React REACT 2

Question 1

What does the useEffect cleanup function run before?

Accepted Answer

Before the next effect re-runs and on unmount

Answer

The cleanup function runs before the effect executes again and when the component unmounts.

Question 2

Which hook should you use to memoize an expensive computed value?

Accepted Answer

useMemo

Answer

useMemo caches a computed value and recomputes only when dependencies change.

Question 3

What is the purpose of the key prop when rendering a list?

Accepted Answer

Help React identify which items changed, added, or removed

Answer

Keys give elements a stable identity so React can efficiently reconcile list changes.

Question 4

Why is using an array index as a key often discouraged?

Accepted Answer

It can cause incorrect state when the list reorders

Answer

Index keys can mismatch component state when items are inserted, deleted, or reordered.

Question 5

What does useCallback return?

Accepted Answer

A memoized version of the callback function

Answer

useCallback returns a memoized callback that changes only when its dependencies change.

React Practice Test