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mediumSystem Design

What is Middleware in Next.js and what can it do?

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01

Understand the problem

Edge code running before a request is completed.

nextjsmiddlewareedgeauth
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

middleware.ts runs on every matching request before rendering, on the Edge runtime. Use it for auth gating, redirects/rewrites, A/B testing, geolocation, setting headers/cookies — but keep it lightweight (no DB, no heavy work) since it runs on the hot path.

Solution ready — 2 min read

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04

Read the code

Auth gate with a matcher
// middleware.ts
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';

export function middleware(req: NextRequest) {
  const token = req.cookies.get('session')?.value;
  if (!token) return NextResponse.redirect(new URL('/login', req.url));
  return NextResponse.next();
}
export const config = { matcher: ['/dashboard/:path*', '/settings/:path*'] };
05

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Transmission complete // awaiting log

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Dossier 34 of 95 decoded in the Next.js track. One more won't hurt.

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