Middleware minangka fungsi-fungsi sing mbacot kanthi urutan sajroning siklus request-response ing Express. Saben middleware nampa (req, res, next) lan bisa ngkaji/ngowah request lan response, mungkasi response, utawa ngetik next() kanggo nerusake kontrol menyang middleware sabanjure. Dheweke dadi inti saka carane Express app distruktur.
app.use((req, res, next) => {
console.log(`${req.method} ${req.url}`); // do something
next(); // ⬅ pass control to the next middleware
});
Peraturan sing penting: tik next() kanggo nerusake chain. Lali (lan ora ngirim response), request bakal ngantung selamene.
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 bisa motong alur (ngirim response lan ora ngetik next(), contone: memblokir request sing ora otorisasi) utawa ngayak request (nglekati req.user kanggo handler sabanjure).
// 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 duwe pat parameter (err pisanan). Sampeyan tekan kono kanthi ngetik next(err); iku kudu didaftar paling pungkasan.
Middleware runs in the ORDER you register it. Body parser before routes that
read the body; auth before protected routes; error handler last.
Middleware minangka tulang punggung arsitektur Express (lan framework-framework sing padha) — authentication, logging, body parsing, CORS, validasi, lan error handling kabeh minangka middleware.
Pamahaman babagan (req, res, next) contract (utamane manawa sampeyan kudu ngetik next() utawa ngirim response), carane middleware bisa motong alur utawa ngayak request, error handler pat-argumen, lan manawa urutan iku penting, iku perlu banget kanggo ngawangun, ngestruktur, lan ngowah-owah Express application.
Pustaka pitakon wawancara IT kanthi jawaban rinci — saka Junior nganti Senior.
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