Router level middleware in Express

Introduction

Router-level middleware helps organize code by running functions only for specific groups of routes. It keeps your app tidy and easier to manage.

You want to apply a check or setup only to routes under a certain path, like '/users'.

You need to add logging or authentication only for a part of your app.

You want to reuse middleware for multiple routes grouped together.

You want to keep your main app file clean by separating route logic.

You want to handle errors or modify requests for a specific router.

Syntax

Express

const router = express.Router();

router.use((req, res, next) => {
  // middleware code here
  next();
});

router.get('/path', (req, res) => {
  res.send('response');
});

Router-level middleware is attached to an instance of express.Router().

Use router.use() to add middleware that runs for all routes in that router.

Examples

This middleware logs a message every time a route in this router is accessed.

Express

const router = express.Router();

// Middleware for all routes in this router
router.use((req, res, next) => {
  console.log('Router middleware running');
  next();
});

A simple route inside the router that sends a greeting.

Express

router.get('/hello', (req, res) => {
  res.send('Hello from router!');
});

This mounts the router on the '/users' path, so all router routes start with '/users'.

Express

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

Sample Program

This example creates a router with middleware that logs a message for all '/users' routes. It has two routes: '/' and '/profile'. The router is mounted on '/users', so the full paths are '/users/' and '/users/profile'.

Express

import express from 'express';

const app = express();
const router = express.Router();

// Router-level middleware
router.use((req, res, next) => {
  console.log('Middleware for /users routes');
  next();
});

// Route inside router
router.get('/', (req, res) => {
  res.send('User list');
});

router.get('/profile', (req, res) => {
  res.send('User profile');
});

// Mount router on /users path
app.use('/users', router);

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always call next() in middleware to pass control to the next handler.

Router-level middleware only affects routes defined on that router.

You can add multiple middleware functions to a router for layered processing.

Summary

Router-level middleware runs only for routes in a specific router.

It helps organize and reuse middleware for grouped routes.

Use router.use() to add middleware to a router.

Practice

1. What is the main purpose of router-level middleware in Express?

easy

A. To replace the main app middleware completely

B. To run middleware only for routes defined in a specific router

C. To handle errors globally across all routers

D. To serve static files from a directory

Solution

1. Step 1: Understand router-level middleware scope

   Router-level middleware runs only for routes defined in that router, not the entire app.

2. Step 2: Compare with other middleware types

   Main app middleware runs for all routes, error middleware handles errors, static middleware serves files. Router middleware is specific to router routes.

3. Final Answer:

   To run middleware only for routes defined in a specific router -> Option B

4. Quick Check:

   Router middleware scope = specific router routes [OK]

Hint: Router middleware affects only its router's routes [OK]

Common Mistakes:

• Confusing router middleware with app-level middleware
• Thinking router middleware handles errors globally
• Assuming router middleware serves static files

2. Which of the following is the correct way to add middleware to an Express router?

easy

A. router.middleware(function(req, res, next) { next(); });

B. router.add(function(req, res, next) { next(); });

C. app.useRouter(function(req, res, next) { next(); });

D. router.use(function(req, res, next) { next(); });

Solution

1. Step 1: Recall Express router middleware syntax

   The correct method to add middleware to a router is router.use().

2. Step 2: Check other options for validity

   router.middleware, app.useRouter, and router.add are not valid Express methods.

3. Final Answer:

   router.use(function(req, res, next) { next(); }); -> Option D

4. Quick Check:

   Use router.use() to add middleware [OK]

Hint: Use router.use() to add middleware to routers [OK]

Common Mistakes:

• Using non-existent methods like router.middleware or router.add
• Confusing app.useRouter with router.use
• Forgetting to call next() inside middleware

3. Consider this Express router code:

const express = require('express');
const router = express.Router();

router.use((req, res, next) => {
  console.log('Middleware A');
  next();
});

router.get('/test', (req, res) => {
  res.send('Hello from test');
});

module.exports = router;

What will be printed in the console when a client requests /test on this router?

medium

A. Middleware A

B. Hello from test

C. Middleware A and then Hello from test

D. Nothing will be printed

Solution

1. Step 1: Identify middleware behavior on route request

   The router middleware logs 'Middleware A' and calls next(), so it runs before the route handler.

2. Step 2: Understand console output vs response

   The console.log prints 'Middleware A' to the server console. The response sends 'Hello from test' to the client but does not print to console.

3. Final Answer:

   Middleware A -> Option A

4. Quick Check:

   Middleware console.log prints 'Middleware A' [OK]

Hint: Middleware logs to console, route sends response [OK]

Common Mistakes:

• Confusing console output with response content
• Expecting both console and response text in console
• Ignoring the next() call in middleware

4. Given this router code:

const router = require('express').Router();

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

router.get('/hello', (req, res) => {
  res.send('Hello World');
});

router.use((req, res) => {
});

What is the problem with this code?

medium

A. The last middleware does not call next(), causing requests to hang

B. The last middleware is missing next() and will cause 404 for all routes

C. The last middleware will never run because it is after a route handler

D. The last middleware should have 4 parameters to handle errors

Solution

1. Step 1: Analyze middleware execution order

   Middleware and routes execute in the order defined. First middleware runs for all requests and calls next().

2. Step 2: Trace unmatched route behavior

   For unmatched routes like /foo: first middleware calls next(), /hello route doesn't match so next(), last middleware runs but does nothing--no response and no next() call.

3. Step 3: Identify the consequence

   The request pipeline stops without a response, causing the client request to hang indefinitely.

4. Final Answer:

   The last middleware does not call next(), causing requests to hang -> Option A

5. Quick Check:

   No response + no next() = request hangs [OK]

Hint: Every middleware must either respond or call next() [OK]

Common Mistakes:

• Thinking empty catch-all middleware is harmless
• Forgetting unmatched routes reach the last middleware
• Assuming the router automatically handles 404s

5. You want to apply a logging middleware only to routes under /api in your Express app. You create a router and add middleware like this:

const express = require('express');
const app = express();
const apiRouter = express.Router();

apiRouter.use((req, res, next) => {
  console.log('API request:', req.method, req.url);
  next();
});

apiRouter.get('/users', (req, res) => {
  res.send('User list');
});

app.use('/api', apiRouter);

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

What will happen when a client requests /api/users and /?

hard

A. Logging middleware does not run for any route

B. Logging middleware runs for both /api/users and /

C. Logging middleware runs only for /api/users, not for /

D. Logging middleware runs only for / route

Solution

1. Step 1: Understand router mounting path

   The router is mounted at '/api', so its middleware runs only for routes starting with '/api'.

2. Step 2: Analyze requests

   Request to '/api/users' triggers the router middleware and route handler. Request to '/' goes directly to app.get('/') and skips router middleware.

3. Final Answer:

   Logging middleware runs only for /api/users, not for / -> Option C

4. Quick Check:

   Router middleware runs only on mounted path routes [OK]

Hint: Router middleware runs only on its mounted path routes [OK]

Common Mistakes:

• Assuming router middleware runs for all app routes
• Confusing app-level and router-level middleware scopes
• Expecting middleware to run for '/' when mounted on '/api'