Advanced interview questions in React & Next.js, page 2
How do nested layouts work in the App Router?
Tests nested layout composition and state persistence in the App Router. Good answers explain root wraps dashboard wraps page, dashboard state survives child navigation, and only the page segment rerenders.
What are Parallel and Intercepting Routes? Describe a photo gallery modal scenario.
This tests App Router layout composition and URL-state. A strong answer defines @modal slots and intercepting (.) routes, explaining modal overlays that preserve page context. Red flag is proposing global state or portals over file-system conventions.
Server Component fetch vs. client-side SWR or React Query
Tests App Router cache boundaries and hydration costs. Strong answers weigh server TTFB and bundle savings against client interactivity, mutations, and background refetching. Red flag: defaulting to client fetching everywhere and ignoring waterfalls.
Explain streaming with Server Components, Suspense, and loading.js
Tests progressive HTML streaming through Suspense boundaries. Strong answers state loading.js auto-wraps a route segment in Suspense so the server can stream the shell instantly and defer slow data without blocking first paint.
How do you implement on-demand revalidation from a CMS webhook?
Tests Next.js cache invalidation and webhook security. Answer: secure Route Handler that calls revalidatePath or revalidateTag; next request rebuilds. Red flag: full redeploys, no secret check, or Server Component revalidate calls.
How would you use generateStaticParams for known products and on-demand unknowns?
Tests App Router hybrid rendering. Export generateStaticParams for known slugs at build time; leave dynamicParams true so unknown slugs server-render on demand. Red flag: citing Pages Router fallback or ignoring route segment config.
How would you use Next.js Middleware to protect /api/admin/* routes?
Match /api/admin/:path*, read the secure cookie, validate the token, return 401 or proceed.
How do you implement response streaming in a Next.js Route Handler?
This tests Web Streams API mastery in Next.js. Strong answer: create a ReadableStream with TextEncoder, return it in a Response, and cite AI chat or large JSON as use cases. Red flag: suggesting Node res.write instead of standard Web Streams.
How to securely set and read httpOnly cookies in Route Handlers
This tests cookie security across the App Router. A strong answer covers setting httpOnly, Secure, SameSite via NextResponse in a Route Handler, reading with cookies() later, and why document.cookie cannot access it. Red flag: client-side JavaScript access.
How do you opt out of static rendering for real-time data?
Tests Next.js App Router caching and dynamic rendering escape hatches. Cover force-dynamic SSR, ISR with revalidate, noStore, and client fetching; weigh server load vs staleness. Red flag: only CDN purges without segment config or data cache fixes.
Implement A/B testing with Middleware rewrites and cookies
Sticky cookie, internal rewrite, vary cache on cookie, server-side analytics.
Implement a custom next/image loader for a self-hosted service
Tests next/image loader contract and URL construction. A strong answer defines a function taking src, width, and quality; returns a URL string for your service; and wires it via the loader prop or loaderFile config.
How can you use Edge Middleware for auth and trade-offs versus getServerSideProps?
Middleware checks cookies/JWT for fast Edge rewrites/redirects; getServerSideProps uses full Node.js for heavy sessions with colder starts.
How would you design auth for ISR pages without losing cache benefits?
Tests cache segmentation and dynamic boundaries in Next.js. Propose a static ISR shell for anonymous users, then fetch personalized data client-side or via dynamic server paths when cookies are present.
What is a Fiber in React Fiber architecture?
Tests reconciler internals. Strong answers define a Fiber as a unit of work with child, sibling, return, and alternate pointers; explain the linked-list tree enables incremental traversal; and note alternate connects current and work-in-progress trees.
How does React's scheduler prioritize updates with Fiber?
Tests cooperative scheduling. Cover Fiber's incremental units, the Scheduler's time-slicing loop, and Lanes where SyncLane and InputContinuousLane outrank DefaultLane. Red flag: claiming React uses a separate thread or that batching alone handles priority.
Tearing and useSyncExternalStore in concurrent React
Tearing is inconsistent UI when an interruptible render reads a changed external store mid-pass; useSyncExternalStore subscribes and forces consistent snapshots, re-rendering synchronously on change.
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