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If useMemo returns outdated data, first check whether its calculation reads a reactive value missing from the dependency list. If it recalculates too often, look for a new object or array being created on every render. And if the result is wrong regardless of caching, fix the underlying data flow: React treats useMemo as a performance optimization, not application state.
What useMemo does—and what it does not do
useMemo caches the result of a calculation between renders while its dependencies remain unchanged. React compares each dependency with its previous value using Object.is. When a dependency changes, React runs the calculation again.
The cache is optional for correctness. React’s documentation says, “You should only rely on useMemo as a performance optimization.” If an application needs the cached value to stay correct, the underlying state or data flow needs fixing.
Memoization can be useful for a genuinely expensive calculation, for keeping a prop stable when passing it to a memo-wrapped child, or for stabilizing a dependency used by another Hook. It adds complexity, though, so use it when the cost or identity requirement justifies it—not automatically.
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Why is my useMemo returning stale values?
A reactive dependency is missing
Every reactive value used by the calculation—such as props, state, or variables declared in the component—belongs in the dependency list. If the calculation reads filter and items, for example, but the list includes only items, a change to filter may leave the memoized result based on an earlier value.
const visibleItems = useMemo(() => filterItems(items, filter), [items, filter]);Do not use an empty dependency list to mean “calculate whenever the inputs change.” It means React has no listed dependencies to compare. The exhaustive-deps lint rule flags missing reactive dependencies because they can cause outdated values. Treat its warning as a prompt to examine the code’s data flow; do not silence it reflexively.
An object was mutated without changing identity
Object.is compares dependency identity, not the contents of an object or array. If you mutate an existing dependency and keep the same reference, React may not see a change and may reuse the previous result.
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Use immutable updates so changed data has a new identity, and make sure the calculation’s inputs are actually represented in its dependencies. For example, create an updated array rather than changing an item in the existing array in place.
Why does useMemo recalculate every render?
A dependency is a fresh object or array
An object or array literal created in the component body has a new identity each render. If the memo depends on it, React sees a changed dependency and recalculates, even when its contents look the same.
function Results({ items, query }) {
const options = { query };
const filtered = useMemo(() => filterItems(items, options), [items, options]);
// ...
}Here, options is new on every render. Prefer creating it inside the calculation so the memo depends on the actual reactive inputs:
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const filtered = useMemo(() => {
const options = { query };
return filterItems(items, options);
}, [items, query]);If a value is truly constant, it may belong outside the component. Memoizing an intermediate object is another option when its stable identity is itself useful, but avoid adding a memo just to preserve an identity that nothing needs.
A dependency changes for a legitimate reason
When every dependency is a real input and it changes on each render, recalculation is expected. First confirm that the calculation is actually expensive or that a downstream identity requirement matters. Ordinary, inexpensive display calculations often do not need useMemo.
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| Situation | Better starting point |
|---|---|
| The display value follows current props or state, and the calculation is inexpensive | Derive it directly during render. |
| The calculation is expensive enough that recalculating is a concern | Measure the cost, then consider useMemo with complete dependencies. |
A stable value is needed by a memo-wrapped child or another Hook |
Consider memoization if the identity change is causing meaningful extra work. |
| Derived state is copied into state and kept in sync with an Effect | Prefer deriving from the current props or state during render; avoid redundant state and synchronization when possible. |
React’s guidance on [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect) explains why an Effect that merely synchronizes derived data can add an unnecessary extra update. Start with the simplest correct data flow; introduce memoization only for a performance or identity reason.
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Other useMemo mistakes that can cause bugs
Putting side effects or mutations in the calculation
The calculation should be pure: given the same inputs, it should return the same result without changing existing values or causing observable effects. In development, Strict Mode may call the calculation twice and ignore one result to help reveal accidental impurity. A calculation that mutates an input or performs a side effect can therefore behave unexpectedly.
Keep render-time calculations pure. Put side effects in the appropriate Effect or event handler, and use immutable data updates rather than changing values that the calculation receives.
Using useMemo to conceal an underlying correctness problem
If the output is incorrect, first inspect where the input comes from, whether it updates correctly, and whether the calculation uses the current values. Caching cannot repair a broken state update, a missing dependency, or a mutation hidden by an unchanged reference. Fix that issue first, then consider whether recalculation is costly enough to optimize.
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Calling the Hook conditionally or in a loop
useMemo is a Hook, so call it at the top level of a component or custom Hook—not inside a condition or loop. If the logic needs conditional structure, extract a component or custom Hook so each Hook call remains at the top level of its function.
A practical troubleshooting checklist
- Read the calculation and list every prop, state value, and component-local variable it uses.
- Include all those reactive inputs in the dependency list, and address any
exhaustive-depswarning by correcting the structure or dependencies. - Check whether an object or array dependency is freshly created each render, or whether an existing object or array is being mutated in place.
- Confirm the calculation is pure and that
useMemois called at the top level. - Make the result correct through ordinary data flow first. Then decide whether an expensive calculation or a specific identity requirement warrants memoization.
React documents dependency comparison and usage in [useMemo](https://react.dev/reference/react/useMemo), the missing-dependency warning in [exhaustive-deps](https://react.dev/reference/eslint-plugin-react-hooks/lints/exhaustive-deps), and render purity in [Components and Hooks must be pure](https://react.dev/reference/rules/components-and-hooks-must-be-pure).
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