Practice sheets are sets of problems with progress you can track. Quizzes award 2 XP for each correct answer on the first attempt.
The playground runs JavaScript entirely in your browser, in a sandboxed worker. Your code never reaches our servers, which is why it is free to use and why it cannot touch anything on the site.
It is a scratchpad, not a full environment: there is no file system, no network and no npm. Long loops are stopped rather than left to hang your tab.