Why Use onload in Cross-domain Ajax


Generally the XMLHttpRequest.onreadystatechange property is used in an Ajax request.

var xhr = new XMLHttpRequest(),
xhr.open("GET", "http://zqzhang.github.io", true);
xhr.onreadystatechange = function () {
    if(xhr.readyState === xhr.DONE && xhr.status === 200) {


The XMLHttpRequest.onload event handler, however, is suggested to be used in cross-domain Ajax with Cross-Origin Resource Sharing (CORS) instead of the onreadystatechange event handler.

var xhr = new XMLHttpRequest();
xhr.open("GET", "http://zqzhang.github.io:8080", true);
xhr.onload = function(){  // instead of onreadystatechange
    // do something


Is onload equal to readyState==4 in XMLHttpRequest?

Stackoverflow records this question and answers here. Copy the most accepted answer here:

This is almost always true. One significant difference, however, is that the onreadystatechange event handler also gets triggered with readyState==4 in the cases where the onerror handler is usually triggered (typically a network connectivity issue). It gets a status of 0 in this case. I’ve verified this happens on the latest Chrome, Firefox and IE.

So if you are using onerror and are targeting modern browsers, you should not use onreadystatechange but should use onload instead, which seems to be guaranteed to only be called when the HTTP request has successfully completed (with a real response and status code). Otherwise you may end up getting two event handlers triggered in case of errors (which is how I empirically found out about this special case.)