Subscribe, then unsubscribe in React
React interview exercise on Effects & refs, 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.
Write OnlineStatus with a `source` prop ({ get(): boolean; subscribe(listener): unsubscribe }): show <span className="status">Online</span> or Offline for the current value, follow every change, and unsubscribe when the component unmounts. If the source prop changes, subscribe to the new one.
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.