Middleware runs before a request is completed, letting you rewrite, redirect, or modify headers. It is a common senior Next.js interview topic.
Common Interview Questions
Q: Where does middleware live?
Answer: Place middleware.ts at root or inside src/. Use matcher to limit which paths run middleware — important for performance.
// middleware.ts at project root (or src/)
export function middleware(request) {
return NextResponse.next();
}
export const config = { matcher: ["/dashboard/:path*"] };
Q: Middleware vs API route?
Answer: Middleware runs on Edge before routing completes — ideal for auth gates and redirects. API routes handle full request/response logic with Node or Edge runtime.
Q: Auth check pattern?
Answer: Read cookies/headers, redirect unauthenticated users, optionally set request headers for downstream Server Components.
const token = request.cookies.get("session");
if (!token) {
return NextResponse.redirect(new URL("/login", request.url));
}
Q: Limitations?
Answer: Cannot access full Node APIs on Edge runtime. Keep middleware fast — heavy work belongs in Server Components or route handlers.
Key Points for Interviews
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Runs before request hits page or API
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Edge runtime by default — lightweight APIs only
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Use matcher to avoid running on static assets
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NextResponse.rewrite for internal rewrites
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Pair with cookies/JWT for auth at the edge