Rewritten answer: see original answer in revision history
You could override the default send method of XMLHttpRequest with one that hijacks the readystatechange handler:
(function ()
{
var xhrSend = XMLHttpRequest.prototype.send;
XMLHttpRequest.prototype.send = function ()
{
var handler = this.onreadystatechange;
this.onreadystatechange = function ()
{
if (handler) {
if (handler.handleEvent) handler.handleEvent.apply(xhr, arguments);
else handler.apply(xhr, arguments);
}
if (this.readyState == 4)
{
// your oncomplete function here
this.onreadystatechange = handler;
}
};
xhrSend.apply(this, arguments);
};
})();
Edit: The above function doesn't work with jQuery requests, and so potentially it could fail with other libraries as well. The revision below addresses the issue with a setTimeout hack to delay the code that overrides the handler. Of course, with jQuery, you can just use the .ajaxSuccess() global handler, but for other libraries with similar behavior, this would be useful.
(function() {
function globalHandler() {
if (this.readyState == 4) {
// your oncomplete code here
}
}
var xhrSend = XMLHttpRequest.prototype.send;
XMLHttpRequest.prototype.send = function() {
var xhr = this;
if (xhr.addEventListener) {
xhr.removeEventListener("readystatechange", globalHandler);
xhr.addEventListener("readystatechange", globalHandler, false);
}
else {
function readyStateChange() {
if (handler) {
if (handler.handleEvent)
handler.handleEvent.apply(xhr, arguments);
else
handler.apply(xhr, arguments);
}
globalHandler.apply(xhr, arguments);
setReadyStateChange();
}
function setReadyStateChange() {
setTimeout(function() {
if (xhr.onreadystatechange != readyStateChange) {
handler = xhr.onreadystatechange;
xhr.onreadystatechange = readyStateChange;
}
}, 1);
}
var handler;
setReadyStateChange();
}
xhrSend.apply(xhr, arguments);
};
})();
http://jsfiddle.net/gilly3/FuacA/5/
I tested this in IE7-9, and the latest versions of Chrome and FF