<html> 
  <head>
    <title>Loading...</title>
  </head>
  <body onload="attachUnloadListener()"> 
    <script> 
      function attachUnloadListener() {
        window.addEventListener('unload', onUnload, false);
        document.title = "set cookie on unload";
      }

      function onUnload() {
        document.cookie = "onunloadCookie=foo";
      }
    </script> 
    <p>Navigate to another page to set cookie</p>
  </body> 
</html>