In Next.js, middleware functions allow you to process requests before they reach your API routes or pages. You can use middleware for authentication, logging, rate limiting, and modifying request/response objects. There are two main ways to implement middleware in Next.js:

1. Global Middleware (middleware.ts or middleware.js)

• Placed in the root directory of your Next.js project.

• Runs on every request before it reaches the API route or page.

• Uses Next.js Edge API (runs on Edge functions, not Node.js).

Example: Redirect unauthorized users

javascript
export const config = {  matcher: ["/dashboard/:path*", "/admin/:path*"],};

This middleware ensures only authenticated users can access /dashboard or /admin.

2. API Route Middleware

• Applied within specific API routes.

• Uses standard Express-like middleware functions.

Example: Middleware for API Authentication

javascript
export function withAuth(handler) {  return async (req, res) => {    const token = req.headers.authorization;    if (!token || token !== "your-secret-token") {      return res.status(401).json({ message: "Unauthorized" });    }    return handler(req, res);  };}// Applying middleware to an API routeexport default withAuth(async function handler(req, res) {  res.status(200).json({ message: "Welcome to the protected API!" });});

This function wraps API handlers to enforce authentication.

3. Custom Express-like Middleware (Using next-connect)

For more complex middleware chains, next-connect helps organize middleware functions.

Example: Using next-connect

javascript
import nextConnect from "next-connect";const middleware = nextConnect();middleware.use((req, res, next) => {  console.log("Request received at:", new Date().toISOString());  next();});export default middleware;

Then apply it to an API route:

javascript
import middleware from "../../middlewares/logger";export default function handler(req, res) {  middleware(req, res, () => {    res.status(200).json({ message: "Middleware applied!" });  });}

When to Use Each Approach?

• Global Middleware (middleware.ts): Best for authentication, redirects, or modifying requests before they hit API routes.

• API Route Middleware: Best for per-route authentication, logging, or validation.

• next-connect Middleware: Useful for complex middlewares with multiple steps.