domOn, domOnce

.js

function domOn(subject, eventName, handler, options = false) {
  // This function is useless except in the case domOn(x, 'e', f); domOn(x, 'e', f); is run
  // where cleanup can remove the distinct 'inner' function from each call, instead of both 'f' listeners.
  function inner(event) {
    return handler.call(this, event);
  }
  
  subject.addEventListener(eventName, inner, options);
  
  const cleanup = () => subject.removeEventListener(eventName, inner, options);
  
  return cleanup;
}

function domOnce(subject, eventName, handler, options = false) {
  const cleanup = domOn(subject, eventName, function (event) {
    cleanup();
    return handler(event);
  }, options);
  
  return cleanup;
}

domOnce(session, 'end', () => {
  // TODO: implement
});