Ajax call - ready state flow confusion

Question

I was developing using Ajax call and while debugging, I found Ajax is sending request/response so many time than I actually imagined.

Long time ago I got good document describing what's happening behind the scene but I lost it.

Nowadays Ajax tutorials on the web only talks about how to code and those IF statement only checks readystate == 4 and status == 200 which does not provide good explanation for someone like me.

I tested the flow with code below and output is I think weird. My confusion is why ready 4 state is showing up twice? According to the definition, ready 4 means completed so there should be no reason to complete twice?

OUTPUT

START
ready 1                //loading
START
ready 2                //loaded
ready 2 status=200     //loaded
START
ready 3                //interactive
ready 3 status=200     //interactive
START
ready 4                //complete
START
ready 4                //complete   ... again???

TEST CODE snippet

xmlHttp.onreadystatechange = function() {

                alert("START");

                if(xmlHttp.readyState == 0) {
                    alert('ready 0');
                    alert('ready 0 status=' + xmlHttp.status);
                }

                if(xmlHttp.readyState == 1) {
                    alert('ready 1');
                    alert('ready 1 status=' + xmlHttp.status);
                }

                if(xmlHttp.readyState == 2) {
                    alert('ready 2');
                    alert('ready 2 status=' + xmlHttp.status);
                }

                if(xmlHttp.readyState == 3) {
                    alert('ready 3');
                    alert('ready 3 status=' + xmlHttp.status);
                }

                if(xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                    alert('ready 4');
                } 
   }                 

Answer 1

You can read in quirksmode about the way different browsers behave with AJAX calls and readyState.

I found this link that claims using the Abort command will issue a readystate 4 (have not tested it) - are you using Abort?