How do you fetch data in React and avoid race conditions?

Question

IT Interview Practice · Accepted Answer

The basic approach uses `useEffect`, but you must handle **cleanup** and **out-of-order responses**, or you'll get stale data and warnings.

```jsx
function User({ id }) {
  const [user, setUser] = useState(null);
  const [error, setError] = useState(null);

useEffect(() => {
    const ctrl = new AbortController();
    fetch(`/api/users/${id}`, { signal: ctrl.signal })
      .then(r => r.json())
      .then(setUser)
      .catch(e => { if (e.name !== "AbortError") setError(e); });

return () => ctrl.abort(); // cancel the in-flight request when id changes/unmounts
  }, [id]);
}
```

## The race condition this prevents

Say `id` changes quickly from `1` to `2`. Without cancellation, both requests are in flight. If request `1` is slow and resolves **after** request `2`, you'd display user `1`'s data for user `2`:

```text
fetch(1) ───────────(slow)──────▶ setUser(user1)  ❌ shown last, but wrong
fetch(2) ──(fast)──▶ setUser(user2)
```

Aborting the previous request (the cleanup) fixes this. An older boolean-flag pattern (`let active = true; ... if (active) setUser(...)`) achieves the same by ignoring stale responses.

## In real apps, use a library

Manual fetching gets repetitive and misses caching/retries. **React Query (TanStack Query)** or **SWR** handle loading/error states, caching, deduplication, background refetch, and stale-while-revalidate for you:

```jsx
const { data, isLoading, error } = useQuery({ queryKey: ["user", id], queryFn: () => getUser(id) });
```

With frameworks like Next.js, server-side data loaders are often preferable.

The race condition this prevents

In real apps, use a library

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