AJAX and PHP

I admit that the following piece of code may have puzzled you:
if (!xmlResponse || !xmlResponse.documentElement)
throw("Invalid XML structure:\n" + xmlHttp.responseText);
Chapter 2
Apparently, if xmlResponse is void, we risk generating another error when trying to read its
documentElement property. In practice, the JavaScript interpreter only evaluates logical expressions
when necessary, and it does so from left to right. In our particular case, if (!xmlResponse) is true,
the second expression isn't evaluated at all, because the end result is true anyway. This feature,
which is implemented in JavaScript and other languages, is called short-circuit evaluation and
you can read more about it here: http://www.webreference.com/javascript/reference/
core/expr.html.
Creating XML Structures
XML and DOM are everywhere. In this chapter, you used the DOM to create HTML elements on
the existing DOM object called document, and you also learned how to read XML documents
received from the server. An important detail that we didn't cover was creating brand new XML
documents using JavaScript's DOM. You may need to perform this kind of functionality if you
want to create XML documents on the client, and send them for reading on the server.
We won't go through more examples, but we will only show you the missing bits. The trick with
creating a brand new XML document is creating the XML document itself. When adding elements
to the HTML output, you used the implicit document object, but this is not an option when you
need to create a new document.
When creating a new DOM object with JavaScript, we're facing the same problem as with creating
XMLHttpRequest objects; the method of creating the object depends on the browser. The following
function is a universal function that returns a new instance of a DOM object:
function createDomObject()
{
// will store reference to the DOM object
var xmlDoc;
// create XML document
if (document.implementation && document.implementation.createDocument)
{
xmlDoc = document.implementation.createDocument("", "", null);
}
// works for Internet Explorer
else if (window.ActiveXObject)
{
xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
}
// returns the created object or displays an error message
if (!xmlDoc)
alert("Error creating the DOM object.");
else
return xmlDoc;
}
After executing this function, you can use the created DOM object to perform whatever actions
you want. For more details about creating the DOM object check the following link:
http://www.webreference.com/programming/javascript/domwrapper/index.html. For details
of using the DOM object, refer to the DOM articles mentioned earlier in this chapter.
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