Separating routes into files in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing the /users route?

Given the Express app setup below with routes separated into files, what will the server respond when a client requests /users?

Express

const express = require('express');
const app = express();
const usersRouter = require('./routes/users');

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

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

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

module.exports = router;

A The server throws a runtime error because of missing middleware.

B The server responds with 'Cannot GET /users'.

C The server responds with 'Welcome to users'.

D The server responds with 'User list'.

💡 Hint

Check how the router handles the / path inside the /users prefix.

The usersRouter handles requests to / relative to /users. So a GET request to /users matches router.get('/', ...) and responds with 'User list'.

📝 Syntax

intermediate

Which option correctly exports an Express router from a separate file?

Choose the correct way to export an Express router from a file named products.js.

Express

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

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

// What is the correct export statement here?

A module.exports = router;

B module.export = router;

C export default router;

D exports = router;

💡 Hint

Remember how CommonJS modules export values.

In CommonJS, module.exports = router; correctly exports the router object. exports = router; only reassigns the local variable and does not change the exported object. export default is ES module syntax and not valid here. module.export is a typo.

🔧 Debug

advanced

Why does the route handler not respond when using separated routes?

Consider this Express app setup:

const express = require('express');
const app = express();
const apiRouter = require('./routes/api');

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

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

router.get('/data', (req, res) => {
  // missing res.send or res.end
});

module.exports = router;

What happens when a client requests /api/data?

A The request hangs and eventually times out because no response is sent.

B The server throws a syntax error on startup.

C The server responds with an empty 200 OK response.

D The server responds with a 404 Not Found error.

💡 Hint

Check if the route handler sends a response.

The route handler does not call res.send() or res.end(), so the request stays open and the client waits indefinitely until timeout.

🧠 Conceptual

advanced

What is the benefit of separating routes into files in Express?

Why do developers separate routes into different files when building Express applications?

A To avoid using middleware functions in the main app file.

B To improve server performance by loading routes faster.

C To organize code better and keep route logic modular and maintainable.

D To prevent Express from handling requests synchronously.

💡 Hint

Think about code organization and teamwork.

Separating routes into files helps keep code organized, easier to read, and maintain. It also allows multiple developers to work on different routes without conflicts. It does not directly affect performance or middleware usage.

❓ state_output

expert

What is the output after mounting nested routers with middleware?

Given the following Express app and route files, what will be the output when a client requests /api/users/profile?

Express

const express = require('express');
const app = express();
const apiRouter = require('./routes/api');

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

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

router.use('/users', (req, res, next) => {
  req.customData = 'middleware data';
  next();
});
router.use('/users', usersRouter);

module.exports = router;

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

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

module.exports = router;

A The server responds with 404 Not Found because /profile route is not found.

B "middleware data" is sent as the response.

C "undefined" is sent as the response because req.customData is missing.

D The server throws an error because middleware is not properly chained.

💡 Hint

Consider how middleware modifies req and how nested routers handle requests.

The middleware on /users adds req.customData. Then the nested usersRouter handles /profile and sends req.customData. So the response is 'middleware data'.

Practice

1. What is the main benefit of separating routes into different files in an Express app?

easy

A. It makes the app run faster.

B. It keeps the app organized and easier to maintain.

C. It automatically secures the routes.

D. It removes the need for middleware.

Solution

1. Step 1: Understand code organization

   Separating routes into files helps keep code clean and manageable as the app grows.

2. Step 2: Identify benefits

   This separation does not affect speed or security directly but improves maintainability.

3. Final Answer:

   It keeps the app organized and easier to maintain. -> Option B

4. Quick Check:

   Organized code = easier maintenance [OK]

Hint: Think about code clarity and maintenance benefits [OK]

Common Mistakes:

• Confusing organization with performance improvements
• Assuming route separation adds security automatically
• Thinking middleware is not needed after separation

2. Which of the following is the correct way to create a router in a separate file named users.js?

easy

A. const router = express.Router(); module.exports = router;

B. const router = new express.Router(); export default router;

C. const router = express.Router; module.exports = router();

D. const router = express.Router(); export router;

Solution

1. Step 1: Create router instance correctly

   Use express.Router() to create a new router instance.

