If you've been following the whole Ajax thing for a while, you�ve certainly seen this few lines of code hundreds of times. You know, the ones that check the readyState and status properties of a xmlHttpRequest.
Things change when you start developing Ajax-style with XmlHttpRequest. You are basically by-passing the browser native handling of HTTP and left dealing with the status code on your own. And, that�s a great opportunity. Why ? Because you can use it to relay information about the execution of your server-side scripts.
The Form Assembly: 
