Why are useState and useEffect not interchangeable?
Because they serve complementary roles. useState stores data and triggers re-renders; useEffect runs side effects in response to changes. You cannot use one to do the other's job without causing bugs.
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More FAQs in useEffect vs useState: Different Roles in a React Component
useState holds data inside a component and triggers a re-render when it changes. useEffect runs side effects after render in response to state or prop changes. useState stores; useEffect reacts.
No. useEffect is for side effects, not for storing state. State belongs in useState. You can update state from inside an effect, but the storage itself is always useState.
No. Fetching is a side effect and belongs in useEffect. useState only holds the data once it is fetched. The common pattern is to fetch in useEffect and store the result in useState.
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