demonstration of what can be done inside an `unload`/`beforeunload` listener

unload-event-listener-actions-limitation-demo.js

window.addEventListener('unload', (e) => {
  function sleep(delay) {
    const start = new Date().getTime();
    while (new Date().getTime() < start + delay);
  }
  
  // unloading won't finish until 10 full seconds pass!
  sleep(10000); 
  
  setTimeout(() => {
    // this part would be ignored since unloading will already
    // finish by now, thus it won't wait for this async call
    sleep(10000); 
  }, 0);
});