Function pooledMap

async/pool.ts:41–117

(
  poolLimit: number,
  array: Iterable<T> | AsyncIterable<T>,
  iteratorFn: (data: T) => Promise<R>,
)

 * @throws {RangeError} If `poolLimit` is not a positive integer.
 */
export function pooledMap<T, R>(
  poolLimit: number,
  array: Iterable<T> | AsyncIterable<T>,
  iteratorFn: (data: T) => Promise<R>,
): AsyncIterableIterator<R> {
  if (!Number.isInteger(poolLimit) || poolLimit < 1) {
    throw new RangeError("'poolLimit' must be a positive integer");
  }

  const res = new TransformStream<Promise<R>, R>({
    async transform(
      p: Promise<R>,
      controller: TransformStreamDefaultController<R>,
    ) {
      try {
        const s = await p;
        controller.enqueue(s);
      } catch (e) {
        if (
          e instanceof AggregateError &&
          e.message === ERROR_WHILE_MAPPING_MESSAGE
        ) {
          controller.error(e as unknown);
        }
      }
    },
  });
  // Start processing items from the iterator
  (async () => {
    const writer = res.writable.getWriter();
    const executing: Array<Promise<unknown>> = [];
    try {
      for await (const item of array) {
        const p = Promise.resolve().then(() => iteratorFn(item));
        // Only write on success. If we `writer.write()` a rejected promise,
        // that will end the iteration. We don't want that yet. Instead let it
        // fail the race, taking us to the catch block where all currently
        // executing jobs are allowed to finish and all rejections among them
        // can be reported together.
        writer.write(p);
        const e: Promise<unknown> = p.then(() =>
          executing.splice(executing.indexOf(e), 1)
        );
        executing.push(e);
        if (executing.length >= poolLimit) {
          await Promise.race(executing);
        }
      }
      // Wait until all ongoing events have processed, then close the writer.
      await Promise.all(executing);
      writer.close();
    } catch {
      const errors = [];
      for (const result of await Promise.allSettled(executing)) {
        if (result.status === "rejected") {
          errors.push(result.reason);
        }
      }