React Essentials · Hooks · Lesson 11 of 26
useMemo and useCallback
Concept
useMemo and useCallback
These hooks memoize expensive calculations and function references so they are not recreated on every render.
By the end of this lesson
- Explain the core idea behind useMemo and useCallback.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
How to study this page
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
ExampleRunnable
const total = useMemo(() => items.reduce((a, b) => a + b, 0), [items]);Self-check before continuing
Without looking at the example, describe what changes when you modify one input in useMemo and useCallback. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Explain in a comment when you would reach for useMemo instead of just recalculating a value.
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Console