weizman

var f = (e, t) => {
    return (5, 6) + 2;
}

var e = function () {
    return f(8,8) + 4;
}

window['a b'] = function () {
    return e(5, 7);

weizman

// run this code in devtools console and than try to shutdown 
// the tab - see how Chrome suddenly respects `debugger;` statement!
window.onunload = (e) => {
  debugger;
};

weizman

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(() => {

weizman

function listener() {}

const frameset = document.createElement('frameset');
const body = document.body;

window.onunload = listener;
window.onbeforeunload = listener;

body.onunload       === listener; // true
body.onbeforeunload === listener; // true

weizman

function successful_listener(e) {
  console.log('would successfully fire!');
}

function failing_listener(e) {
  console.log('would never fire - this message would never be logged to console');
}

// both ways to register both beforeunload and unload events would work with window

weizman

const ifr = document.createElement('iframe');
document.body.appendChild(ifr);

// top window's beforeunload event would be the first one to fire
window.onbeforeunload = function(e) {
	console.log('first event to fire (1)');
};

// other windows beforeunload events would fire after top window
ifr.contentWindow.onbeforeunload = function(e) {

weizman

window.addEventListener('unload', (e) => {
  console.log('unloading!');            // would successfully log to console
  localStorage.setItem('key', 'value'); // would successfully set new item to local storage
  debugger;                             // would fail to break at this point!
});

weizman

// by https://developer.mozilla.org/en-US/docs/Web/Events/beforeunload#Example
window.addEventListener("beforeunload", function (event) {
  event.preventDefault(); // Cancel the event as stated by the standard.
  event.returnValue = ''; // Chrome requires returnValue to be set.
});