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The Document Object Model Dissected

TOC

1. Introduction
2. Gray's Anatomy\ Master of Your DOM
3. Use the DOM, Luke
4. Netscape: The DOM
5. Microsoft: DOM, the Sequel
6. DOM of the Future

Microsoft: DOM, the Sequel

Historically speaking, Netscape was the first out of the gate with their Document Object Model, originally implemented for developers in Navigator 2 and accompanying the birth of JavaScript 1.0. Microsoft essentially attempted to replicate Netscape's DOM for use in their Internet Explorer 3 browser. Because Netscape did not publish specifications for their early DOM or for JavaScript, and because Microsoft needed IE 3 to be compatible with Navigator (which was then the de facto standard), Microsoft was left to basically reverse engineer the Netscape DOM and JavaScript language, somewhat like re-creating a building without the original blueprint in hand. The result, unfortunately, was less than stellar compatibility: Internet Explorer 3 possessed many bugs and incomplete implementations of various DOM components.

After receiving heated negative energy from developers, Microsoft dug in their heels in preparing Internet Explorer 4 and re-approached their DOM with a new outlook. Whereas Netscape was planning several modest and a couple of significant additions to their DOM for their fourth generation browser (most notably, style sheets and layers, which are arguably one in the same), Microsoft went on the offensive. Rather than a tepid upgrade, the Internet Explorer 4 DOM was extremely ambitious, attempting to break down the anatomy of the page into many more, smaller, more manipulable components than the Netscape DOM attempted. Both companies touted their new DOM's as the bearers of "Dynamic HTML," but in reality Microsoft's implementation was far more "dynamic."

Microsoft's IE 4 Document Object Model was a hit with many developers, who began to migrate towards MSIE as their web development environment of choice. Combined with the proliferation of the MSIE browsers among users (since it became part of the operating system, nudge nudge), Microsoft was in control of the browser market. With the release of IE 5, Microsoft built upon the DOM foundation laid in version 4, with a variety of refinements. Overall, however, the structure of Microsoft's DOM has remained intact between IE 4 and IE 5.

To further encourage developers, Microsoft has published its DOM documentation in a very accessible format on the Web. Microsoft's MSDN Web Workshop contains a clean reference of Document Object Model components. Unlike Netscape, Microsoft has kept their DOM reference distinct from their JavaScript (a.k.a. "JScript") reference material, the latter which can be found at the Microsoft Scripting Technologies site. Unfortunately, Microsoft has a habit of shuffling their web pages frequently, so direct bookmarks to these sites may become extinct.

Turning to Microsoft's DOM reference, you will find several subheadings beneath the "DHTML References" topic: objects, properties, methods, and events should all ring familiar by now. The convenient structure of Microsoft's documentation allows you to cross-reference information via each of these indices -- thus, if you want to look up a property whose name you recall but whose owning object you do not, use the properties index. Once you click on a chosen property, the information sheet will report the objects to which it can belong (under the subheading "Applies To")..

Microsoft also indexes an additional topic which they call collections. A "collection," in Microsoft verbiage, is an array-based object. For example, the applet object contains properties and methods reflecting Java applets embedded into a page using the <applet> tag. However, the applet objects are contained within an array called applets[], and so you refer to a specific applet object via its index in this array; e.g. applets[2]. Thus, the entry for the applets array is listed in Microsoft's "collections" index, while the entry for the applet itself resides in the "objects" index.

Surfing through the index of objects in Microsoft's DOM, you can quickly see that it supports many more than the Netscape DOM.

Cleverly and conveniently, Microsoft has published their DOM so that it is easiest to reference while using MSIE 4. For instance, we'll look up the table object (an example, by the way, of an object which is not supported in the Netscape DOM). The reference sheets, seen below, contain quite a bit of information -- using the drop down menu you can switch between views of supported properties, methods, and events. The documentation will also indicate whether the component is new to IE 5, as compared to IE 4, via a note "This feature requires Microsoft® Internet Explorer 5 or later" on the reference page.

Armed with the above documentation, the DOM's are at your fingertips. Despite the fact that the entries may appear long and intimidating, most objects are constructed along the same patterns. In practice, reading the DOM reference typically boils down to verifying what properties and methods do or do not belong to a particular object and, in some cases, how exactly they behave.