow the user to enter his order and could use JavaScript to read the input from that form in order to compute the cost of the order, the sales tax, and the shipping charge. JavaScript programs like this are, in fact, very common on the Web. We'll see a program shortly that uses an HTML form and JavaScript to allow the user to compute monthly payments on a home mortgage or other loan. JavaScript has an obvious advantage over server-based scripts for applications like these: JavaScript code is executed on the client, so the form's contents don't have to be sent to the server in order for relatively simple computations to be performed.

Another common use of client-side JavaScript with forms is for validating form data before it is submitted. If client-side JavaScript is able to perform all necessary error checking of a user's input, no round trip to the server is required to detect and inform the user of trivial input errors. Client-side JavaScript can also perform preprocessing of input data, which can reduce the amount of data that must be transmitted to the server. In some cases, client-side JavaScript can eliminate the need for scripts on the server altogether! (On the other hand, JavaScript and server-side scripting do work well together. For example, a server-side program can dynamically create JavaScript code on the fly, just as it dynamically creates HTML.)

1.6.6. Still More Features

In addition to the features I have already discussed, JavaScript has many other capabilities, including the following:

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