Protecting user routes in modern Next.js web applications should be executed with minimum latency. Next.js Edge middleware allows you to intercept incoming requests at the network edge, validating tokens and redirecting unauthorized sessions before pages begin rendering.

1. Edge Runtime Constraints

Edge middleware runs in a lightweight JavaScript V8 engine container rather than a complete Node.js server. This means standard Node.js APIs (such as filesystem access or heavy crypto modules) are unavailable. We must use Web APIs like crypto.subtle for JWT verification.

2. Writing the Middleware Interceptor

A typical middleware file resides in your root source folder. It reads HTTP cookies, decodes jwt segments, and conditionally redirects users if the validation fails.

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

export function middleware(request: NextRequest) {
  const token = request.cookies.get('auth-token')?.value;
  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  return NextResponse.next();
}

3. Custom Routing Rules

By defining matching paths, you specify exactly which routes the middleware should filter. This prevents script executions on static assets like images, manifests, and favicon files, saving edge compute resources.