Wrap a child in memo to let it skip a parent-triggered render when its props have not changed. If the parent recreates a function, object, or array prop on every render, stabilize that value with useCallback or useMemo—but only when profiling shows the extra work matters. These tools are performance optimizations, not guarantees: a child’s own state and context changes can still make it render.
First, determine whether the rerender is a problem
A render is not automatically a performance issue. Before changing the component tree, use React Developer Tools to profile the interaction that feels slow and identify which component work is costly. Memoization adds complexity, so it is most useful when it addresses a measured cost rather than a render count in isolation.
Keep rendering pure: given the same inputs, a component should produce the same output without causing side effects. Also check whether an unnecessary Effect is updating state and creating an avoidable chain of renders. Fixing the state flow may be clearer than memoizing around it.
Use memo when a child receives unchanged props
memo creates a memoized component. React will usually skip a render caused by the parent when every prop compares equal to its previous value. By default, React compares each prop with Object.is. The React documentation emphasizes that memo is a performance optimization, not a guarantee.
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For example, a list component can be memoized when its inputs remain the same during unrelated parent updates:
import { memo } from 'react';
const Results = memo(function Results({ items, onSelect }) {
return items.map(item => (
<button key={item.id} onClick={() => onSelect(item)}>
{item.name}
</button>
));
});This boundary helps only if the props remain equal. It does not prevent updates caused by the child’s own state or by context values the child reads.
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Stabilize props that are recreated by the parent
Functions, objects, and arrays are compared by identity. Creating an inline function or a new object or array during every parent render produces a different reference, even when the function behaves the same or the value contains the same data. That changed prop defeats the default memo comparison.
Cache a function with useCallback
Use useCallback to retain a function reference while its dependencies remain unchanged. Include every reactive value used by the function in the dependency list. When a dependency changes, React returns a new function.
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const handleSelect = useCallback(item => {
console.log('Selected', item.id);
}, []);The empty dependency list is appropriate here because the callback reads no reactive values from the component. If it used a prop or state value, that value would need to be included. See the React useCallback reference.
Cache a calculated value with useMemo
Use useMemo to retain the result of a calculation while its dependencies remain unchanged. For a filtered list, this can keep the array reference stable during unrelated parent renders:
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const visibleItems = useMemo(
() => items.filter(item =>
item.name.toLowerCase().includes(query.toLowerCase())
),
[items, query]
);Here, a change to items or query recalculates the array. That is necessary because the displayed results may have changed. Do not omit reactive dependencies merely to preserve a cache; stale values can make the code incorrect. React says to rely on useMemo only as a performance optimization.
Putting the pieces together
This example lets the memoized child avoid work during unrelated parent updates when its props are stable. It is an illustrative pattern, not a benchmark:
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import { memo, useCallback, useMemo, useState } from 'react';
const Results = memo(function Results({ items, onSelect }) {
return items.map(item => (
<button key={item.id} onClick={() => onSelect(item)}>
{item.name}
</button>
));
});
function SearchPage({ items }) {
const [query, setQuery] = useState('');
const visibleItems = useMemo(
() => items.filter(item =>
item.name.toLowerCase().includes(query.toLowerCase())
),
[items, query]
);
const handleSelect = useCallback(item => {
console.log('Selected', item.id);
}, []);
return (
<>
<input value={query} onChange={event => setQuery(event.target.value)} />
<Results items={visibleItems} onSelect={handleSelect} />
</>
);
}When query changes, visibleItems changes and Results should render to show the updated matches. The goal is to avoid repeating that work when the parent updates for an unrelated reason and both child props remain stable.
Know which updates memoization cannot block
The child’s own state changes
A memoized component still renders when its own state changes. memo compares props; it does not freeze the component.
A context value changes
A component that reads a changed context updates even when wrapped in memo. Context changes propagate to consumers; memoization does not shield a consumer from a fresh provider value. If only part of a context consumer needs to respond, read the context in an outer component and pass the specific value to a memoized inner component. See React’s useContext reference.
A dependency changes
useCallback and useMemo refresh their cached result when a listed dependency changes under Object.is. That is expected behavior, not a failure. Include the values the calculation or function actually uses.
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- Keep transient state close to where it is used. A local state update then need not make a broad parent component render.
- Pass JSX as children to visual wrappers when appropriate. This can keep the wrapper’s contents from being recreated as part of the wrapper’s own rendering work.
- Remove unnecessary Effects that update state. Avoiding an update chain can eliminate the renders at their source.
- Simplify props. Avoid passing freshly created objects or arrays when a smaller set of stable values will do.
- Use manual memoization selectively. React notes that most
useCallbackanduseMemocalls have no benefit unless they support a memoized child, a Hook dependency, or an expensive calculation. Extra caches can reduce readability, and one always-new prop can defeat a memoized boundary.
Check whether React Compiler is enabled
React Compiler can automatically handle many memoization cases when it is installed, configured, and compiling the component. Do not assume it is active simply because the project uses React; confirm the project’s build and compiler setup. The current React Compiler documentation explains its role and setup.
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