Cad a úsáidtear middleware Next.js air?

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.

## Sainmhíniú ar 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*"] };
```

## Cad is féidir le middleware a dhéanamh

```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.
```

Úsáidí coitianta: **authentication gating**, **redirects** (bunaithe ar locale/geo, www→apex), **A/B testing** trí rewrites, socrú **security headers**, agus bot/feature flags.

## Srianta príomhúla

```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
```

Tá an config `matcher` tábhachtach: cuir theorainn ar middleware go dtí na bealaí ar bith ar iad a bheidh de dhíth air, ionas nach rith sé ar shocmhainní statach agus go mall a dhéanann sé gach rud.

## Cad is fiú a thuiscint

Is é middleware an pointe singil smachta do loighic na trasghearrtha iarrataí — redirects fíordheimhniúcháin, ródaireamh locale, rewrites, agus ceannáil — feidhmítear ar go leor bealaí gan an cód a dhéanamh arís in gach leathanach.

A bheith ar an eolas ar a bhaill Edge (tapa, gan API Node, seiceanna éadrom amháin) agus ar scópadh `matcher` atá de dhíth ort a úsáid go ceart i gcás bailithe gatekeeping agus ródaireamh.