What is Next.js middleware used for?

Question

Accepted Answer

**Middleware** is code that runs **before a request is completed** — on the Edge, before the page or route handler renders. It lets you inspect and rewrite/redirect requests, set headers, or check auth for many routes in one place.

## Defining middleware

```ts
// middleware.ts (at the project root)
import { NextRequest, NextResponse } from "next/server";

export function middleware(req: NextRequest) {
  const token = req.cookies.get("session")?.value;

// protect /dashboard — redirect unauthenticated users to login
  if (req.nextUrl.pathname.startsWith("/dashboard") && !token) {
    return NextResponse.redirect(new URL("/login", req.url));
  }
  return NextResponse.next(); // allow the request to proceed
}

// only run on these paths (performance — don't run on every asset)
export const config = { matcher: ["/dashboard/:path*", "/account/:path*"] };
```

## What middleware can do

```ts
NextResponse.redirect(url);          // send the user elsewhere (auth gates)
NextResponse.rewrite(url);           // serve different content, URL unchanged (A/B, i18n)
NextResponse.next();                  // continue, optionally with modified headers
// req.geo / req.cookies / req.headers — inspect location, auth, etc.
```

Common uses: **authentication gating**, **redirects** (locale/geo-based, www→apex), **A/B testing** via rewrites, setting **security headers**, and bot/feature flags.

## Key constraints

```text
⚠️ Runs on the EDGE runtime → no Node.js APIs (no fs, limited libraries)
⚠️ Must be FAST — it runs before every matched request; heavy work adds latency
⚠️ Can't access the database directly for complex auth — do lightweight checks
   (e.g. verify a JWT/cookie exists); do full auth in the page/handler
```

The `matcher` config is important: scope middleware to only the routes that need it, so it doesn't run on static assets and slow everything down.

## Why it matters

Middleware is the single choke point for cross-cutting request logic — auth redirects, locale routing, rewrites, and headers — applied across many routes without repeating code in each page.

Knowing its Edge constraints (fast, no Node APIs, lightweight checks only) and the `matcher` scoping is what lets you use it correctly for gatekeeping and routing concerns.