XMLHttpRequestEventTarget.onprogress

The XMLHttpRequestEventTarget.onprogress is the function called periodically with information when an XMLHttpRequest before success completely.

XMLHttpRequest.onprogress = callback;

• callback is the function to be called periodically before the request is completed.

• event.loaded  the amount of data currently transfered.
• event.total  the total amount of data to be transferred.

XMLHttpRequest.onprogress = function (event) {
  event.loaded;
  event.total;
};

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

xmlhttp.open(method, url, true);
xmlhttp.onprogress = function () {
  //do something
};
xmlhttp.send();