What is Express router and how to use it for modular routing?

Express Router

express.Router() is a mini Express application that handles only routing and middleware. It lets you split your routes into separate files, keeping your codebase modular and maintainable.

The Problem Without Router

// app.js — everything in one file (bad for large apps)
app.get('/users', ...)
app.post('/users', ...)
app.get('/users/:id', ...)
app.get('/products', ...)
app.post('/products', ...)
// hundreds of routes...

Solution: Express Router

src/
  app.js
  routes/
    users.routes.js
    products.routes.js
    auth.routes.js
  controllers/
    users.controller.js

Creating a Router

// routes/users.routes.js
const express = require('express');
const router = express.Router();

// Routes are relative to the mount point
router.get('/', (req, res) => {
  res.json({ users: [] });
});

router.get('/:id', (req, res) => {
  res.json({ id: req.params.id });
});

router.post('/', (req, res) => {
  res.status(201).json({ created: req.body });
});

router.put('/:id', (req, res) => {
  res.json({ updated: req.params.id });
});

router.delete('/:id', (req, res) => {
  res.status(204).send();
});

module.exports = router;

Mounting the Router

// app.js
const express = require('express');
const usersRouter = require('./routes/users.routes');
const productsRouter = require('./routes/products.routes');
const authRouter = require('./routes/auth.routes');

const app = express();
app.use(express.json());

// Mount routers at prefixes
app.use('/api/v1/users', usersRouter);
app.use('/api/v1/products', productsRouter);
app.use('/api/v1/auth', authRouter);

// GET /api/v1/users      → usersRouter GET /
// GET /api/v1/users/42   → usersRouter GET /:id
// POST /api/v1/users     → usersRouter POST /

app.listen(3000);

Router-Level Middleware

// routes/admin.routes.js
const express = require('express');
const router = express.Router();
const { requireAdmin } = require('../middleware/auth');

// Apply middleware to ALL routes in this router
router.use(requireAdmin);

router.get('/users', getAllUsers);
router.delete('/users/:id', deleteUser);

module.exports = router;

Chained Route Handlers (.route())

router.route('/users/:id')
  .get(getUser)
  .put(updateUser)
  .patch(patchUser)
  .delete(deleteUser);
// Cleaner than repeating router.get, router.put, etc.

Router with Controller Pattern

// controllers/users.controller.js
const getUsers  = (req, res) => res.json({ users: [] });
const getUser   = (req, res) => res.json({ id: req.params.id });
const createUser = (req, res) => res.status(201).json(req.body);

module.exports = { getUsers, getUser, createUser };

// routes/users.routes.js
const { getUsers, getUser, createUser } = require('../controllers/users.controller');
const router = express.Router();

router.get('/', getUsers);
router.get('/:id', getUser);
router.post('/', createUser);

module.exports = router;

Express Router is the key to scalable Express apps. Each router is a self-contained module for a feature area. Mount routers with app.use(prefix, router) to build clean, maintainable route hierarchies.

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