onbeforeunload_example.js

 window.addEventListener("beforeunload", function(e) {
  // check to cancel
  console.log("checking if we should display confirmation dialog");
  var shouldCancel = true;
  if (shouldCancel) {
    console.log("displaying confirmation dialog");
    e.preventDefault(); // this will display the confirmation dialog
  }
  // Chrome requires returnValue to be set
  e.returnValue = "";
});
window.addEventListener("unload", function(event) {
  // user confirmed they are sure
  // perform cleanup
  console.log("cleaning up");
});