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.