How do you handle middleware in Next.js?

Middleware in Next.js enables you to run code on incoming requests before they are completed, ideal for authentication, bot detection, A/B testing, and URL rewrites:

### Creating Middleware:
Create a single middleware.ts (or .js) file at the root of your project (or inside src/):

// middleware.ts
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/request';

export function middleware(request: NextRequest) {
  const token = request.cookies.get('auth_token')?.value;

  // Protect admin dashboard routes
  if (request.nextUrl.pathname.startsWith('/admin') && !token) {
    return NextResponse.redirect(new URL('/login', request.url));
  }

  return NextResponse.next();
}

// Matcher config: run only on specific routes
export const config = {
  matcher: ['/admin/:path*', '/dashboard/:path*'],
};

### Key Capabilities:

• Operates on the lightweight Next.js Edge Runtime.
• Can modify request headers, response headers, or set cookies.
• Can rewrite URLs (NextResponse.rewrite()) or redirect users (NextResponse.redirect()).