mapLimit (concurrency)

Concurrent async map with a concurrency cap — apply an async fn across an array running at most N at a time, preserving order. A tiny worker pool for fan-out fetches/jobs. No deps.

map-limit.ts

// Concurrent async map with a concurrency cap. Applies `fn` across `items`, running at
// most `limit` at a time, preserving input order. A tiny worker-pool — no deps.
// Extracted from WC26 (fetching ~40 match details in parallel without hammering the upstream).
//
//   const details = await mapLimit(matchIds, 6, (id) => fetchMatch(id));

export async function mapLimit<T, R>(
  items: T[],
  limit: number,
  fn: (item: T, index: number) => Promise<R>,
): Promise<R[]> {
  const out: R[] = new Array(items.length);
  let i = 0;
  const workers = Array.from({ length: Math.max(1, Math.min(limit, items.length)) }, async () => {
    while (i < items.length) {
      const idx = i++;
      out[idx] = await fn(items[idx], idx);
    }
  });
  await Promise.all(workers);
  return out;
}

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