Next.js interviews centre on rendering strategies, the App Router, server components, and caching — the parts that separate Next from plain React. Below are the most common questions with model answers. Senior: architecture, trade-offs, mentoring, and decision-making.
By how fresh the content must be and how many distinct versions exist. Marketing pages and articles are static with revalidation, since a minute of staleness costs nothing. Anything personalised is dynamic, because a per-user cache is not a cache. The trap is making a page dynamic accidentally — reading cookies or headers anywhere in the tree opts the whole route out of static rendering.
Push 'use client' as far down as possible and keep data fetching on the server, since a component that never reaches the browser costs nothing. Then the usual: dynamic imports for heavy widgets, and checking what a dependency pulls in — a client component importing a date library is how a page gains a hundred kilobytes. The bundle analyser in CI is what keeps it honest.
Image optimisation needs a loader you configure, ISR needs shared storage so every instance revalidates consistently, and middleware runs on Node rather than an edge runtime. None is a blocker, but each is a decision that Vercel makes silently and self-hosting makes explicit. Finding out after the migration is the expensive order.
Read the session on the server — in a layout, a Server Component or middleware — so protected content is never sent to a browser that should not have it. A client-side redirect after render leaks the page first. Middleware handles the coarse gate; the fine-grained check belongs next to the data, because a route guard alone protects the page and not the endpoint.
When the app is behind a login and has no SEO or first-paint requirement — a plain SPA is simpler to build and to host, and much of Next's value is server rendering you would not use. Also when the team cannot operate a Node server and a static host is the constraint. Choosing it by default is how a dashboard acquires a rendering model it never needed.
In a real interview you speak under pressure. Cam asks these same questions, scores every answer, and shows exactly what to fix.
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