Custom event emitter using native DOM APIs. Read more here: https://cameronnokes.com/blog/build-your-own-event-emitter-using-only-native-dom-apis/

event-emitter.js

class EventEmitter {
  constructor() {
    this.target = new EventTarget();
  }
  on(eventName, listener) {
    return this.target.addEventListener(eventName, listener);
  }
  once(eventName, listener) {
    return this.target.addEventListener(eventName, listener, { once: true });
  }
  off(eventName, listener) {
    return this.target.removeEventListener(eventName, listener);
  }
  emit(eventName, detail) {
    return this.target.dispatchEvent(
      new CustomEvent(eventName, { detail, cancelable: true })
    );
  }
}