HttpRequest And Timeout

I use an HttpRequest in my web application to launch an heavy
computation server side. The result is then send back to the browser.
The problem I got is that when the computation took too much time, then
I never get the result. I have to reload the page to get it.

I assume it's because my socket connection has time out, but my event
listener does not seems to get any event. Maybe I miss something:

function xmlhttpChange()
{
// looking fo a xmlhttp "loaded"
for (var i in nodeId2xmlHttp) {
if ((nodeId2xmlHttp[i] != null) && nodeId2xmlHttp[i].readyState ==
4) {
if (nodeId2xmlHttp[i].status==200) {
nodeId2xmlHttp[i] = null;
} else {
alert("Problem retrieving XML data (nodeId: " + i + ")");
nodeId2xmlHttp[i] = null;
}
}
}
}

My question is : how to handle XMLHttpRequest timeout. Any pointers?