/**
 * Runs tasks with a strict concurrency limit using modern Promise pooling.
 */
async function asyncPool(limit, items, iteratorFn) {
  const results = [];
  const executing = new Set();

  for (const item of items) {
    // 1. Kick off the task and store its promise:
    const p = Promise.resolve().then(() => iteratorFn(item));
    results.push(p);
    executing.add(p);

    // Clean up when settled:
    const clean = () => executing.delete(p);
    p.then(clean, clean);

    // 2. If pool limit reached, wait for at least one executing task to finish:
    if (executing.size >= limit) {
      await Promise.race(executing);
    }
  }

  // 3. Wait for all queued results to settle:
  return Promise.all(results);
}

// Verification:
const urls = [1, 2, 3, 4, 5, 6, 7, 8];
const downloadFile = id => new Promise(res => {
  console.log(`Starting file #${id}`);
  setTimeout(() => {
    console.log(`Finished file #${id}`);
    res(`Data #${id}`);
  }, 200);
});

// Run with max 2 concurrent downloads at a time:
asyncPool(2, urls, downloadFile).then(allResults => {
  console.log('All files downloaded:', allResults);
});