Middleware in Express.js – A Complete Guide

Middleware are function that runs between request from the client and response from the server.

Each middleware function has access to :

• req - request object
• res- response object
• next() - a function that pass control to next middleware.

Signature of middleware function

function middlewareName(req, res, next) {
  // Do something
  next(); // move to next middleware
}

if forgot to call next(), the request-response cycle will hang.

Express has 5 types of middleware:

• Application-level
• Router-level
• Built-in
• Third-party
• Error-handling

Application-level middleware

• Bound to an app object using app.use() or app.METHOD().
• Runs for every request or specific routes.

app.use((req, res, next) => {
  console.log("Application middleware executed");
  next();
});

---

Router-level Middleware

• Works the same as application-level, but bound to an express.Router() instance.
• Useful for modular route handling.

const router = express.Router();
router.use((req, res, next) => {
  console.log("Router middleware executed");
  next();
});
app.use("/api", router);

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Built-in Middleware

Comes with Express by default.

Common ones are:

• express.json() → parse JSON body
• express.urlencoded() → parse form data
• express.static() → serve static files

app.use(express.json());
app.use(express.static("public"));

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Third-party Middleware

Installed via npm to add extra features.

Examples:

• morgan → request logging
• cookie-parser → parse cookies
• cors → handle cross-origin requests
• helmet → security headers

const morgan = require("morgan");
app.use(morgan("dev"));

---

Error-handling Middleware

• Special middleware with 4 parameters: (err, req, res, next).
• Used to handle errors globally.

app.use((err, req, res, next) => {
  console.error(err.message);
  res.status(500).send("Something broke!");
});