Middleware huma funzjonijiet li jaħdmu f'sekwenza matul iċ-ċiklu request-response f'Express. Kull wieħed jirċievi (req, res, next) u jista' jinspettja/jimmodifika r-request u r-response, jtemm ir-response, jew ijħaddi next() biex jgħaddi l-kontroll lill-middleware li jmiss. Huma l-qalba ta' kif jiġu strutturati l-applikazzjonijiet ta' Express.
app.use((req, res, next) => {
console.log(`${req.method} ${req.url}`); // do something
next(); // ⬅ pass control to the next middleware
});
Ir-regola kritika: irja' next() biex tikkontinwa l-katina. Jekk tinsa (u ma tibgħatx response), ir-request jibqa' mixi għaliba eternament.
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.
// 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));
Middleware jista' qassar iċ-ċirkwitu (ibgħat response u ma jħaddix next(), pereż biex jblokka requests li mhumiex awtorizzati) jew jtaħħan r-request (bit-talq ta' req.user għal handlers li jmiss).
// 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
Error handler għandu erba' parametri (err l-ewwel). Wasal tiegħu billi tħaddi next(err); għandu jiġi reġistrat l-aħħar.
Middleware runs in the ORDER you register it. Body parser before routes that
read the body; auth before protected routes; error handler last.
Middleware huwa l-qalba arkitettunika ta' Express (u frameworks simili) — authentication, logging, body parsing, CORS, validation, u error handling huma kollha middleware.
Il-fehim tal-kuntratt ta' (req, res, next) (speċjalment li għandek titħaddi next() jew tibgħat response), kif middleware jista' qassar iċ-ċirkwitu jew jtaħħan requests, l-error handler bi erba' argumenti, u li l-ordni importanti, hija essenzjali għall-bnija, struttura, u debug ta' kwalunkwe applikazzjoni ta' Express.