export default <T, U>(
  sourceIterator: AsyncIterator<T>,
  transform: (payload: T) => U | Promise<U>,
): AsyncIterator<U> & AsyncIterable<U> => {
  const applyTransformToResult = async (
    nextResult: IteratorResult<T>,
  ): Promise<IteratorResult<U>> => {
    return {
      done: nextResult.done,
      value: nextResult.done ? undefined as any : await transform(nextResult.value),
    };
  };

  return {
    next(): Promise<IteratorResult<U>> {
      return sourceIterator.next().then(applyTransformToResult);
    },

    return(arg?: T): Promise<IteratorResult<U>> {
      if (sourceIterator.return)
        return sourceIterator.return(arg).then(applyTransformToResult);
      return Promise.resolve({
        done: true,
        value: arg as any,
      });
    },

    throw(e?: any): Promise<IteratorResult<U>> {
      if (sourceIterator.throw)
        return sourceIterator.throw(e).then(applyTransformToResult);
      return Promise.reject(e);
    },

    [Symbol.asyncIterator]() {
      return this;
    },
  };
};