When should a value be derived during render instead of stored in React state?

Avoid duplicated derived state in React

Explains when to derive values during render instead of storing duplicate state that can drift.

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TL;DR

If a value can be calculated from current props or state, I usually derive it during render instead of storing it separately. For example, filtered items can be derived from items and filter text. Storing both the raw list and filtered list creates a synchronization problem. State is best for values that change independently, such as user input, selected id, open panels, or server data.

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Derived values, single source of truth, stale state, memoization, form state, and UI consistency.

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Core answer
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Why this question matters
A lot of subtle React bugs come from storing data that could have been calculated from existing props or state. This drill checks whether the candidate keeps state minimal.
If a value can be calculated from current props or state, I usually derive it during render instead of storing it separately. For example, filtered items can be derived from items and filter text. Storing both the raw list and filtered list creates a synchronization problem. State is best for values that change independently, such as user input, selected id, open panels, or server data.
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Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would use useMemo only when the derived calculation is expensive or when referential stability matters for child components. I would not use memoization to hide unclear state ownership. Some derived values become state when the user can edit them independently, such as a draft field initialized from server data. In that case I would name the draft state clearly and define how reset, save, and server refresh behave.

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Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Storing filtered or counted data when it can be derived cheaply.
Using useEffect to keep duplicate state in sync.
Memoizing unclear logic instead of simplifying state ownership.
Forgetting reset behavior for draft state initialized from props.

When should a value be derived during render instead of stored in React state?

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When should a value be derived during render instead of stored in React state?

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