Updating arrays immutably in React
React interview exercise on State, 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 TodoList with an `initial` prop ({ id, title, done }[]): each todo is a <li> with a checkbox toggling `done` (class "done" on the li) and a <button className="remove">; <input className="new"> adds a todo on Enter (next id = max id + 1, blank ignored); <p className="left"> says "N left". Never mutate the array or a todo: the `initial` array must stay as it was.
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.