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Working with JavaScript's Document Object Model

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The true power of JavaScript comes not from the language itself but from the DOM, or Document Object Model. This is a set of objects that allow your script to integrate with the browser and work with documents, windows, forms, and other components. This lesson introduces the DOM and shows how to use its objects to work with documents and browser history.

When I first wrote about JavaScript in 1996, I was excited about its potential for the Web. While it had its limitations, it went beyond simple HTML and offered true interactivity to Web pages. In the years since then, JavaScript has lost most of its limitations and has become an essential part of the Web. Image rollovers, pop-up messages, and form validation are now commonplace, and JavaScript is still the language of choice for these and many other applications.

My book, "Sams Teach Yourself JavaScript in 24 Hours," is now in its third edition, revised and updated to cover the latest browsers and features. This book starts with the fundamentals of JavaScript—statements, variables, functions, strings, and objects—and moves on to a wide variety of complete examples, including sophisticated techniques such as Dynamic HTML.

While JavaScript is widely used on today's Web, it is by no means widely understood—a great many pages are created by copying scripts from other pages, or from books. While this book certainly has some great scripts to copy, it also encourages you to step beyond simple cut-and-paste scripting and really get a feel for scripting from scratch. Even if you've never written anything but HTML so far, you'll find it easy to learn JavaScript, and you'll be creating interactive pages with scripts in no time.

—Michael Moncur

This chapter is from the book

You've arrived at Part III of this book. (If you've been reading nonstop, it's been eight hours, so you might want to get some sleep before you continue.) In this part, you'll explore some of the most important objects used with JavaScript.

In Hour 8, "Using Math and Date Functions," you learned about JavaScript's support for objects, which allow you to store data in all sorts of interesting ways. However, the objects you'll use the most are those in the Document Object Model (DOM), which let your scripts manipulate Web pages, windows, and documents.

In this hour, you will explore this hierarchy of objects. Hour 9 covers the following topics:

  • How to access the various objects in the DOM

  • Working with windows using the window object

  • Working with Web documents with the document object

  • Using objects for links and anchors

  • Using the location object to work with URLs

  • Getting information about the browser with the navigator object

  • Creating JavaScript-based Back and Forward buttons

Understanding the Document Object Model

One advantage that JavaScript has over basic HTML is that scripts can manipulate the Web document and its contents. Your script can load a new page into the browser, work with parts of the browser window and document, open new windows, and even modify text within the page dynamically.

To work with the browser and documents, JavaScript uses a hierarchy of parent and child objects called the Document Object Model, or DOM. These objects are organized into a tree-like structure, and represent all of the content and components of a Web document.

Like other objects you've explored, the objects in the DOM have properties, which describe the Web page or document, and methods, which allow you to work with parts of the Web page.

When you refer to an object, you use the parent object name followed by the child object name or names, separated by periods. For example, JavaScript stores objects to represent images in a document as children of the document object. The following refers to the image9 object, a child of the document object, which is a child of the window object:

window.document.image9

The window object is the parent object for all the objects we will be looking at in this hour. Figure 9.1 shows this section of the DOM object hierarchy and a variety of its objects.

Figure 9.1. The JavaScript browser object hierarchy.

NOTE

This diagram only includes the basic browser objects that will be covered in this hour. These are actually a small part of the Document Object Model, which you'll learn more about starting with Hour 18, "Working with Style Sheets."

History of the DOM

Starting with the introduction of JavaScript 1.0 in Netscape 2.0, JavaScript has included objects to represent parts of a Web document and other browser features. However, there was never a true standard. While both Netscape and Microsoft Internet Explorer included many of the same objects, there was no guarantee that the same objects would work the same way in both browsers, let alone in less common browsers.

The bad news is that there are still differences between the browsers—but here's the good news. Since the release of Netscape 3.0 and Internet Explorer 4.0, all the basic objects (those covered in this hour) are supported in much the same way in both browsers, and new DOM standards are supported by the latest versions of Netscape and Internet Explorer.

While all this standardization doesn't change how the objects described in this hour work, you'll be thankful for it as you move into the advanced features of the DOM later in this book.

DOM Levels

The W3C (World-Wide Web Consortium) has recently developed the DOM level 1 standard. This standard defines not only basic objects, but an entire set of objects that encompass all parts of an HTML document. A level 2 DOM standard is also under development.

The basic object hierarchy described in this hour is informally referred to as DOM level 0, and the objects are included in the DOM level 1 standard. You'll learn how to use the full set of Level 1 DOM objects in Part V of this book.

TIP

The Level 1 and Level 2 DOM objects allow you to modify a Web page in real time after it has loaded. This is called dynamic HTML (DHTML) and you'll learn more about it in Part V.

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