iterateOverSyncOrAsyncGenerator.ts

type GetFn = () => {
  next(iteration: IteratorResult<any, any>): IteratorResult<any, any>;
};

function iterateOverSyncOrAsyncGenerator<T extends Iterable<any>>(getIterator: GetFn, gen: T): Iterable<any>;
function iterateOverSyncOrAsyncGenerator<T extends AsyncIterable<any>>(getIterator: GetFn, gen: T): AsyncIterable<any>;
function iterateOverSyncOrAsyncGenerator<T extends Iterable<any> | AsyncIterable<any>>(
  getIterator: GetFn,
  gen: T
): Iterable<any> | AsyncIterable<any> {
  if (Symbol.asyncIterator in gen) {
    const iterator = getIterator();
    return {
      [Symbol.asyncIterator]() {
        const genIterator = gen[Symbol.asyncIterator]();
        return {
          async next(...args) {
            const iteration = await genIterator.next(...args);
            console.log('async', args, '->', iteration);
            return iterator.next(iteration);
          },
        };
      },
    };
  }
  if (Symbol.iterator in gen) {
    const iterator = getIterator();
    return {
      [Symbol.iterator]() {
        const genIterator = gen[Symbol.iterator]();
        return {
          next(...args) {
            const iteration = genIterator.next(...args);
            console.log('sync', args, '->', iteration);
            return iterator.next(iteration);
          },
        };
      },
    };
  }
  throw new Error('Value is not an iterator or asyncIterator');
}