scenario withX

withX.ts

export type ReturnValue<T> = T extends (...args: any[]) => infer RETURN
  ? RETURN
  : T;
export type Args<T> = T extends (...args: infer ARGS) => any
  ? ARGS
  : [];
export type CallAsStep<C, T> = (...args: Args<T>) => Step<C, Promise<Resolve<ReturnValue<T>>>>;
export type StepGenerator<C, T> = { [K in keyof T]: CallAsStep<C, T[K]> };

export type Resolve<T> = T extends Promise<infer U> ? U :T;
export function withX<C, T>(f: (c: C) => T, tPrototype: object): StepGenerator<C, Resolve<T>> {
  function asStep<K extends keyof T>(k: K): CallAsStep<C, T[K]> {
    return  (...args: Args<T[K]>): Step<C, Promise<Resolve<ReturnValue<T[K]>>>> => {
      return async (c: C): Promise<Resolve<ReturnValue<T[K]>>> => {
        const target = await f(c);
        const value = target[k];
        return typeof value === 'function'
          ? await (value as any).call(target, ...args) as Resolve<ReturnValue<T[K]>>
          : await value as Resolve<ReturnValue<T[K]>>;
      }
    };
  }

  return Object.getOwnPropertyNames(tPrototype).reduce(
    (ret: StepGenerator<C, T>, k: any) => {
      ret[k as keyof T] = asStep(k);
      return ret;
    },
    {}) as any;
}

export function withToaster(id?: string): StepGenerator<ScenarioContext, Toaster> {
  return withX((c) => c.toaster(id), Toaster.prototype);
}
scenario(context,
  withToaster("mytoaster").addToast()
)