type StringKeyOf<T> = Extract<keyof T, string>
type CallbackType<T extends Record<string, any>, EventName extends StringKeyOf<T>> = T[EventName] extends any[]
  ? T[EventName]
  : [T[EventName]]
type CallbackFunction<T extends Record<string, any>, EventName extends StringKeyOf<T>> = (
  ...props: CallbackType<T, EventName>
) => any

export class EventEmitter<T extends Record<string, any>> {
  private callbacks: { [key: string]: Array<(...args: any[]) => void> } = {}

  public on<EventName extends StringKeyOf<T>>(event: EventName, fn: CallbackFunction<T, EventName>): this {
    if (!this.callbacks[event]) {
      this.callbacks[event] = []
    }

    this.callbacks[event].push(fn)

    return this
  }

  public emit<EventName extends StringKeyOf<T>>(event: EventName, ...args: CallbackType<T, EventName>): this {
    const callbacks = this.callbacks[event]

    if (callbacks) {
      callbacks.forEach(callback => callback.apply(this, args))
    }

    return this
  }

  public off<EventName extends StringKeyOf<T>>(event: EventName, fn?: CallbackFunction<T, EventName>): this {
    const callbacks = this.callbacks[event]

    if (callbacks) {
      if (fn) {
        this.callbacks[event] = callbacks.filter(callback => callback !== fn)
      } else {
        delete this.callbacks[event]
      }
    }

    return this
  }

  public once<EventName extends StringKeyOf<T>>(event: EventName, fn: CallbackFunction<T, EventName>): this {
    const onceFn = (...args: CallbackType<T, EventName>) => {
      this.off(event, onceFn)
      fn.apply(this, args)
    }

    return this.on(event, onceFn)
  }

  public removeAllListeners(): void {
    this.callbacks = {}
  }
}