Creating an Express Router - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing the '/users' route?

Given the Express router code below, what response will the server send when a client requests the '/users' path?

Express

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

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

module.exports = router;

A The server responds with 'User list'.

B The server responds with a 404 Not Found error.

C The server responds with 'users'.

D The server crashes with a runtime error.

💡 Hint

Check the route path and the response sent inside the router.get handler.

The router defines a GET handler for '/users' that sends the string 'User list' as the response.

📝 Syntax

intermediate

Which option correctly creates and exports an Express router?

Select the code snippet that correctly creates an Express router and exports it for use in other files.

A

const express = require('express');
const router = new express.Router();
module.exports = router;

B

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

C

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

D

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

💡 Hint

Remember to call express.Router() as a function to create the router instance.

Option B correctly calls express.Router() to create a router instance and exports it. Option B calls express() which creates an app, not a router. Option B misses parentheses after Router. Option B incorrectly uses 'new' with Router.

🔧 Debug

advanced

Why does this Express router code cause a runtime error?

Examine the code below. Why will this cause an error when the server runs?

Express

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

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

module.exports = router;

A The variable 'items' is not defined, causing a ReferenceError.

B The route path '/items' is invalid and causes a syntax error.

C The router is not exported correctly, causing a module error.

D The res.send method is used incorrectly and causes a TypeError.

💡 Hint

Check if all variables used inside the route handler are declared.

The code tries to send 'items' but this variable is not declared anywhere, causing a ReferenceError at runtime.

❓ state_output

advanced

What is the response after adding middleware to the router?

Consider this router code with middleware. What will the client receive when requesting '/data'?

Express

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

router.use((req, res, next) => {
  req.customData = 'Hello';
  next();
});

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

module.exports = router;

A The server throws a TypeError.

B The client receives undefined.

C The client receives 'Hello'.

D The client receives an empty response.

💡 Hint

Middleware adds a property to the request object before the route handler runs.

The middleware sets req.customData to 'Hello'. The route handler sends this value, so the client receives 'Hello'.

🧠 Conceptual

expert

What happens if you mount the same router twice with different base paths?

If you have a router with a route '/info' and you mount it on an Express app at '/a' and '/b', what URLs will respond with the router's '/info' handler?

Express

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

router.get('/info', (req, res) => {
  res.send('Info page');
});

app.use('/a', router);
app.use('/b', router);

A Neither will respond; mounting twice causes a conflict error.

B Only '/a/info' will respond; '/b/info' will 404.

C Only '/b/info' will respond; '/a/info' will 404.

D Both '/a/info' and '/b/info' will respond with 'Info page'.

💡 Hint

Think about how Express mounts routers at different base paths.

Mounting the same router at '/a' and '/b' means its routes are accessible under both base paths. So '/a/info' and '/b/info' both work.

Practice

1. What is the main purpose of using express.Router() in an Express app?

easy

A. To handle database connections

B. To start the Express server

C. To create a modular set of routes that can be mounted separately

D. To serve static files

Solution

1. Step 1: Understand what express.Router() does

   This function creates a new router object that can hold routes separately from the main app.

2. Step 2: Identify the purpose of modular routes

   Modular routes help organize code by grouping related routes, making the app easier to maintain.

3. Final Answer:

   To create a modular set of routes that can be mounted separately -> Option C

4. Quick Check:

   Router = modular routes [OK]

Hint: Router organizes routes separately for cleaner code [OK]

Common Mistakes:

• Thinking router starts the server
• Confusing router with database handling
• Assuming router serves static files

2. Which of the following is the correct way to create a new Express router?

easy

A. const router = express.Router();

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

C. const router = express.createRouter();

D. const router = Router();

Solution

1. Step 1: Recall the Express Router creation syntax

   The correct method is calling Router() as a function on the express object.

2. Step 2: Check each option for syntax correctness

   const router = express.Router(); uses express.Router() correctly. const router = new express.Router(); wrongly uses new. const router = express.createRouter(); uses a non-existent method. const router = Router(); calls Router() without express..

3. Final Answer:

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

4. Quick Check:

   Correct syntax = express.Router() [OK]

Hint: Use express.Router() without new keyword [OK]

Common Mistakes:

• Using new keyword with Router
• Calling a non-existent createRouter method
• Calling Router without express prefix

3. Given this code snippet, what will be the output when a GET request is made to /api/users?

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

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

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

app.listen(3000);

medium

A. Cannot GET /api/users

B. Error: router not defined

C. 404 Not Found

D. User list

Solution

1. Step 1: Understand route mounting with app.use('/api', router)

   This mounts the router under the path /api, so routes inside router are prefixed with /api.

2. Step 2: Check the router route /users

   The router handles GET requests to /users, so combined with the mount path, the full path is /api/users.

3. Final Answer:

   User list -> Option D

4. Quick Check:

   Mounted path + router path = /api/users [OK]

Hint: Router path combines with app.use prefix [OK]

Common Mistakes:

• Forgetting the mount path prefix
• Expecting route at root instead of /api
• Assuming router not mounted

4. Identify the error in this Express router code:

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

router.get('/info', (req, res) => {
  res.send('Info page');
});

module.exports = router;

medium

A. Missing parentheses when creating router instance

B. Route path should be '/info/' with trailing slash

C. res.send should be res.json

D. module.exports should export an object, not router

Solution

1. Step 1: Check router creation syntax

   The code uses express.Router without parentheses, so it assigns the function itself, not an instance.

2. Step 2: Confirm correct router instantiation

   It should be express.Router() to create a router instance that supports route methods.

3. Final Answer:

   Missing parentheses when creating router instance -> Option A

4. Quick Check:

   Router needs () to create instance [OK]

Hint: Router must be called as a function with () [OK]

Common Mistakes:

• Forgetting () after express.Router
• Thinking trailing slash is required
• Confusing res.send with res.json

5. You want to create two routers: one for /users and one for /products. How do you correctly mount them in your Express app so that requests to /users/list and /products/list work as expected?

hard

A. app.use(usersRouter); app.use(productsRouter);

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

C. app.use('/list', usersRouter); app.use('/list', productsRouter);

D. app.get('/users', usersRouter); app.get('/products', productsRouter);

Solution

1. Step 1: Understand router mounting with path prefixes

   To handle routes under /users and /products, mount each router with those paths using app.use().

2. Step 2: Check each option for correct usage

   app.use('/users', usersRouter); app.use('/products', productsRouter); mounts routers correctly with prefixes. app.use(usersRouter); app.use(productsRouter); mounts without prefixes, so routes won't be under desired paths. app.use('/list', usersRouter); app.use('/list', productsRouter); mounts both under /list, causing conflicts. app.get('/users', usersRouter); app.get('/products', productsRouter); uses app.get incorrectly with routers.

3. Final Answer:

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

4. Quick Check:

   Mount routers with correct path prefixes [OK]

Hint: Use app.use with path prefix for each router [OK]

Common Mistakes:

• Mounting routers without path prefix
• Using app.get instead of app.use for routers
• Mounting multiple routers on same path