Type-safe EventEmitter

typed-emitter.ts

type Arguments<T> = [T] extends [(...args: infer U) => any]
  ? U
  : [T] extends [void] ? [] : [T]

/**
 * Type-safe event emitter.
 *
 * Use it as:
 *
 * interface MyEvents {
 *   error: (error: Error) => void
 *   message: (from: string, content: string) => void
 * }
 *
 * const myEmitter = new EventEmitter() as TypedEmitter<MyEvents>
 *
 * myEmitter.on("message", (from, content) => {
 *   // ...
 * })
 *
 * myEmitter.emit("error", "x")  // <- Will catch this type error
 */
export interface TypedEventEmitter<Events> {
  addListener<E extends keyof Events> (event: E, listener: Events[E]): this
  on<E extends keyof Events> (event: E, listener: Events[E]): this
  once<E extends keyof Events> (event: E, listener: Events[E]): this
  prependListener<E extends keyof Events> (event: E, listener: Events[E]): this
  prependOnceListener<E extends keyof Events> (event: E, listener: Events[E]): this

  removeAllListeners<E extends keyof Events> (event: E): this
  removeListener<E extends keyof Events> (event: E, listener: Events[E]): this

  emit<E extends keyof Events> (event: E, ...args: Arguments<Events[E]>): boolean
  eventNames (): (keyof Events)[]
  listeners<E extends keyof Events> (event: E): Events[E][]
  listenerCount<E extends keyof Events> (event: E): number

  getMaxListeners (): number
  setMaxListeners (maxListeners: number): this
}