Lesson 25 — Ultimate: todo app

Boss level: a complete feature

A real todo app needs state for the list and the input, adding with validation, toggling an item immutably (map to a new array and copy only the changed item), and a derived "remaining" count. Each item is an object like { text, done }, and the style changes when it is done.

setItems(items.map((x, j) => (j === idx ? { ...x, done: !x.done } : x)));

Why it matters: this is a full feature end to end — the same steps you will use for carts, inboxes and task boards.