The XMLHttpRequestEventTarget.onerror is the function called when an XMLHttpRequest transaction fails due to an error.
Syntax
XMLHttpRequest.onerror = callback;
Values
callback is the function to be executed when the request fails.
Example
var xmlhttp = new XMLHttpRequest(),
method = 'GET',
url = 'https://developer.mozilla.org/';
xmlhttp.open(method, url, true);
xmlhttp.onerror = function () {
console.log("** An error occurred during the transaction");
};
xmlhttp.send();