What are useMemo and useCallback used for? Is it always rational to use them?

useMemo and useCallback are React hooks that help optimize component performance.

• useMemo memorizes the result of a function calculation and recalculates it only when dependencies change. This is useful for expensive calculations to avoid performing them on every render.

• useCallback returns a memoized version of a function that is preserved between renders if dependencies do not change. This is important when passing functions to child components to prevent unnecessary re-renders.

However, using them all the time is not always rational, as memoization itself has overhead. They should be used only when there are real performance issues or when passing functions causes unnecessary re-renders.