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15.1 Overview of the DOM

15.1 Overview of the DOM

The Document Object Model, or DOM, is the fundamental API for representing and manipulating the content of HTML and XML documents. The API is not particularly complicated, but there are a number of architectural details you need to understand. First, you should understand that the nested elements of an HTML or XML document are represented in the DOM as a tree of objects. The tree representation of an HTML document contains nodes representing HTML tags or elements, such asand

, and nodes representing strings of text. An HTML document may also contain nodes representing HTML comments. Consider the following simple HTML document:

Sample Document

An HTML Document

This is a simple document.

The DOM representation of this document is the tree pictured in Figure 15-1 .

If you are not already familiar with tree structures in computer programming, it is helpful to know that they borrow terminology from family trees. The node directly above a node is the parent of that node. The nodes one level directly below another node are the children of that node. Nodes at the same level, and with the same parent, are siblings. The set of nodes any number of levels below another node are the descendants of that node. And the parent, grandparent, and all other nodes above a node are the ancestors of that node.

Each box in Figure 15-1 is a node of the document and is represented by a Node object. We’ll talk about the properties and methods of Node in some of the sections that follow, and you can look up those properties and methods under Node in Part IV . Note that the figure contains three different types of nodes. At the root of the tree is the Document node that represents the entire document. The nodes that represent HTML elements are Element nodes, and the nodes that represent text are Text nodes. Document, Element, and Text are subclasses of Node and have their own entries in the reference section. Document and Element are the two most important DOM classes, and much of this chapter is devoted to their properties and methods.

Node and its subtypes form the type hierarchy illustrated in Figure 15-2 . Notice that there is a formal distinction between the generic Document and Element types, and the HTMLDocument and HTMLElement types. The Document type represents either an HTML or an XML document, and the Element class represents an element of such a document. The HTMLDocument and HTMLElement subclasses are specific to HTML documents and elements. In this book, we often use the generic class names Document and Element, even when referring to HTML documents. This is true in the reference section as well: the properties and methods of the HTMLDocument and the HTMLElement types are documented in the Document and Element reference pages.

It is also worth noting in Figure 15-2 that there are many subtypes of HTMLElement that represent specific types of HTML elements. Each defines JavaScript properties to mirror the HTML attributes of a specific element or group of elements (see §15.4.1 ). Some of these element-specific classes define additional properties or methods that go beyond simple mirroring of HTML syntax. These classes and their additional features are covered in the reference section.

Finally, note that Figure 15-2 shows some node types that haven’t been mentioned so far. Comment nodes represent HTML or XML comments. Because comments are basically strings of text, these nodes are much like the Text nodes that represent the

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displayed text of a document. CharacterData, the common ancestor of both Text and Comment, defines methods shared by both nodes. The Attr node type represents an XML or HTML attribute, but it is almost never used because the Element class defines methods for treating attributes as name/value pairs rather than document nodes. The DocumentFragment class (not pictured) is a kind of Node that never exists in an actual document: it represents a sequence of Nodes that do not have a common parent. DocumentFragments are useful for some document manipulations and are covered in §15.6.4 . The DOM also defines infrequently used types to represent things like doctype declarations and XML processing instructions.

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