Conas a oibríonn middleware Express?

Question

Accepted Answer

**Middleware** a fheidhmeanna atá ag rith **in ord** le linn an timthriall iarratais-freagartha in Express. Gabhann gach ceann `(req, res, next)` agus is féidir leo an t-iarratas agus an freagra a scrúdú/a athrú, an freagra a bhualadh i ndiaidh a chéile, nó `next()` a ghlaoch chun smacht a thabhairt don chéad middleware eile. Is iad an croílár a bhíonn ann i dtaobh conas a bhíonn feidhmchláir Express struchtúrtha.

## An síniú agus an slabhra

```js
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`); // do something
  next();                                   // ⬅ pass control to the next middleware
});
```

An riail chriticiúil: **`next()` a ghlaoch** chun an slabhra a leanúint. Má dhéanann tú dearmad air (agus níl freagra á sheoladh agat), fanainn an t-iarratas go brách.

## An sreabhadh

```text
Request → [logger] → [auth] → [body parser] → [route handler] → Response
            next()    next()     next()         res.send()
Each middleware runs in order; any can short-circuit by sending a response.
```

## Cineálacha coitianta middleware

```js
// 1. Application-level — runs for all (or path-matched) requests
app.use(express.json());                       // built-in: parse JSON bodies
app.use(cors());                                // third-party

// 2. Custom — e.g. authentication that can SHORT-CIRCUIT
function requireAuth(req, res, next) {
  if (!req.headers.authorization) {
    return res.status(401).json({ error: "Unauthorized" }); // STOP — don't call next()
  }
  req.user = decodeToken(req.headers.authorization);  // attach data for later handlers
  next();                                              // authorized → continue
}

// 3. Route-specific
app.get("/profile", requireAuth, (req, res) => res.json(req.user));
```

Is féidir le middleware an slabhra a **ghiorrú** (freagra a sheoladh agus gan `next()` a ghlaoch, m.sh. iarratais dhíobhálacha a bhacadh) nó an t-iarratas a **shaibhriú** (ag ceangal `req.user` do láimhdeoirs thíos síos).

## Middleware láimhdeora earráide (ceithre argóint)

```js
// special signature with 4 params → Express treats it as an error handler
app.use((err, req, res, next) => {
  console.error(err);
  res.status(500).json({ error: "Server error" });
});
// triggered by next(err) or a thrown error; place it LAST
```

Tá **ceithre** pharaiméadar ag láimhdeoir earráide (`err` ar dtús). Sroichíonn tú é trí `next(err)` a ghlaoch; ba chóir é a chlárú ar deireadh.

## Déanann ord difríochtaí

```text
Middleware runs in the ORDER you register it. Body parser before routes that
read the body; auth before protected routes; error handler last.
```

## Cén fáth a bhíonn tábhacht ann

Is é middleware an droichead ailtireachta de Express (agus freamharcanna cosúil) — fíordheimhniú, logáil, parsáil choirp, CORS, bailíochtú, agus láimhdeáltas earráide ar fad middleware a bhíonn ann.

An gconarthadh `(req, res, next)` a thuiscint (go háirithe gur ghá duit `next()` a ghlaoch nó freagra a sheoladh), conas ar féidir le middleware an t-iarratas a ghiorrú nó a shaibhriú, an ceithre-argóint láimhdeora earráide, agus go bhfuil **ord tábhachtach**, tá sé riachtanach do aon fheidhmchlár Express a thógáil, a struchtúrú, agus a dhífhabhtú.