Stable handlers with useCallback in React
React interview exercise on Performance, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
SearchForm keeps a `query` in state and renders SubmitButton (memoized, calls onRender() each render) with an onSubmit that calls the `onSearch` prop with the current query. Typing must not re-render the button; clicking it must search for what was typed at click time.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.