Question 1

What does useMemo do?

Accepted Answer

Memoizes a calculated value until dependencies change. Use for expensive pure calculations or referential stability for children. Don't memoize everything by default.

Question 2

When is useMemo unnecessary?

Accepted Answer

Cheap calculations; premature optimization without measured cost. React Compiler may memoize automatically in supporting setups—follow team guidance.

Question 3

useMemo for object identity — why?

Accepted Answer

Keep the same reference so memoized children skip re-renders when contents unchanged. Alternative: move object creation higher or split state. Measure before sprinkling memos.

Question 4

Dependency mistakes with useMemo?

Accepted Answer

Omitting deps yields stale values; unstable deps (new object each render) defeat memoization. Stabilize inputs. Prefer primitive deps when possible.

Question 5

useMemo vs computing during render?

Accepted Answer

Always prefer simple render computation first; memoize when profiling shows cost or identity needs. Clarity beats micro-optimization. Expensive filters on large lists are a good candidate.

useMemo React Interview Questions

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