Sams Teach Yourself XML in 21 Days

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Sams Teach Yourself XML in 21 Days, 3rd Edition

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Recommended Book

Recommended Book 

Sams Teach Yourself XML in 21 Days, 3rd Edition

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Introducing the W3C DOM

When you're dealing with programming, you need some way of working with the data in XML documents, and the W3C DOM gives you that. The DOM lets you consider an XML document as a tree of nodes. In the DOM, every data item is a node, and a child element or enclosed text becomes a subnode. When everything in a document—text, attributes, elements, and so on—becomes a node, you can use some well-defined functions to access and work with those nodes in a standard way. These are the node types in the DOM:

• Element
• Attribute
• Text
• CDATA section
• Entity reference
• Entity
• Processing instruction
• Comment
• Document
• Document type
• Document fragment
• Notation

Let's take a look at a sample XML document for today's work:

<?xml version="1.0" encoding="UTF-8"?>
<document>
    <title>
        The Report
    </title>
    <text>
        All clear on the Western front.
    </text>
</document>

This document has an element node, <document>, that has two child nodes, <title> and <text>, and these two nodes are siblings of each other. Each of the <title> and <text> element nodes contains text, which is treated as a child node. Here's what this document looks like as a tree of nodes:

         <document>
            |
   ----------------------------------
   |                 |
 <title>              <text>
   |                 |
The Report       All clear on the Western front.

When you use the methods defined in the W3C DOM, you can navigate along the various branches of a document's tree by using methods such as nextChild to move to the next child node. Using methods in this way in the W3C DOM and becoming familiar with how they work are a good part of today's work.

The DOM Levels

There are four different levels of the DOM specification:

• Level 0— There actually is no DOM Level 0, but that's how W3C refers to the DOM in early versions of the popular browsers—Netscape Navigator 3.0 and Microsoft Internet Explorer 3.0.
• Level 1— This level is a W3C recommendation. You can find the documentation for this level at http://www.w3.org/TR/REC-DOM-Level-1.
• Level 2— Now a recommendation, this level of the DOM is more advanced than Level 1 and includes a style sheet object model. It also lets you traverse a document, has a built-in event model, and supports XML namespaces. You can find the documentation for this level at http://www.w3.org/TR/DOM-Level-2.
• Level 3— This level is still in the working draft stage and will address document loading and saving. It will also address document views and formatting, event groups, and more. You can find the core working draft at http://www.w3.org/TR/DOM-Level-3-Core.

From a JavaScript perspective, the only complete DOM implementation is in Internet Explorer 6 and later, which supports DOM Level 1. You can read all about Internet Explorer's support for the DOM at http://msdn.microsoft.com/library/default.asp?url=/workshop/author/dom/domoverview.asp. (Note, however, that Microsoft has the habit of changing its URLs very often, so by the time you read this book, this URL may be out-of-date.)

Today you'll be working with DOM Level 1—the only level of the DOM in widespread use today. You'll also be working with objects in JavaScript today. For example, there's an object that corresponds to the document itself, an object that corresponds to elements, one that corresponds to attributes, and so forth. When you've have an object corresponding to an element, for example, you can use the object's methods and properties to learn all about that element—what its name is, how many children it has, and so on. To dig into the programming, then, you need to understand the objects in the DOM.