EventEmitterII

// You are free to use alternative approaches of
// instantiating the EventEmitter as long as the
// default export is correct.
export default class EventEmitter {
  constructor() {
    // eventName -> [listener1, listener2, ...]
    // Map gives O(1)-ish lookup by event name.
    this.events = new Map();
  }

  /**
   * @param {string} eventName
   * @param {Function} listener
   * @returns {{off: Function}}
   */
  on(eventName, listener) {
    if (!this.events.has(eventName)) {
      this.events.set(eventName, []);
    }

    const listeners = this.events.get(eventName);
    // Keep insertion order so listeners run in subscribe order.
    listeners.push(listener);

    // Common interview API shape: return a handle with `off()`.
    // This closure keeps access to the exact listeners array.
    return {
      off: () => {
        const index = listeners.indexOf(listener);
        if (index !== -1) {
          // Remove only this listener instance.
          listeners.splice(index, 1);
        }
      },
    };
  }

  /**
   * @param {string} eventName
   * @param {...any} args
   * @returns boolean
   */
  emit(eventName, ...args) {
    // Match common EventEmitter behavior: return false if nothing is emitted.
    if (!this.events.has(eventName)) {
      return false;
    }

    const listeners = this.events.get(eventName);
    if (listeners.length === 0) {
      return false;
    }

    // Fan out payload to each subscriber.
    for (const listener of listeners) {
      listener(...args);
    }

    // At least one listener ran.
    return true;
  }
}