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Reconciliation, hook rules and dependency correctness, server components, and state management.
5 questions in Frontend Engineering.
React useEffect runs twice on mount in development when Strict Mode deliberately runs one extra setup-cleanup-setup cycle. It does not happen in production builds, and the fix is to write the cleanup the effect was missing rather than to guard the effect or remove Strict Mode.
A React index key bug happens when list positions are used as identity. Deleting, sorting or filtering shifts positions, so React reuses the wrong component instance and local state appears beside the wrong row.
React.memo still re-renders when shallow prop comparison fails because parents recreate objects, callbacks or children, or when context and local state update. Stabilise references with useMemo/useCallback only where it removes real render cost.
Assess the candidate's expertise in distributed systems within the browser, focusing on Conflict-free Replicated Data Types, memory optimisation, and peer-to-peer state synchronisation. Use this frontend answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects architecture to the point an interviewer is testing.
React Server Components differ from SSR because they keep server-only work out of the client bundle instead of only rendering HTML on the server. The migration works when the use client boundary stays low and data fetching stays on the server.