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.