Middlewares in Express

Middleware functions run during the request-response cycle, sitting between the incoming request and the final route handler. They can read or change req/res, end the cycle by sending a response, or hand off control with next().

Syntax

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

How middleware executes

1. A request reaches the server.
2. Middleware functions run in the order they were registered.
3. Each one either sends a response or calls next().
4. If nothing ends the cycle, the matching route handler runs and sends the final response.

Types of middleware

1. Application-level — runs for the whole app.

app.use(express.json());
app.use((req, res, next) => {
    console.log(`${req.method} ${req.url}`);
    next();
});

2. Router-level — scoped to one router instance.

const router = express.Router();

router.use((req, res, next) => {
    console.log("Router-specific middleware");
    next();
});

router.get("/dashboard", (req, res) => {
    res.send("Dashboard Page");
});

app.use("/account", router);

3. Error-handling — takes four arguments, defined after routes.

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

4. Built-in — ships with Express.

app.use(express.static("public"));
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

5. Third-party — installed from npm.

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

Putting it together

const express = require("express");
const app = express();
const port = process.env.PORT || 5100;

app.use((req, res, next) => {
    console.log(`Request received: ${req.method} ${req.url}`);
    next();
});

app.get("/", (req, res) => {
    res.send("<h2>Bookstore API</h2><p>Middleware tutorial</p>");
});

app.listen(port, () => {
    console.log(`Server is running on http://localhost:${port}`);
});

Visiting / logs the request in the terminal and shows the HTML page in the browser.