Event Emitter (Pub/Sub)

class EventEmitter {
  #events = {};

  on(event, listener) {
    (this.#events[event] ??= []).push(listener);
    return () => this.off(event, listener);
  }

  once(event, listener) {
    const wrapper = (...args) => { listener(...args); this.off(event, wrapper); };
    return this.on(event, wrapper);
  }

  off(event, listener) {
    this.#events[event] = (this.#events[event] ?? []).filter((l) => l !== listener);
  }

  emit(event, ...args) {
    (this.#events[event] ?? []).forEach((l) => l(...args));
  }
}

// Usage
const bus = new EventEmitter();
const off = bus.on('data', (d) => console.log('received:', d));
bus.emit('data', { id: 1 }); // received: { id: 1 }
off(); // unsubscribe