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devexamples

Promise Pool With Ordered Results

Run many async tasks with a fixed concurrency limit while keeping results in input order and rejecting invalid pool sizes.

JavascriptResilience

Code

snippet
export function mapPool(items, mapper, concurrency = 4) {
  if (!Array.isArray(items)) throw new TypeError("items must be an array");
  if (typeof mapper !== "function") throw new TypeError("mapper must be a function");
  if (!Number.isInteger(concurrency) || concurrency < 1) {
    throw new RangeError("concurrency must be a positive integer");
  }

  const results = new Array(items.length);
  let nextIndex = 0;

  async function worker() {
    while (nextIndex < items.length) {
      const index = nextIndex;
      nextIndex += 1;
      results[index] = await mapper(items[index], index);
    }
  }

  const workerCount = Math.min(concurrency, items.length);
  const workers = Array.from({ length: workerCount }, () => worker());
  return Promise.all(workers).then(() => results);
}

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