Code review interviews test candidate engineering maturity better than traditional algorithm puzzles. Candidates analyze realistic React pull requests containing concurrency bugs, performance bottlenecks, and hook misuse.
Bug 1: Stale Closure in useEffect Timer
JSX
Pairlet Snippet
// ❌ Flawed Implementation
function Timer() {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => {
setCount(count + 1); // Stale closure over initial count (0)!
}, 1000);
return () => clearInterval(id);
}, []); // Empty dependency array
return <div>{count}</div>;
}
// ✅ Corrected Implementation
function TimerFixed() {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => {
setCount((prev) => prev + 1); // Functional state update
}, 1000);
return () => clearInterval(id);
}, []);
return <div>{count}</div>;
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