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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesIf a value can be calculated from the current props or state, calculate it during rendering instead of storing it in another useState variable. Keeping both the source and a copy creates two values that must stay synchronized—and the copy can become stale. State is still right for information that changes independently, or when you deliberately want to preserve an initial value.
What “derived state” means in React
Derived state is a value that can be worked out from a component’s current props or other state. For example, if a component has a first name and a last name, the full name is determined by those two values. Storing all three means the component must keep the full name in sync whenever either name changes.
React’s “Choosing the State Structure” guidance recommends calculating information during rendering when it can be derived from props or existing state. The practical test is simple: if the value has no independent source of change, it generally does not need its own state.
Calculate a display value directly
function Greeting({ firstName, lastName }) {
const fullName = firstName + ' ' + lastName;
return <p>Hello, {fullName}!</p>;
}Here, fullName always reflects the current names. There is no extra setter to call and no second value that can drift out of sync.
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Why copying a prop into state goes stale
useState(messageColor) uses messageColor to initialize state on the component’s first render. It does not make that state a live copy of the prop. If the parent later passes a different color, the local state remains unchanged unless your component explicitly updates it.
If the component should always use the parent’s latest value, read the prop directly. If the component is meant to start with a value and then let the user change it independently, name the prop to show that one-time intent, such as initialColor or defaultColor.
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Use a prop directly when it remains parent-owned
function Message({ color }) {
return <p style={{ color }}>Message</p>;
}The parent supplies the current color, so there is no local mirror to keep synchronized.
Initialize local state only when later prop changes should be ignored
function ColorPicker({ initialColor }) {
const [color, setColor] = useState(initialColor);
// The component owns color after initialization.
}This pattern is intentional only if the component should preserve its own value when initialColor later changes. The prop name makes that contract clearer to callers.
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Store an ID, not a copied selected record
For a selected item in a list, keep the item’s ID in state and find the current item from the current list during rendering. Storing a whole item object can leave the selection holding old data after the corresponding list entry changes.
function ItemList({ items }) {
const [selectedId, setSelectedId] = useState(null);
const selectedItem = items.find(item => item.id === selectedId);
return (
<>
<ul>
{items.map(item => (
<li key={item.id}>
<button onClick={() => setSelectedId(item.id)}>
{item.name}
</button>
</li>
))}
</ul>
{selectedItem && <p>Selected: {selectedItem.name}</p>}
</>
);
}The ID records the user’s selection; the object is derived from the latest list. If the selected ID no longer matches an item, selectedItem is undefined, which the rendering logic can handle.
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Do not use an Effect just to keep derived state synchronized
An Effect is for synchronizing with something outside React. If the job is only to transform props or state into a value for rendering, calculate that value in the component instead. Adding an Effect to update one state variable when another changes introduces an extra synchronization step and can leave the rendered value behind its source.
React’s “You Might Not Need an Effect” guidance specifically distinguishes external synchronization from deriving state based on props or other state. Use an Effect when there is an external system to coordinate with; do not add one merely to maintain a redundant copy.
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Choose the pattern that matches who owns the value
| Need | Pattern | Trade-off |
|---|---|---|
| The value is determined by current props or state | Calculate during rendering | Always reflects current inputs; computation runs as part of rendering. |
| The calculation is expensive | Consider useMemo |
Can reduce repeated computation; the result is still derived, not independently owned state. |
| The child should always follow the parent | Read the prop directly or use a controlled component | The parent remains the source of truth. |
| The child should preserve only its starting value | Initialize local state from a clearly named initial or default prop |
Later prop changes are intentionally not followed. |
| A new identity should reset all child state | Change the component’s key |
React resets the keyed component tree. |
| A selection refers to an item in a changing collection | Store the item ID and derive the current object | The selected record reflects the current collection. |
| The component must synchronize with a non-React system | Use an Effect where appropriate | Suitable for external synchronization, not routine render calculations. |
When a prop change should reset or adjust local state
Reset the whole child with a key
If a change means the child should start over entirely, give it a different key when its identity changes. React then resets that component’s state tree. This is often clearer than watching a prop in an Effect and manually clearing several state variables.
Use a controlled component when the parent should own the value
If the parent decides the value and the child should display its latest version, pass the value down and report user changes back through a callback. This keeps a single source of truth rather than maintaining a local mirror.
Adjusting state during render is a rare option
React also documents conditionally adjusting a component’s own state during rendering when a prop or other state changes and simpler approaches do not fit. This is an uncommon pattern: it makes data flow harder to follow and must be guarded by a condition so it does not trigger repeated updates. Prefer a controlled design or a key-based reset when either expresses the intended behavior.
Quick Recap
A quick decision checklist
- If the value is fully determined by current props or state, calculate it in the component body.
- If performance is the concern, consider memoizing the calculation rather than storing a separately updated copy.
- If a child should follow the parent’s latest value, use the prop or a controlled component.
- If a child should keep only its starting value, make that intent explicit with an initial/default prop name.
- If changing identity should clear the child’s state, change its key.
- If you are synchronizing with an external system, use an Effect for that synchronization—not merely to copy one React value into another.
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