How do useMemo and useCallback help with performance?

Short answer: useMemo: useMemo is a hook that memoizes the result of an expensive function call and only recomputes the value when one of the dependencies changes. Use case: Expensive calculations or operations that don’t need to be recalculated on every render.

Example code

const expensiveValue = useMemo(() => computeExpensiveValue(a, b), [a, b]); // expensiveValue will only recompute when `a` or `b` changes useCallback: useCallback is used to memoize a function so it doesn’t get recreated on every render. Use case: Prevents function re-creation when passing functions down to child components (important when using React.memo or PureComponent). Example: const handleClick = useCallback(() => { // handle the click event }, [dependencies]); // Recreate the function only when dependencies change

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.