17.4 DOM Compatibility with Netscape 4

Netscape 4 does not even come close to implementing the DOM standard. In particular, Netscape 4 provides no way to access or set attributes on arbitrary elements of a document. Netscape 4 supports the Level 0 DOM API, of course, so elements such as forms and links can be accessed through the forms[] and links[] arrays, but there is no general way to traverse the children of these elements or set arbitrary attributes on them. Furthermore, Netscape 4 does not have the ability to "reflow" document content in response to changes in element size.

Despite these restrictions, Netscape 4 does provide an API that allows access to and manipulation of the crucial "dynamic elements" used to implement DHTML effects. In the Netscape 4 API, these elements are known as layers; they float above the rest of the document and can be moved, resized, and modified independently of the other elements of the document. Layers are typically implemented using CSS style sheets, and the Netscape 4 Layer API is discussed in detail in Chapter 18.

What follows is simply an overview that explains how you can create, access, and modify the content of individual layer elements within a document. Although Netscape 4 does not support anything like the DOM standard, its Layer API allows you to achieve some of the same dynamic effects that are possible with the standard API. Note that the Layer API was submitted to the W3C for consideration as part of the DOM standard, but no part of this API was ever standardized. Because Netscape 6 is based on a complete rewrite of Netscape 4, the Layer API has been abandoned and is not supported in Netscape 6 (or in Mozilla).

Layers can be created in a document using the <layer> tag, a proprietary Netscape extension to HTML. More commonly, however, you create a layer in a Netscape 4 document using standard CSS positioning attributes (which will be explained in detail in Chapter 18). Any element made dynamic with CSS style attributes is treated as a layer by Netscape 4 and can be manipulated using the Layer API. (Note, though, that Netscape 4 does not allow all elements to be made dynamic. To be safe, a <div> wrapper element is usually used around any element that is to be dynamic.) JavaScript can also dynamically create layers using the Layer( ) constructor, which you can read about in the client-side reference section of this book.

Once you've created dynamic elements, or layers, in your document, Netscape 4 allows you to access them via a simple extension of the Level 0 DOM API. Just as you access form elements through a forms[] array and image elements through an images[] array, so do you access layers through a layers[] array of the Document object. If the first layer in a document has a name attribute of "layer1", you can refer to that layer element with any of the following expressions:

document.layers[0]         // Index the array with a number
document.layers['layer1']  // Index the array with an element name
document.layer1            // Named layers become a document property 

If a layer has no name attribute but has an id attribute, the value of this attribute is used as the layer name instead.

Layers in your documents are represented by Layer objects that define a number of useful properties and methods you can use to move, resize, show, hide, and set the stacking order of the layer. These properties and methods are related to CSS style attributes and will be discussed in Chapter 18. The most interesting thing about the Layer object is that it contains a Document object of its own: the content of a layer is treated as an entirely separate document from the document that contains the layer. This allows you to modify the content displayed by a layer by dynamically rewriting the content of the layer using the document.write( ) and document.close( ) methods. You can also dynamically load documents into a layer using Layer's load( ) method. Finally, note that layers may themselves contain layers, and you can refer to such nested layers with expressions like this:

// The second layer nested within the layer named "mylayer"
document.mylayer.document.layers[1];