Why does window.open(…).onunload = function () { … } not work as I expect?

Question

I want to be able to tell when a window that I open is closed by the user. This is the code of my attempt at monitoring this:

  
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Answer 1

The window first loads with a blank page and then unloads the page, causing the unload event.
Your page then loads. Try attaching the event when the onload event fires to avoid this.

Simple demo

document.getElementById('openWindow').onclick = function () {
      var windowref = window.open('tests2.html');
      windowref.onload = function() {
            windowref.onunload =  function () {
                window.alert('hola!');
            };
      }
};