Node.js · 03/19 6 min

Async pitfalls catalogue

Every async bug that shows up in debugging rounds, with the one-line fix.

Bug	Symptom	Fix
Missing `await`	value is `Promise { <pending> }`, checks always truthy	`await` it
`forEach(async…)`	function returns before work finishes	`for…of` + `await`, or `Promise.all(map)`
Sequential awaits	correct but slow	`Promise.all` independent calls
`Promise.all` fail-fast	one rejection loses all results	`Promise.allSettled` when partial success is OK
Unawaited `return` inside try	catch never sees the rejection	`return await p` inside try/catch
Timer leak	test hangs / never exits	`clearTimeout`, `clearInterval` in cleanup
Shared mutable state	flaky under concurrency	atomic DB ops, locks, or per-request state
Retry without backoff	thundering herd	exponential backoff + jitter, max attempts

async function getUser(id) {
  try {
    return await db.users.findById(id); // 'await' here makes the catch work
  } catch (e) {
    throw new NotFound(id);
  }
}

Practise it now

Async lab Retry lab

Next in Node.js

JavaScript gotchas