Next.js middleware inatumika kwa nini?

Question

Accepted Answer

**Middleware** ni kod inayofanya kazi **kabla ombi halijafungwa** — kwenye Edge, kabla kurasa au route handler kujifanya. Inaruhusu kufanya uchunguzi na kuandika upya/kuelekeza maombi, kusanidi mipangilio, au kuangalia uthibitisho kwa njia nyingi katika sehemu moja.

## Kufafanua 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*"] };
```

## Kile middleware kinachoweza kufanya

```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.
```

Matumizi ya kawaida: **kuzuia uthibitisho**, **kuelekeza upya** (kulingana na lugha/eneo, www→apex), **jaribio la A/B** kupitia kuandika upya, kusanidi **mipangilio ya usalama** na bot/feature flags.

## Vikomo muhimu

```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
```

Mbinu ya `matcher` ni muhimu: zuia middleware kwa njia za haraka tu, ili isiweze kufanya kazi kwenye mali tuli na kupunguza kasi ya kila kitu.

## Kwa nini ni muhimu

Middleware ni mahali pa kupiga kelele kwa mantiki ya ombi inayoeneza — kuelekeza upya uthibitisho, kueneza lugha, kuandika upya na mipangilio — inayotumiwa kwa njia nyingi bila kurudia kod kwenye kila kurasa.

Kujua vikomo vyake vya Edge (haraka, hakuna Node APIs, uchunguzi wa mwanga tu) na upeo wa `matcher` ndilo kinachokuruhusu kuitumia kwa usahihi kwa kuzuia na matatizo ya kueneza.