Next.js Middleware Error Fix

In this tutorial, you'll learn about Next.js Middleware Error Fix. We cover key concepts, practical examples, and best practices.

The Problem

Error: The edge runtime does not support Node.js 'fs' module.

Next.js middleware runs on the Edge Runtime, which does not support Node.js built-in modules.

Wrong

// middleware.ts
import fs from 'fs'

export function middleware(request: NextRequest) {
  const config = fs.readFileSync('./config.json', 'utf8')
  return NextResponse.next()
}

Output: Error: The edge runtime does not support <a href="/backend/nodejs/">Node.js</a> 'fs' module.

Right

// middleware.ts
import { NextRequest, NextResponse } from 'next/server'

export function middleware(request: NextRequest) {
  const token = request.cookies.get('token')
  const url = request.nextUrl.clone()

  if (!token && url.pathname.startsWith('/dashboard')) {
    url.pathname = '/login'
    return NextResponse.redirect(url)
  }

  return NextResponse.next()
}

export const config = {
  matcher: '/dashboard/:path*',
}

Expected output: middleware redirects unauthenticated users to /login.

Prevention

• Do not import Node.js modules in middleware
• Use Web APIs like Request, Response, and URL instead
• Keep middleware logic light — it runs on every matching request

Common Mistakes with middleware error

1. Using foldl instead of foldl' causing stack overflow on large lists
2. Forgetting deriving (Show, Eq) on custom data types needed for debugging
3. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable

These mistakes appear frequently in real-world NEXTJS code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