The answer
Each piece of data should have one owner. Do not duplicate data between props and state, or across stores. Duplication causes sync bugs when one copy updates and another does not.
React State and Data Flow Interview Questions
What is the single source of truth principle in React?
More FAQs in React State and Data Flow Interview Questions
Props are inputs from a parent, read-only in the child. State is internal data the component owns and can change, triggering re-renders. Props are external; state is internal.
When two or more siblings need the same data and must stay in sync. Lift it to their common parent, which owns the state and passes it down to both as props, creating a single source of truth.
Context for widely-shared, rarely-changed data like auth and theme. Redux for complex, frequently-updated shared state with many interactions. Reach for Redux only when Context is genuinely insufficient.
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