onerror.js

let frame = document.body.appendChild(document.createElement('iframe'));
frame.addEventListener('load', ()=>{
  let w = frame.contentWindow;
  
  let evt;
  w.addEventListener('error', e=>{evt = e;});
  
  let scr = w.document.createElement('script');
  scr.text = 'throw 42;';
  
  w.document.body.appendChild(scr); // this line executes synchronously and triggers w's new 'error' event handler synchronously?
  
  console.log(evt); // so that by the time this line is hit, 'evt' will have been assigned?
});
frame.src='';