const triggerEvent = (el, eventType, detail) =>
  el.dispatchEvent(new CustomEvent(eventType, { detail }));

const myElement = document.getElementById('my-element');
myElement.addEventListener('click', e => console.log(e.detail));

triggerEvent(myElement, 'click');
// The event listener will log: null

triggerEvent(myElement, 'click', { username: 'bob' });
// The event listener will log: { username: 'bob' }