objects that support beforeunload and unload events

unload-supporting-objects-demo.js

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

window.addEventListener('beforeunload', successful_listener);
window.addEventListener('unload', successful_listener);

window.onbeforeunload = successful_listener;
window.onunload = successful_listener;

// addEventListener won't work with document.body whereas direct registration would

document.body.addEventListener('beforeunload', failing_listener);
document.body.addEventListener('unload', failing_listener);

document.body.onbeforeunload = successful_listener;
document.body.onunload = successful_listener;

// addEventListener won't work with frameset whereas direct registration would

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

frameset.addEventListener('beforeunload', failing_listener);
frameset.addEventListener('unload', failing_listener);

frameset.onbeforeunload = successful_listener;
frameset.onunload = successful_listener;