Hi,
I'll start at the beginning of the Ajax sequence: creating and dispatching an XMLHttpRequest from the browser. Unfortunately, the method to create an XMLHttpRequest differs from browser to browser. The JavaScript function in Listing 2 smoothes out these browser-dependent wrinkles, detecting the correct approach for the current browser and returning an XMLHttpRequest ready to use. It's best to think of this as boilerplate code: simply copy it into your JavaScript library and use it when you need an XMLHttpRequest.

/*
* Returns a new XMLHttpRequest object, or false if this browser
* doesn't support it
*/
function newXMLHttpRequest() {

var xmlreq = false;

if (window.XMLHttpRequest) {

// Create XMLHttpRequest object in non-Microsoft browsers
xmlreq = new XMLHttpRequest();

} else if (window.ActiveXObject) {

// Create XMLHttpRequest via MS ActiveX
try {
// Try to create XMLHttpRequest in later versions
// of Internet Explorer

xmlreq = new ActiveXObject("Msxml2.XMLHTTP");

} catch (e1) {

// Failed to create required ActiveXObject

try {
// Try version supported by older versions
// of Internet Explorer

xmlreq = new ActiveXObject("Microsoft.XMLHTTP");

} catch (e2) {

// Unable to create an XMLHttpRequest with ActiveX
}
}
}

return xmlreq;
}