demonstration of how the priority of beforeunload and unload events works

unload-priority-demo.js

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) {
	console.log('second event to fire (2)');
};

// top window's unload event would fire after all beforeunload events have been fired
window.onunload = function(e) {
	console.log('third event to fire (3)');
};

// other windows unload events would fire after top window
ifr.contentWindow.onunload = function(e) {
	console.log('fourth event to fire (4)');
};