An event handler 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 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.


XMLHttpRequest.onreadystatechange = callback;


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


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

xhr.open(method, url, true);
xhr.onreadystatechange = function () {
  // In local files, status is 0 upon success in Mozilla Firefox
  if(xhr.readyState === XMLHttpRequest.DONE) {
    var status = xhr.status;
    if (status === 0 || (status >= 200 && status < 400)) {
      // The request has been completed successfully
    } else {
      // Oh no! There has been an error with the request!


Specification Status Comment
XMLHttpRequest Living Standard WHATWG living standard

Browser compatibility

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