What is middleware in Express.js?

Node.jsJunior~2 min read

1. What middleware is in Express.js

Middleware is a function that runs between receiving the request (Request) and sending the response (Response).

Every middleware can:

Read or modify the req object (the request);
Read or modify the res object (the response);
Stop the request (e.g. return an error or a response);
Pass control onward, by calling the next() function.

You can picture it like this:

javascript

Client → [Middleware 1] → [Middleware 2] → [Route Handler] → Response

2. A simple middleware example

javascript

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

// Middleware: logging every request
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // pass control to the next handler
});

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

app.listen(3000, () => console.log('Server running'));

On every request:

javascript

GET /
→ The method and URL get logged
→ The response is sent

3. What a middleware's structure looks like

javascript

function middleware(req, res, next) {
  // 1. Do something with the request
  // 2. Optionally modify res (e.g. add a header)
  // 3. Must call next() if we want to continue the chain
}

If next() is not called, the chain breaks, Express never reaches the next middleware or route.

4. Types of middleware in Express

Type	Where it's used	Example
Application-level	Applied to the whole app	`app.use(...)`
Router-level	Only for a specific router	`router.use(...)`
Built-in	Built into Express (`express.json()`, `express.static()`)
Third-party	External packages (e.g. `morgan`, `cors`, `helmet`)
Error-handling	Handle errors, take 4 arguments (`err, req, res, next`)

5. An example with several middleware

javascript

app.use(express.json()); // a built-in middleware for JSON parsing
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('Third: the route');
});

Output for GET /:

javascript

First middleware
Second middleware
Third: the route

6. Error-handling middleware

Error-handling middleware in Express always takes 4 arguments: (err, req, res, next)

javascript

app.use((err, req, res, next) => {
  console.error('Error:', err.message);
  res.status(500).send('Something went wrong');
});

This middleware is called whenever an error happens somewhere in the code, or you call next(err).

7. A practical middleware example

javascript

// Checking a token
function authMiddleware(req, res, next) {
  if (req.headers.authorization === 'secret123') {
    next(); // all good, continue
  } else {
    res.status(401).send('Access denied');
  }
}

app.get('/private', authMiddleware, (req, res) => {
  res.send('Secret information');
});

Now /private is only reachable with the correct header.

8. Express's built-in middleware

Middleware	Purpose
`express.json()`	Parses a JSON request body
`express.urlencoded()`	Parses data from HTML forms
`express.static()`	Serves static files (HTML, CSS, JS, images)

9. Popular third-party middleware

Package	Purpose
morgan	Request logging
cors	Allowing cross-origin requests
helmet	Hardening HTTP headers
express-session	User sessions
body-parser (deprecated, now built in)	Request-body parsing

10. Summary

Middleware in Express.js is functions that run in the HTTP request-processing chain.

They can:

modify req and res;

perform checks, logging, parsing, etc.;

pass control onward via next();

handle errors.

Middleware is what makes Express a flexible, modular, and extensible framework.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.