Lazy initial state in React
React interview exercise on Performance, 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.
Report receives `build()` (expensive; returns the report's lines) and a `ticks` counter button that re-renders it. Keep the lines in state initialized from build() so that build runs exactly once per mounted Report, and render a <li> per line.
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.