maybe you take a look at this one and a look at W3C: XMLHttpRequest it’s the same if your browser supports xhr.onload. Requires XMLHttpRequest 2)
You can also write a wrapping function that emulates xhr.onload if it’s not present.
(I think you need to override XMLHttpRequest.prototype.onload = function(args){//calling onreadystatechanges somehow}). If you only support modern browsers using xhr.onload should be available – the best solution is using a framework (like jquery or mootools that provides wrapping functionality for that.