# Lesson 2 complete: keep / flatten / throw

Learner finished lesson 2 with all quiz answers correct, including the second pass at lesson 1 B (clone is still JS; REST still needs JSON text) and D (class instances flatten; custom serializer for revival). Treat as known: three buckets; Date/Map/cycles keep; functions throw; class instances become plain objects; JSON is a document algorithm beside clone. Next ZPD: transfer vs clone — `structuredClone(value, { transfer })`, transfer-list rules, detachment, and APIs that cannot transfer.