Why is duplicating data between props and state bad?
Because the two copies can go out of sync, producing bugs that are hard to trace. Keeping a single source of truth for each piece of data is a core React principle that prevents these issues.
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More FAQs in Props vs State in React: When to Use Which
Ask who owns and changes the data. If the value comes from outside the component and is read-only, it is props. If the component owns the data and needs to change it over time, it is state.
Copying a prop into state is usually a mistake because the two can drift apart. If the value needs to change, the parent should keep it in state and pass it down as a prop with a callback to update it. This is the controlled component pattern.
No. If a value can be computed from props or existing state, compute it during render instead of storing it. Storing derived values creates synchronization bugs when the source changes.
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