State that persists in React
React interview exercise on Custom hooks, difficulty 2 of 3. Solve it in the editor beside a worked example; the tests run the moment you do.
Write useStoredState<T>(key, initial, storage): like useState, but the initial value comes from storage.getItem(key) (JSON) when present, every set writes the JSON back, and the setter accepts a value or an updater function. Read storage only once, when the hook mounts.
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.