Array.fromAsync — Sequential vs Parallel

Array.fromAsyncsequential

0 mstotal = sum

await Array.fromAsync(
  tasks, t => t()
);
// each awaits the previous

Promise.allparallel

0 mstotal = slowest

await Promise.all(
  tasks.map(t => t())
);
// all fire at once

Promise.all can't do this. A paginated API modelled as an async generator doesn't know its length upfront. Array.fromAsync walks it lazily, collecting every item across every page into one array.

async function* fetchAllPages(url) {
  let next = url;
  while (next) {
    const page = await fetch(next).then(r => r.json());
    for (const item of page.items) yield item;
    next = page.nextPage;   // null when done
  }
}
const all = await Array.fromAsync(fetchAllPages('/api/products'));

idle — 0 items collected

// items appear here as each page arrives…

Two gotchas that bite in production. Sequential naturally throttles a rate-limited API; parallel hammers it. And Array.fromAsync always returns a Promise — forget await and you get the wrong type.

Rate-limit simulator (API allows 2 concurrent)

// fire requests and watch for 429s…

The forgotten-await gotcha

❌ Missing await

Array.fromAsync(gen())

// —

✅ With await

await Array.fromAsync(gen())

// —