demonstration of how onbeforeunload and onunload properties all point to the same place

onunload-properties-same-location-in-memory-demo.js

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

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