Middleware ni funguo zinazoendeshwa kwa mfululizo wakati wa mzunguko wa ombi-jibu katika Express. Kila moja hupokea (req, res, next) na inaweza kukagua/kubadilisha ombi na jibu, kumaliza jibu, au kuita next() ili kupitisha udhibiti kwa middleware inayofuata. Ndizo msingi wa jinsi programu za Express zinavyojengwa.
app.( {
.();
();
});
Kanuni muhimu: ita next() ili kuendeleza mnyororo. Ukiisahau (na usitume jibu), ombi litabaki limening'inia milele.
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 inaweza kukata mnyororo (kutuma jibu na kutoita next(), mfano kuzuia maombi yasiyoidhinishwa) au kuboresha ombi (kuambatanisha req.user kwa ajili ya handlers za baadaye).
// 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
Handler ya makosa ina parameta nne (err kwanza). Unaifikia kwa kuita next(err); inapaswa kusajiliwa mwisho.
Middleware runs in the ORDER you register it. Body parser before routes that
read the body; auth before protected routes; error handler last.
Middleware ni uti wa mgongo wa kimuundo wa Express (na frameworks zinazofanana) — authentication, logging, body parsing, CORS, validation, na kushughulikia makosa yote ni middleware.
Kuelewa mkataba wa (req, res, next) (hasa kwamba lazima uite next() au utume jibu), jinsi middleware inavyoweza kukata mnyororo au kuboresha maombi, handler ya makosa yenye hoja nne, na kwamba mpangilio ni muhimu, ni jambo la lazima kwa kujenga, kuunda muundo, na kutatua matatizo ya programu yoyote ya Express.