import type { Awaitable } from "./Awaitable";

export function mapAddFields<
  K extends string,
  T extends Record<string, any>,
  R,
>(key: K, fn: (x: T, i: number) => Awaitable<R>) {
  let i = 0;
  return new TransformStream<
    T,
    Omit<T, K> & {
      [key in K]: R;
    }
  >({
    transform: async (chunk, ctrl) =>
      ctrl.enqueue({ ...chunk, [key]: await fn(chunk, i++) }),
  });
}