Router level middleware in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does router-level middleware affect request handling?

Consider an Express app with a router that has middleware attached. What happens when a request matches a route inside that router?

Express

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

router.use((req, res, next) => {
  req.customValue = 'hello';
  next();
});

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

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

// What will be the response when a client requests GET /api/test ?

A The response will be empty because the middleware does not call next().

B The response will be undefined because router middleware does not modify req object.

C The response will be an error because middleware cannot set properties on req.

D The response will be 'hello' because the router middleware sets req.customValue before the route handler.

💡 Hint

Router middleware runs before route handlers inside that router and can modify req and res objects.

Router-level middleware runs for all routes defined on that router. It can modify the request object, and since it calls next(), the route handler receives the modified req object and can use the new property.

📝 Syntax

intermediate

Identify the correct syntax to apply router-level middleware

Which option correctly applies middleware only to routes inside a router in Express?

Express

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

// Middleware to log request method
function logMethod(req, res, next) {
  console.log(req.method);
  next();
}

// Apply middleware here

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

A router.use(logMethod);

B app.use('/router', logMethod);

C router.get(logMethod);

D app.use(logMethod);

💡 Hint

Router-level middleware is applied with router.use() before defining routes.

Using router.use(logMethod) applies the middleware to all routes inside that router. app.use applies middleware globally or to a path on the app, not just the router.

🔧 Debug

advanced

Why does this router-level middleware not run?

Given this code, why does the middleware not log anything when requesting /api/data? const express = require('express'); const app = express(); const router = express.Router(); router.get('/data', (req, res) => { res.send('Data'); }); router.use((req, res, next) => { console.log('Middleware ran'); next(); }); app.use('/api', router);

A Middleware is missing the next parameter.

B Middleware must call res.send() to run.

C Middleware is defined after the route, so it never runs for matched routes.

D Middleware must be defined on app, not router.

💡 Hint

Order of middleware and routes matters in Express.

Express runs middleware and routes in the order they are defined. Since the route is defined before the middleware, the route matches and sends a response before middleware runs.

❓ state_output

advanced

What is the value of req.processed after these middlewares?

Analyze the following router-level middleware sequence: const express = require('express'); const router = express.Router(); router.use((req, res, next) => { req.processed = 1; next(); }); router.use((req, res, next) => { req.processed += 2; next(); }); router.get('/check', (req, res) => { res.send(String(req.processed)); });

A '3'

B '1'

C '2'

D undefined

💡 Hint

Middleware runs in order and can modify req object cumulatively.

The first middleware sets req.processed to 1, the second adds 2, so the final value is 3 sent as a string.

🧠 Conceptual

expert

Which statement about router-level middleware is true?

Select the correct statement about router-level middleware in Express.

A Router-level middleware runs for every request to the app regardless of the path.

B Router-level middleware runs only if the request path matches the router's mount path and the middleware is defined before the route handlers.

C Router-level middleware cannot modify the request or response objects.

D Router-level middleware must always end the response and cannot call next().

💡 Hint

Think about when router middleware runs and how it interacts with routes.

Router-level middleware runs only when the request path matches the router's mount path and is defined before route handlers to affect them. It can modify req/res and must call next() to continue.

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'