Use useMemo when profiling shows that a render-time calculation is meaningfully expensive and its inputs often remain unchanged—or when a stable calculated value helps a memoized child or another Hook avoid repeated work. It is a performance optimization, not a requirement for correct code. Keep the calculation pure, list every reactive dependency, and make sure the app still works if React recalculates or discards the cached value.
What useMemo does
useMemo(calculateValue, dependencies) caches the result of a calculation between renders. React calls the calculation during rendering and compares each dependency with its previous value using Object.is. If every dependency is equal, React can reuse the cached result; if one changes, React recalculates it. See the official useMemo reference.
The calculation should be pure: it takes no arguments, reads its inputs from the component’s scope, and returns a value without causing side effects. Hooks must be called at the top level of a component or custom Hook—not inside a condition or loop—and the dependency list must have a constant number of entries written inline.
When useMemo is worth trying
A costly calculation repeats with the same inputs
Memoize filtering, sorting, or transforming a large collection when profiling shows that the work is noticeably slowing an interaction and the inputs commonly stay the same across renders. For example:
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const visibleItems = useMemo(
() => filterItems(items, query),
[items, query]
);This can avoid repeating the calculation on renders where both items and query are unchanged. It does not speed up the first render, and it cannot avoid recalculation when either dependency changes. There is no universal component-count or millisecond threshold: measure the interaction that feels slow.
A stable value enables a memoized child to skip work
A calculated object or array is a new reference when recreated on every render. If you pass it to a child wrapped in memo, that changing reference can make the child render again even when its contents are equivalent. Memoizing the value may help when the child’s render is expensive and its other props also remain stable.
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const chartData = useMemo(
() => buildChartData(records, range),
[records, range]
);
return <Chart data={chartData} />;memo can skip a child render when its props have not changed, but it is itself a performance optimization, not a guarantee. A single always-new prop can defeat the benefit. React explains the behavior in its memo reference.
Another Hook needs a stable calculated value
A value used as a dependency of another Hook may need a stable reference to prevent unnecessary work. First consider restructuring the code so the dependency is a simpler reactive value. React’s useMemo guidance illustrates moving an options object inside an Effect so the Effect can depend directly on the meaningful value instead of on a memoized object.
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How to decide whether to add it
- Fix avoidable work at its source. Keep render logic pure, avoid unnecessary Effects that set state, keep transient state local, and avoid lifting state farther than necessary. A component should not need memoization to be correct.
- Profile the slow interaction. Use the React Developer Tools Profiler to identify which calculation or component is taking time. React recommends checking a production build on hardware representative of your users; development measurements can be misleading.
- Check whether inputs stay the same.
useMemohelps on renders where all dependencies compare equal. If dependencies change on nearly every render, the calculation will usually run again. - Try the smallest targeted change. Memoize the expensive calculation or stabilize the specific prop that matters, then profile the interaction again. Keep the change only if it helps without making the code harder to understand.
React’s memo reference recommends using the Profiler when an interaction still feels laggy and adding memoization where it helps most.
Dependencies, object identity, and correctness
Include every reactive value referenced by the calculation in the dependency list. React’s Hooks linting can flag missing dependencies. Omitting one may leave the calculation using an outdated value; memoization is not a reason to suppress or work around dependency checks.
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Watch for objects and functions created during rendering. A freshly created object has a different identity each time, so using it as a dependency can cause the calculation to rerun on every render. Prefer simpler dependencies or restructure the code when possible rather than adding layers of memoization to stabilize an avoidable value.
Do not rely on the cache for correctness. React may discard it—for example, during development edits or if the component suspends during its initial mount. In development Strict Mode, React calls the calculation twice to help expose impurities and ignores one result. A pure calculation makes that extra call harmless.
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Choose the right React tool
| Need | Use | Why |
|---|---|---|
| Reuse a calculated value while its dependencies stay equal | useMemo |
Caches a calculation result between renders. |
| Keep a function reference stable | useCallback |
Designed to cache a function definition; see the useCallback reference. |
| Skip rendering a child when its props are unchanged | memo, possibly with stable calculated props |
Can skip a child render when props remain the same, but is not a correctness guarantee. |
| Synchronize with an external system | useEffect |
Effects are for synchronization; useMemo is for computing and caching a value. |
| Find the source of a laggy interaction | React Developer Tools Profiler | Shows which components may benefit from targeted memoization. |
How React Compiler affects manual memoization
React Compiler can automatically memoize values and functions, reducing the need to add useMemo, useCallback, or memo manually. The React Compiler guide describes this behavior. Manual calls remain useful for targeted, measured performance cases and for understanding existing code, but they should not be added by default.
Quick Recap
What not to use useMemo for
- Side effects: Do not perform them inside the calculation. The use-memo lint guidance says the Hook is for computing and caching a value; use an Effect to synchronize with external systems.
- Correctness or persistent storage: Code must continue to work if React recalculates the value or drops the cache.
- Every calculation: Memoization has its own complexity and only helps when recalculating or passing a changed reference causes meaningful work.
- Making the initial render faster: The calculation still runs when there is no cached result.
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