Star rating, nested comments (Reddit-style), tic-tac-toe, stopwatch, calculator, password strength meter, drag and drop list, tooltip, breadcrumb, and infinite scroll. These test recursion, game logic, APIs, and algorithms.

Create 5 star icons. On hover, highlight up to the hovered star. On click, lock the rating. Support half-stars with a clip or two separate icons. Add keyboard support (arrow keys to change, enter to select). Reset on mouse leave if not clicked.

Use a tree data structure. Write a recursive render function. Each comment has reply, vote, and collapse handlers. Indent nested comments by depth. Handle reply (add a child), vote (update score), and collapse (toggle children).