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WebReference.com - Chapter 17 of JavaScript: The Definitive Guide (4th Ed), from O'Reilly & Associates (5/15)

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JavaScript: The Definitive Guide (4th Ed)

Language-Independent DOM Interfaces

Although the DOM standard grew out of a desire to have a common API for dynamic HTML programming, the DOM is not of interest only to web scripters. In fact, the standard is currently most heavily used by server-side Java and C++ programs that parse and manipulate XML documents. Because of its many uses, the DOM standard is defined to be language-independent. This book describes only the JavaScript binding of the DOM API, but you should be aware of a few other points. First, note that object properties in the JavaScript binding are typically mapped to pairs of get/set methods in other language bindings. Thus, when a Java programmer asks you about the getFirstChild( ) method of the Node interface, you need to understand that the JavaScript binding of the Node API doesn't define a getFirstChild( ) method. Instead, it simply defines a firstChild property, and reading the value of this property in JavaScript is equal to calling getFirstChild( ) in Java.

Another important feature of the JavaScript binding of the DOM API is that certain DOM objects behave like JavaScript arrays. If an interface defines a method named item( ), objects that implement that interface behave like read-only numerical arrays. For example, suppose you've obtained a NodeList object by reading the childNodes property of a node. You can obtain the individual Node objects in the list by passing the desired node number to the item( ) method, or, more simply, you can simply treat the NodeList object as an array and index it directly. The following code illustrates these two options:

var n = document.documentElement;  // This is a Node object.
var children = n.childNodes;       // This is a NodeList object.
var head = children.item(0);       // Here is one way to use a NodeList.
var body = children[1];            // But this way is easier!

Similarly, if a DOM object has a namedItem( ) method, passing a string to this method is the same as using the string as an array index for the object. For example, the following lines of code are all equivalent ways to access a form element:

var f = document.forms.namedItem("myform");
var g = document.forms["myform"];
var h = document.forms.myform;

Because the DOM standard may be used in a variety of ways, the architects of the standard were careful to define the DOM API in a way that would not restrict the ability of others to implement the API as they saw fit. Specifically, the DOM standard defines interfaces instead of classes. In object-oriented programming, a class is a fixed data type that must be implemented exactly as specified. An interface, on the other hand, is a collection of methods and properties that must be implemented together. Thus, an implementation of the DOM is free to define whatever classes it sees fit, but those classes must define the methods and properties of the various DOM interfaces.

This architecture has a couple of important implications. First, the class names used in an implementation might not correspond directly to the interface names used in the DOM standard (and in this book). Second, a single class may implement more than one interface. For example, consider the Document object. This object is an instance of some class defined by the web browser implementation. We don't know what the specific class is, but we do know that it implements the Document interface; that is, all methods and properties defined by Document are available to us through the Document object. Since web browsers work with HTML documents, we also know that the Document object implements the HTMLDocument interface and that all methods and properties defined by that interface are available to us as well. Furthermore, if a web browser supports CSS style sheets and implements the DOM CSS module, the Document object also implements the DocumentStyle and DocumentCSS DOM interfaces. And if the web browser supports the Events and Views modules, Document implements the DocumentEvent and DocumentView interfaces as well.

Because the DOM is broken into independent modules, it defines a number of minor add-on interfaces, such as DocumentStyle, DocumentEvent, and DocumentView, that define only one or two methods each. Interfaces such as these are never implemented independently of the core Document interface, and for this reason, I do not document them independently. When you look up the Document interface in the DOM reference section, you'll find that it also lists the methods and properties of its various add-on interfaces. Similarly, if you look up one of the add-on interfaces, you'll simply find a cross-reference to the core interface with which it is associated. The exception to this rule is when the add-on interface is a complex one. For example, the HTMLDocument interface is always implemented by the same object that implements the Document object, but because it adds substantial new functionality, I have given it a reference page of its own.

Another important fact you need to understand is that since the DOM standard defines interfaces instead of classes, it does not define any constructor methods. If you want to create a new Text object to insert into a document, for example, you cannot simply say:

var t = new Text("this is a new text node");  // No such constructor! 

Since it cannot define constructors, the DOM standard instead defines a number of useful factory methods for creating objects in the Document interface. So, to create a new Text node for a document, you would write the following:

var t = document.createTextNode("this is a new text node"); 

Factory methods defined by the DOM have names that begin with the word "create". In addition to the factory methods defined by Document, a few others are defined by DOMImplementation and available as document.implementation.


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Created: November 28, 2001
Revised: November 28, 2001

URL: http://webreference.com/programming/javascript/definitive/chap17/5.html