On Exit alert/popup unblockable

<html>
<body>
<script>
var warning = true;
window.onbeforeunload = function() {
  if (warning) {
    return "You have made changes on this page that you have not yet confirmed. If you navigate away from this page you will lose your unsaved changes";
    }
}

$('form').submit(function() {
   window.onbeforeunload = null;
});
</script>
</body>
</html>