XML and DHTML’s XMLHttpRequest Object

“Microsoft first implemented the XMLHttpRequest object in Internet Explorer 5 for Windows as an ActiveX object. Engineers on the Mozilla project implemented a compatible native version for Mozilla 1.0 (and Netscape 7). Apple has done the same starting with Safari 1.2.

Similar functionality is covered in a proposed W3C standard, Document Object Model (DOM) Level 3 Load and Save Specification. In the meantime, growing support for the XMLHttpRequest object means that is has become a de facto standard that will likely be supported even after the W3C specification becomes final and starts being implemented in released browsers (whenever that might be).
Creating the Object

Creating an instance of the XMLHttpRequest object requires branching syntax to account for browser differences in the way instances of the object are generated. For Safari and Mozilla, a simple call to the object’s constructor function does the job:

var req = new XMLHttpRequest();

For the ActiveX branch, pass the name of the object to the ActiveX constructor:

var req = new ActiveXObject(”Microsoft.XMLHTTP”);

The object reference returned by both constructors is to an abstract object that works entirely out of view of the user. Its methods control all operations, while its properties hold, among other things, various data pieces returned from the server. ”

Read the full article from Apple’s own Developer corner.