Middlewares¶

Middleware functions in Express.js are functions that execute before the final route handler. They can modify the request (req), response (res), or even end the request-response cycle.

Basic Example¶

const express = require("express");
const app = express();

// Middleware function
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // Passes control to the next middleware/route
});

app.get("/", (req, res) => {
  res.send("Home Page");
});

app.listen(3000, () => console.log("Server running on port 3000"));

app.use() → Adds middleware to the entire app
next() → Moves to the next middleware or route handler

Types of Middleware¶

1. Application-Level Middleware¶

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

Applies to all routes unless restricted by path:

app.use("/api", (req, res, next) => { ... });

2. Router-Level Middleware¶

Used with express.Router() to apply middleware only to specific routers.

const router = express.Router();

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

router.get("/", (req, res) => res.send("Users"));

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

3. Built-in Middleware¶

express.json() → Parses JSON data
express.urlencoded({ extended: true }) → Parses URL-encoded form data
express.static("public") → Serves static files

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

4. Third-Party Middleware¶

You can install and use external middleware packages.

npm install morgan cors

const morgan = require("morgan");
const cors = require("cors");

app.use(morgan("dev")); // Logs requests
app.use(cors());        // Enables CORS

5. Error-Handling Middleware¶

Used to handle errors globally. Must have four parameters.

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

Middleware Flow Example¶

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

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

app.get("/", (req, res) => {
  res.send("Final Route Handler");
});

Output:

First middleware
Second middleware

Then the response “Final Route Handler” is sent.