Question 1

What is Next.js Middleware?

Accepted Answer

A function running before a request is completed, often at the Edge, for redirects, rewrites, headers, and coarse auth. Configured via middleware.ts and a matcher. Keep it light — not a full app server. It cannot replace server-side authorization for data access.

Question 2

What should you avoid doing in Middleware?

Accepted Answer

Heavy DB calls, complex business logic, large dependencies, and assuming it is the only security layer. Cold start and CPU limits matter on Edge. Prefer JWT/session cookie checks that are cheap. Defer deep authz to server code.

Question 3

How do matchers work?

Accepted Answer

They specify which paths invoke middleware using path patterns. Exclude static assets and Next internals carefully. Over-broad matchers add latency to every request.

Question 4

Can Middleware read the request body?

Accepted Answer

Generally not for arbitrary body parsing the way route handlers can — it's geared to URL/headers/cookies. Design auth and redirects around headers/cookies. Body-dependent logic belongs in handlers/actions.

Question 5

How do you implement locale detection in Middleware?

Accepted Answer

Inspect Accept-Language/cookies/path, rewrite or redirect to locale-prefixed routes. Avoid redirect loops. Persist locale cookie. Coordinate with next-intl or custom i18n routing.

Question 6

Senior: middleware auth redirect vs server component redirect — when each?

Accepted Answer

Middleware for early anonymous gates; server redirects when decision needs DB/session details. Hybrid is common: middleware ensures cookie presence; RSC loads user and enforces roles. Measure latency impact of middleware chains.

Middleware Next.js Interview Questions

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