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Use JavaScript to update your Web pages on the fly.

Last month Brett introduced the Document Object Model, whose elements work behind the scenes to define your Web pages. This month he dives even deeper into the DOM. Learn how to create, remove, and change the parts of a DOM tree, and take the next step toward updating your Web pages on the fly!

If you followed my discussion in this series last month, then you got a first-hand look at what goes on when a Web browser displays one of your Web pages. As I explained then, when the HTML and CSS you've defined for your page is sent to a Web browser, it's translated from text to an object model. This is true whether the code is simple or complex, housed all in one file or in separate files. The browser then works directly with the object model, rather than the text files you supplied. The model the browser uses is called the Document Object Model. It connects objects representing the elements, attributes, and text in your documents. All the styles, values, and even most of the spaces in your HTML and CSS are incorporated into the object model. The specific model for a given Web page is called the page's DOM tree.

Understanding what a DOM tree is, and even knowing how it represents your HTML and CSS, is just the first step in taking control of your Web pages. Next, you need to learn how to work with the DOM tree for a particular Web page. For instance, if you add an element to the DOM tree, that element immediately appears in a user's Web browser -- without the page reloading. Remove some text from the DOM tree, and that text vanishes from the user's screen. You can change and interact with the user interface through the DOM, which gives you tremendous programming power and flexibility. Once you learn how to work with a DOM tree you've taken a huge leap toward mastering rich, interactive, dynamic Web sites.

Note that the following discussion builds on last month's "Exploiting the DOM for Web response;" if you haven't read that article, you might want to do so before you proceed here.

Cross browser, cross language

The Document Object Model is a W3C standard (see Resources for links to the W3C). Because of that, all modern Web browsers support the DOM, at least to some degree. While there is some variance among browsers, if you use core DOM functionality -- and pay attention to a few special cases and exceptions -- your DOM code will work on any browser in the same way. The code you write to modify a Web page in Opera will work on Apple's Safari, Firefox, Microsoft Internet Explorer, and Mozilla.

The DOM is also a cross-language specification; in other words, you can use it from most of the popular programming languages. The W3C defines several language bindings for the DOM. A language binding is simply an API defined to let you use the DOM for a specific language. For example, you can find well-defined DOM language bindings for C, Java, and JavaScript. So you can use the DOM from any of these languages. Language bindings are also available for several other languages, although many of these are not defined by the W3C, but instead by third parties.

In this series I'll focus on the JavaScript bindings into the DOM. That makes sense because most asynchronous application development is based on writing JavaScript code to run in a Web browser. With JavaScript and the DOM, you can modify the user interface on the fly, respond to user events and input, and more -- all using fairly standardized JavaScript.

All that said, I do encourage you to check out the DOM language bindings in other languages. For instance, you can use the Java language bindings to work not only with HTML, but also XML, as I'll discuss in a later article. So the lessons you'll learn here apply to far more than HTML, in many more environments than just client-side JavaScript.



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