how to identify onbeforeunload was caused by clicking close button

Question

How do I determine if onbeforeunload was caused by clicking the close button or a page refresh or generally what event caused onbeforeunload?

here is a snippet:

Answer 1

If the close button was pressed the value of e.clientY is negative. For the other possible sources i doubt there is a solution.

window.onbeforeunload = function() {
   var e = window.event;
   alert(e.clientX + " / " + e.clientY);
}