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

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

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