The XMLHttpRequestEventTarget.onload is the function called when an XMLHttpRequest transaction completes successfully.
Syntax
XMLHttpRequest.onload = callback;
Values
callback is the function to be executed when the request completes successfully.
Example
var xmlhttp = new XMLHttpRequest(),
method = 'GET',
url = 'https://developer.mozilla.org/';
xmlhttp.open(method, url, true);
xmlhttp.onload = function () {
// Do something with the retrieved data
};
xmlhttp.send();