emitter-1.ts

// A typed event emitter that uses promises and async iterables instead of
// callbacks to manage event listeners. #events #promise #rewrite

export class Emitter<T extends EventMap = EventMap> {
  readonly listeners = new Map<string, Set<(data: any) => void> | undefined>()

  once<K extends keyof T & string>(event: K) {
    return new Promise<Parameters<T[K]>[0]>((resolve) => {
      const listeners = this.listeners.get(event) || new Set()
      this.listeners.set(event, listeners)

      listeners.add(function get(data) {
        listeners.delete(get)
        resolve(data)
      })
    })
  }

  async *on<K extends keyof T & string>(event: K) {
    try {
      while (true) {
        yield await this.once(event)
      }
    } finally {
      if (this.listeners.get(event)?.size === 0) {
        this.listeners.delete(event)
      }
    }
  }

  emit<K extends keyof T & string>(event: K, ...data: Parameters<T[K]>) {
    this.listeners.get(event)?.forEach((fn) => (fn as any)(...(data as any)))
  }
}

export type EventMap = Record<string, (data?: any) => void>