The XMLHttpRequest object was introduced by Microsoft as an ActiveX control for Internet Explorer 5. It was later added to Mozilla 1.0 and Safari 1.2. Internet Explorer 7 includes it as a native script object, rather than as an ActiveX control. Now, all serious browsers include support for this object. In Internet Explorer before version 7, you pass the name of the object to the ActiveXObject function:
var request = new ActiveXObject("Msxml2.XMLHTTP");
var request = new XMLHttpRequest();
Because ASP.NET AJAX gives you a set of standard classes to ensure consistent behavior when accessing Web resources, rather than using the browser-specific XMLHttpRequest object directly, I won't go into the details of the XMLHttpRequest object, except to point out some security and architectural issues.
The browsers all enforce ...