javascript to check when the browser window is closed

by 
window.onbeforeunload = function (e) {
  var e = e || window.event;

  //IE & Firefox
  if (e) {
    e.returnValue="Are you sure?";
  }

  // For Safari
  return 'Are you sure?';
};

https://developer.mozilla.org/en/DOM/window.onbeforeunload

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