Fixing Broken Event Emitter in Typescript

fixing-broken-event-emitter.ts

type Listener<T extends Array<any>> = (...args: T) => void;

class EventEmitter<EventMap extends Record<string, Array<any>>> {
  private listeners: { [K in keyof EventMap]?: Set<Listener<EventMap[K]>>} = {};

  private getListeners<K extends keyof EventMap>(eventName: K): Set<Listener<EventMap[K]>> {
    return this.listeners[eventName] ?? new Set();
  }

  on<K extends keyof EventMap>(eventName: K, listener: Listener<EventMap[K]>) {
    const listeners = this.getListeners(eventName);
    listeners.add(listener);
    this.listeners[eventName] = listeners;
  }

  emit<K extends keyof EventMap>(eventName: K, ...args: EventMap[K]) {
    const listeners = this.getListeners(eventName);
    for (const listener of listeners) listener(...args);
  }
}

type User = { id: number; name: string }

type EventMap = {
  login: [user: User],
  logout: [],
}

const ev = new EventEmitter<EventMap>();

ev.on('login', (user) => {
  console.log('user...', user)
})

ev.emit('login', { id: 1, name: 'shahan' });

ev.on('logout', () => {
  console.log('logout')
})

ev.emit('logout')