title: Attach arbitrary data to a XMLHttpRequest object
body: <p>I want to use one callback for many different XHR objects, however I also want to know what made the XHR call when it sends a message via onload/onerror/onprogress.
  That is, assuming I get some message through either of the callbacks, I want the message to contain a member that I assigned when making the request.
  Logically, I could simply set a field in the XHR object, and then use the target member of the event that points to the original XHR object...except my member is gone.
  Nothing that I set manually stays set in the target member of the event.</p>
  
  <p>The only other way I can think of is to keep binding new copies of the callbacks, e.g. onload.bind(whatever), but that is really not optimal.</p>
  
  <p>So, long story short, is there a way to attach data to a XHR request, so it can later on be identified?</p>