Lists and keys in React
React interview exercise on Components & JSX, 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 Playlist with a `tracks` prop ({ id, title, seconds }[]). Render a <ul> with one <li> per track reading "1. Intro (92s)" (position from 1), keyed by id, the last one with the class "last". With no tracks render <p className="empty">No tracks yet</p> instead.
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.