Concatenate any kind of iterable or iterator in typescript

concat-iterator.ts

export type IterableOrIterator<T> = Iterable<T> | Iterator<T> | AsyncIterable<T> | AsyncIterator<T>;

/**
 * Concatenate any kind of iterable.
 */
export async function* concatIterable<T>(...iterators: IterableOrIterator<T>[]) {
  for (let i of iterators) {
    if (isIterable(i)) {
      i = i[Symbol.iterator]();
    } else if (isAsyncIterable(i)) {
      i = i[Symbol.asyncIterator]();
    }

    let f: IteratorResult<T> | Promise<IteratorResult<T>>;
    while (true) {
      f = i.next();
      if (isPromiseLike(f)) {
        f = await f;
      }
      if (f.done) {
        break;
      }
      yield f.value;
    }
  }
}

function isPromiseLike<T>(object: any): object is Promise<T> {
  return object && 'then' in object && typeof object.then === 'function';
}

function isIterable<T>(object: any): object is Iterable<T> {
  return object && Symbol.iterator in object;
}

function isAsyncIterable<T>(object: any): object is AsyncIterable<T> {
  return object && Symbol.asyncIterator in object;
}