What is Middleware in Express.js? | Node.js Interview Questions

What You'll Learn

What middleware is
Types of middleware
Creating custom middleware

What is Middleware?

Middleware functions have access to request, response objects, and the next middleware function. They can execute code, modify req/res, end the request, or call next().

Middleware Flow

Request → Middleware 1 → Middleware 2 → Route Handler → Response

Types of Middleware

code.jsJavaScript

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

// 1. Application-level middleware
app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next();
});

// 2. Router-level middleware
const router = express.Router();
router.use(authMiddleware);

// 3. Built-in middleware
app.use(express.json());
app.use(express.static('public'));

// 4. Error-handling middleware
app.use((err, req, res, next) => {
  res.status(500).json({ error: err.message });
});

Custom Auth Middleware Example

code.jsJavaScript

const authMiddleware = (req, res, next) => {
  const token = req.headers.authorization;

  if (!token) {
    return res.status(401).json({ error: 'No token' });
  }

  try {
    req.user = jwt.verify(token, SECRET);
    next();
  } catch {
    res.status(401).json({ error: 'Invalid token' });
  }
};

app.get('/protected', authMiddleware, (req, res) => {
  res.json({ user: req.user });
});