All 5 Middleware questions
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How do you limit which routes your middleware actually applies to? How would you use middleware to redirect users based on their location or device type? How do you rewrite a request to a different path using middleware without changing the URL the user sees? What performance considerations should you keep in mind when writing middleware, since it runs on every matched request?
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Next.js › Middleware

What is middleware in Next.js, and when does it run?

Beginner

Answer

Middleware is a special function defined in a middleware.js file at the root of your project that runs before a request completes, letting you inspect, modify, redirect, or block requests before they reach your actual pages or API routes.
// middleware.js
import { NextResponse } from 'next/server';

export function middleware(request) {
  console.log('Request received:', request.nextUrl.pathname);
  return NextResponse.next();
}
Real-world example An application logs every incoming request's path using middleware, giving the team visibility into traffic patterns before any page even starts rendering.

Common follow-ups: Does middleware run on every single request by default?;What is the difference between middleware and a route handler?

Edge Runtime vs Node.js Runtime;Routing (App/Pages Router)
All 5 Middleware questions