Explain React's useEffect cleanup function and common race condition scenarios in data fetching

Question

At a ride-sharing company, the interviewer presented:

> "Our search page fetches results as the user types. Sometimes old results appear after newer ones. Here's the code:"

```jsx
function SearchResults({ query }) {
  const [results, setResults] = useState([]);

useEffect(() => {
    fetch(`/api/search?q=${query}`)
      .then(r => r.json())
      .then(data => setResults(data));
  }, [query]);

return <ResultList results={results} />;
}
```

> "Type 'abc' then quickly change to 'xyz'. Sometimes 'abc' results show after 'xyz' results. Why?"

Interview OS Community · Accepted Answer

## The Race Condition ``` Typing 'abc' → fetch('abc') starts → takes 800ms Typing 'xyz' → fetch('xyz') starts → takes 200ms → 'xyz' returns first → setResults(xyz data) ✅ → 'abc' returns later → setResults(abc data) ❌ OVERWRITES! ``` The older, slower request overwrites the newer results. ## Fix 1: Cleanup with AbortController ```jsx function SearchResults({ query }) { const [results, setResults] = useState([]); useEffect(() => { const controller = new AbortController(); fetch(`/api/search?q=${query}`, { signal: controller.signal }) .then(r => r.json()) .then(data => setResults(data)) .catch(err => { if (err.name !== 'AbortError') throw err; }); return () => controller.abort(); // Cancel previous request }, [query]); return ; } ``` ## Fix 2: Boolean Flag ```jsx useEffect(() => { let cancelled = false; fetch(`/api/search?q=${query}`) .then(r => r.json()) .then(data => { if (!cancelled) setResults(data); }); return () => { cancelled = true; }; }, [query]); ``` ## Fix 3: use() hook with Suspense (React 19+) ```jsx function SearchResults({ query }) { const results = use(fetch(`/api/search?q=${query}`)); return ; } // React automatically handles cancellation via Suspense boundaries ``` ## Other Common useEffect Cleanup Scenarios | Scenario | What to Clean Up | |----------|-----------------| | **Event listeners** | `removeEventListener()` | | **Intervals/Timeouts** | `clearInterval()` / `clearTimeout()` | | **WebSocket** | `ws.close()` | | **Subscriptions** | `unsubscribe()` | | **Third-party libs** | Destroy instances (`.destroy()`) | ## Rule of Thumb If your effect creates something that persists after it runs, you need a cleanup function. Ask: "What would leak if this component unmounted right now?"

Interview OS

Explain React's useEffect cleanup function and common race condition scenarios in data fetching

Question Details

Suggested Solution

The Race Condition

Fix 1: Cleanup with AbortController

Fix 2: Boolean Flag

Fix 3: use() hook with Suspense (React 19+)

Other Common useEffect Cleanup Scenarios

Rule of Thumb

Discussion (0)