hack2.js

(function () {
  var XHREventTargetPrototype = Object.getPrototypeOf(XMLHttpRequest.prototype);
  var desc = Object.getOwnPropertyDescriptor(XHREventTargetPrototype, "onload");
  var real_set_onload = desc.set;
  Object.defineProperty(XHREventTargetPrototype, "onload", {
    set: function onload(real_callback) {
      real_set_onload.call(this, function fake_callback() {
        console.log("loaded:", arguments);
        return real_callback.apply(this, arguments);
      });
    }
  });
})();