class EventBus {
  #listeners = new Map();
  on(event, cb) {
    if (!this.#listeners.has(event)) this.#listeners.set(event, []);
    this.#listeners.get(event).push(cb);
    return this;
  }
  emit(event, ...args) {
    const cbs = this.#listeners.get(event) || [];
    return cbs.map(cb => cb(...args));
  }
}

const bus = new EventBus();
bus.on('data', x => x * 2).on('data', x => x + 1);
const results = bus.emit('data', 5);
console.log(results, bus.emit('missing', 1));