React — Interview Questions & Mental Models

Short 'round' questions they often ask

• Virtual DOM — why? → Declarative UI + batched updates; diffing reduces direct DOM work; not 'faster than vanilla DOM' in every case.
• What triggers a re-render? → State/context change in this tree, or parent re-render (unless memo + stable props).
• Hooks rules? → Only call at top level of React functions; same order every render — no hooks in loops/conditions.
• Refs vs state? → State triggers re-render; ref updates don't.
• SSR/hydration mismatch? → Don't use Date.now() / random in initial HTML without suppressing or matching server output.
• How would you share logic between components? → Extract custom hooks, composition, or render props — not inheritance.