useOptimistic in React
React interview exercise on React 19, difficulty 2 of 3. Solve it in the editor beside a worked example; the tests run the moment you do.
Write LikeButton({ likes, onLike }): a button reading "♥ N". A click shows N + 1 immediately through useOptimistic while onLike() (async) runs inside a transition; when the parent later passes the real count, that is what shows. Two quick clicks show N + 2 while both are pending.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Progress is saved in your browser, and in your account if you sign in.