createEventEmmiter.js

function createEventEmmiter() {
  let events = new Map();

  function addEventListener(type, callback) {
    if (!events.has(type)) {
      events.set(type, new Map());
    }

    events.get(type).set(callback);
  }

  function removeEventListener(type, callback) {
    let items;
    if ((items = events.get(type))) {
      items.delete(callback);
    }
  }

  function dispatchEvent(type, detail) {
    let items;
    if ((items = events.get(type))) {
      for (let [callback] of items) {
        callback(detail);
      }
    }
  }
  return { addEventListener, removeEventListener, dispatchEvent };
}