TOPIC OVERVIEW

nextjs/Middleware

Code that runs before a request reaches its route, on every matching request.

10 QuestionsAdvancedHigh Priority ~11 min read

A single middleware.ts file at the project root runs before a request completes routing, for every request that matches its configured matcher. It's the right place for logic that needs to happen before a page even starts rendering — redirects, rewrites, authentication gating, A/B test bucketing, or adding response headers uniformly across many routes.

Middleware runs exclusively on the Edge runtime, which means it inherits the same restricted API surface as any Edge function — no direct database connections through Node-specific drivers, no filesystem access. Because it sits on the hot path of every matching request, it needs to stay fast and lightweight; heavier logic, like a full auth check against a database, usually belongs in the actual route or a Route Handler, with Middleware handling only the cheap, request-level decision.

What you'll walk away knowing

What kinds of tasks belong in Middleware — redirects, rewrites, auth gating, header manipulation

Why Middleware runs exclusively on the Edge runtime, and what that restricts it from doing

How the matcher config controls which requests actually run through Middleware

Why heavy logic should stay out of Middleware even though it technically could run there

1Describe middleware.js.ExpertHigh 2What is Next.js Middleware? Where does it run, and what can it do that API routes cannot?ExpertHigh 3How do you write middleware that applies to some routes but not others? Explain the matcher config.ExpertHigh 4What are the limitations of Middleware in Next.js? What can't you do in middleware?ExpertHigh 5How do you implement role-based route protection — what's the difference between doing it in middleware vs layout vs page?ExpertHigh 6What is the difference between middleware rewrite and redirect? How does each affect the browser URL?ExpertHigh 7How does middleware interact with static pages? Does it run for SSG pages?ExpertHigh 8How would you implement a feature flag system that routes users to different page variants using middleware?ExpertHigh 9How would you implement A/B testing using Next.js middleware and routing?ExpertHigh 10How would you implement geo-based routing (serve different content by country) using middleware?ExpertHigh

All Topics