class EventEmitter {
  constructor() {
    this.mapping = new Map();
  }

  subscribe(event, cb) {
    if (!this.mapping.has(event)) {
      this.mapping.set(event, new Array());
    }

    this.mapping.get(event).push(cb);

    return {
        unsubscribe: () => {
          this.mapping.set(event, this.mapping.get(event).filter(s => s !== cb));
        }
    };
  }

  emit(event, args = []) {
    if (!this.mapping.has(event)) {
      return [];
    }

    return this.mapping.get(event).map(cb => cb(...args));
  }
}

/**
 * const emitter = new EventEmitter();
 *
 * // Subscribe to the onClick event with onClickCallback
 * function onClickCallback() { return 99 }
 * const sub = emitter.subscribe('onClick', onClickCallback);
 *
 * emitter.emit('onClick'); // [99]
 * sub.unsubscribe(); // undefined
 * emitter.emit('onClick'); // []
 */