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Expected answer

Server Components render on the server and send HTML to the client — they can access databases and APIs directly without exposing secrets. Client Components run in the browser and handle interactivity (state, effects, event handlers). The key distinction is that Server Components cannot use hooks or browser APIs, but they reduce the JavaScript bundle sent to the client. In Next.js App Router, components are Server Components by default; you opt into Client Components with the "use client" directive.

Expected answer

First, I would profile with React DevTools to identify unnecessary re-renders. Key optimizations: use React.memo for expensive pure components, useMemo/useCallback for expensive computations and stable references, virtualize long lists with react-window, implement code splitting with dynamic imports, optimize images with next/image, and leverage Server Components to move logic off the client. For state management, I would ensure state is colocated (not lifted unnecessarily) and consider using external stores like Zustand for frequently-updated global state.

Expected answer

The cleanup function runs before the component unmounts and before every re-execution of the effect. It prevents memory leaks and stale behavior. Real-world example: subscribing to a WebSocket — the effect opens the connection, and the cleanup closes it. Without cleanup, switching between pages would accumulate open connections, causing memory leaks and duplicate event handlers.

Expected answer

I would use middleware (middleware.ts) to check for a session token on protected routes. For the auth flow: NextAuth.js or a custom JWT solution with httpOnly cookies. The middleware runs on the edge and redirects unauthenticated users to /login. Server Components can read the session directly via cookies() without client-side JavaScript. For client components that need auth state, I would use a React context provider that reads from the server-validated session.

Expected answer

Error boundaries are class components that implement getDerivedStateFromError and componentDidCatch. They catch rendering errors in their child tree and display a fallback UI instead of crashing the whole app. In Next.js, you can use error.tsx files for route-level error handling. Limitations: they do NOT catch errors in event handlers, async code (promises), server-side rendering, or errors thrown in the boundary itself. For those cases, you need try/catch blocks and global error handlers.