JavaScript and XML - 21.1.4 XML Documents from Data Islands
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Microsoft has extended HTML with an <xml> tag that creates an XML data island within the surrounding “sea” of HTML markup. When IE encounters this <xml> tag, it treats its contents as a separate XML document, which you can retrieve using document.getElementById() or other HTML DOM methods. If the <xml> tag has a src attribute, the XML document is loaded from the URL specified by that attribute instead of being parsed from the content of the <xml> tag.
If a web application requires XML data, and the data is known when the application is first loaded, there is an advantage to including that data directly within the HTML page: the data is already available, and the web application does not have to estab lish another network connection to download the data. XML data islands can be a useful way to accomplish this. It is possible to approximate IE data islands in other browsers using code like that shown in Example 21-5.
Example 21-5. Getting an XML document from a data island
/**
* Return a Document object that holds the contents of the <xml> tag
* with the specified id. If the <xml> tag has a src attribute, an XML
* document is loaded from that URL and returned instead.
*
* Since data islands are often looked up more than once, this function caches
* the documents it returns.
*/
XML.getDataIsland = function(id) {
var doc;
// Check the cache first
doc = XML.getDataIsland.cache[id];
if (doc) return doc;
// Look up the specified element
doc = document.getElementById(id);
// If there is a "src" attribute, fetch the Document from that URL
var url = doc.getAttribute('src');
if (url) {
doc = XML.load(url);
}
// Otherwise, if there was no src attribute, the content of the <xml>
// tag is the document we want to return. In Internet Explorer, doc is
// already the document object we want. In other browsers, doc refers to
// an HTML element, and we've got to copy the content of that element
// into a new document object
else if (!doc.documentElement) {// If this is not already a document...
// First, find the document element within the <xml> tag. This is
// the first child of the <xml> tag that is an element, rather
// than text, comment, or processing instruction
var docelt = doc.firstChild;
while(docelt != null) {
if (docelt.nodeType == 1 /*Node.ELEMENT_NODE*/) break;
docelt = docelt.nextSibling;
}
// Create an empty document
doc = XML.newDocument();
// If the <xml> node had some content, import it into the new document
if (docelt) doc.appendChild(doc.importNode(docelt, true));
}
// Now cache and return the document
XML.getDataIsland.cache[id] = doc;
return doc;
};
XML.getDataIsland.cache = {}; // Initialize the cache
This code does not perfectly simulate XML data islands in non-IE browsers. The HTML standard requires browsers to parse (but ignore) tags such as <xml> that they don’t know about. This means that browsers don’t discard XML data within an <xml> tag. It also means that any text within the data island is displayed by default. An easy way to prevent this is with the following CSS stylesheet:
<style type="text/css">xml { display: none; }</style>
Another incompatibility is that non-IE browsers treat the content of XML data islands as HTML rather than XML content. If you use the code in Example 21-5 in Firefox, for example, and then serialize the resulting document (you’ll see how to do this later in the chapter), you’ll find that the tag names are all converted to upper case because Firefox thinks they are HTML tags. In some cases, this may be problematic; in many other cases, it is not. Finally, notice that XML namespaces break if the browser treats the XML tags as HTML tags. This means that inline XML data islands are not suitable for things like XSL stylesheets (XSL is covered in more detail later in this chapter) because those stylesheets always use namespaces.
If you want the network benefits of including XML data directly in an HTML page, but don’t want the browser incompatibilities that come with using XML data islands and the <xml> tag, consider encoding your XML document text as a JavaScript string and then parsing the document using code like that shown in Example 21-4.
Please check back tomorrow for the continuation of this article.
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 This article is excerpted from chapter 21 of the book JavaScript: The Definitive Guide, Fifth Edition, written by David Flanagan (O'Reilly; ISBN: 0596101996). Check it out today at your favorite bookstore. Buy this book now.
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