An EventHandler that is called whenever the readyState attribute changes. The callback is called from the user interface thread. The XMLHttpRequest.onreadystatechange property contains the event handler to be called when the readystatechange (en-US) event is fired, that is every time the readyState property of the XMLHttpRequest changes.

Warning: This should not be used with synchronous requests and must not be used from native code.

The readystatechange event will not be fired when an XMLHttpRequest request is cancelled with the abort() method.

UPDATE: it's firing in the latest version of browsers (Firefox 51.0.1, Opera 43.0.2442.991, Safari 10.0.3 (12602.4.8), Chrome 54.0.2840.71, Edge, IE11). Example here - just reaload page few times.


XMLHttpRequest.onreadystatechange = callback;

  • callback is the function to be executed when the readyState changes.


var xhr = new XMLHttpRequest(),
    method = "GET",
    url = "https://developer.mozilla.org/";

xhr.open(method, url, true);
xhr.onreadystatechange = function () {
        if(xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {


Specification Status Comment
XMLHttpRequest Living Standard WHATWG living standard


We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help! (en-US)

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1 1.0 (1.7 or earlier) 7[1] (Yes) 1.2
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? 1.0 (Yes) ? ? ?

[1] Internet Explorer version 5 and 6 supported ajax calls using ActiveXObject().