useSyncExternalStore in React
React interview exercise on Patterns, difficulty 3 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.
Write createStore(initial) returning { get, set(updater), subscribe }, where set takes a function from the current state to the next and notifies subscribers, and useStore(store, selector) that returns selector(state) and re-renders a component only when its selected value changes.
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.