title: What are the limitation using before unload event?
body: <p>What is and what is not possible to do inside the <code>beforeunload</code> callback ?</p>
  
  <p>Is it possible to open an XHR/fetch and send data to the server ?
   If no, is it possible to just send data, without any success callback blocking ?</p>
  
  <p>Is it possible to change the location of the page with <code>window.location</code>? 
  How long does the function continue to execute? </p>
  
  <pre><code>window.addEventListener("beforeunload", function (event) {
      // code
  });
  </code></pre>