2. Step 2: Export the router properly

   Use module.exports = router; to export it for use in other files.

3. Final Answer:

   const router = express.Router(); module.exports = router; -> Option A

4. Quick Check:

   Correct router creation and export = const router = express.Router(); module.exports = router; [OK]

Hint: Remember to call Router() and use module.exports [OK]

Common Mistakes:

• Forgetting parentheses after Router
• Mixing CommonJS and ES module syntax
• Exporting router incorrectly

3. Given the following code in users.js:

const express = require('express');
const router = express.Router();
router.get('/profile', (req, res) => {
  res.send('User Profile');
});
module.exports = router;

And in app.js:

const express = require('express');
const app = express();
const usersRouter = require('./users');
app.use('/users', usersRouter);
app.listen(3000);

What will be the response when a client requests /users/profile?

medium

A. User Profile

B. Cannot GET /users/profile

C. 404 Not Found error

D. Server crashes with error

Solution

1. Step 1: Understand route prefixing

   The router is mounted at /users, so /profile inside router becomes /users/profile.

2. Step 2: Check route handler response

   The route /profile sends 'User Profile' as response.

3. Final Answer:

   User Profile -> Option A

4. Quick Check:

   Route prefix + handler response = 'User Profile' [OK]

Hint: Remember mounted path prefixes router routes [OK]

Common Mistakes:

• Forgetting the prefix from app.use
• Expecting route at root instead of prefixed path
• Assuming 404 without checking router setup

4. You have this code in products.js:

const express = require('express');
const router = express.Router();
router.get('/list', (req, res) => {
  res.send('Product List');
});
// Missing module.exports here

And in app.js:

const express = require('express');
const app = express();
const productsRouter = require('./products');
app.use('/products', productsRouter);
app.listen(3000);

What error will occur when starting the app or accessing /products/list?

medium

A. SyntaxError due to missing export

B. TypeError: Cannot read property 'use' of undefined

C. No error, but route won't respond

D. productsRouter is undefined, causing an error

Solution

1. Step 1: Check module export in products.js

   Since module.exports is missing, require('./products') returns undefined.

2. Step 2: Understand impact in app.js

   Using undefined as router in app.use causes an error when app tries to use it.

3. Final Answer:

   productsRouter is undefined, causing an error -> Option D

4. Quick Check:

   Missing export = undefined import error [OK]

Hint: Always export router to avoid undefined imports [OK]

Common Mistakes:

• Assuming app runs without export
• Confusing syntax error with runtime error
• Not checking module.exports presence

5. You want to separate routes for /api/users and /api/products into two files: users.js and products.js. Each file exports a router with routes starting at /. How should you mount these routers in app.js to correctly handle requests to /api/users/profile and /api/products/list?

hard

A. app.use('/api', usersRouter); app.use('/api', productsRouter);

B. app.use('/users', usersRouter); app.use('/products', productsRouter);

C. app.use('/api/users', usersRouter); app.use('/api/products', productsRouter);

D. app.use('/', usersRouter); app.use('/', productsRouter);

Solution

1. Step 1: Understand router base paths

   Each router handles routes starting at /, so mounting them at specific paths adds prefixes.

2. Step 2: Mount routers with full API paths

   Mount usersRouter at /api/users and productsRouter at /api/products to match desired URLs.

3. Final Answer:

   app.use('/api/users', usersRouter); app.use('/api/products', productsRouter); -> Option C

4. Quick Check:

   Mount routers at full paths for correct URL handling [OK]

Hint: Mount routers at full URL prefixes to match routes [OK]

Common Mistakes:

• Mounting routers at wrong or incomplete paths
• Mounting both routers at same path causing conflicts
• Assuming routers handle full paths internally