class EventBus {
  #listeners = new Map();
  on(event, fn) {
    if (!this.#listeners.has(event)) this.#listeners.set(event, new Set());
    this.#listeners.get(event).add(fn);
    return () => this.#listeners.get(event).delete(fn);
  }
  emit(event, payload) {
    this.#listeners.get(event)?.forEach(fn => fn(payload));
  }
}
const bus = new EventBus();
const log = [];
const unsub = bus.on('data', v => log.push(`A:${v}`));
bus.on('data', v => log.push(`B:${v}`));
bus.emit('data', 1);
unsub();
bus.on('data', v => log.push(`C:${v}`));
bus.emit('data', 2);
console.log(log.join(','));