Super simple and elegant implementation of an event emitter that follows the EventTarget interface of browsers using ES6 goodness.

EventEmitter.js

// store for private variables
let _ = new WeakMap();

/**
 * Event emitter following browser's EventTarget interface
 */
export class EventEmitter {

  constructor() {
    _.set(this, { callbacks: new Map() });
  }

  addEventListener(type, listener) {
    let callbacks = _.get(this).callbacks;
    if (!callbacks.has(type))
      callbacks.set(type, new Set());
    if (typeof listener == 'function')
      callbacks.get(type).add(listener);
  }

  dispatchEvent(type, ...args) {
    let listeners = _.get(this).callbacks.get(type);
    if (listeners) {
      for (let listener of listeners) {
        listener.apply(this, args);
      }
    }
  }

  removeEventListener(type, listener) {
    let listeners = _.get(this).callbacks.get(type);
    if (listeners)
      listeners.delete(listener);
  }
}