Virtual path prefixes in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing a virtual path?

Consider this Express setup using a virtual path prefix. What will be the response when a client requests /api/users?

Express

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);

A The server responds with 404 Not Found.

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

C The server responds with 'User list'.

D The server crashes with an error.

💡 Hint

Remember that the router is mounted on '/api', so routes inside the router are relative to that prefix.

The router handles requests starting with '/api'. The route '/users' inside the router matches '/api/users' on the main app, so the response is 'User list'.

❓ state_output

intermediate

What is the value of req.baseUrl inside a virtual path router?

Given this Express code, what will req.baseUrl be when the route handler runs for a request to /admin/dashboard?

Express

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

adminRouter.get('/dashboard', (req, res) => {
  res.send(req.baseUrl);
});

app.use('/admin', adminRouter);

app.listen(3000);

A "/admin"

B "/dashboard"

C "/admin/dashboard"

D "/"

💡 Hint

req.baseUrl shows the path where the router is mounted.

req.baseUrl contains the path prefix where the router is mounted, which is '/admin'.

📝 Syntax

advanced

Which option correctly mounts a router with a virtual path prefix?

Select the code snippet that correctly mounts a router on the path '/api/v1' so that routes inside the router are prefixed accordingly.

A app.use(router, '/api/v1');

B app.use('/api/v1', router);

C app.use('/api/v1' => router);

D app.use(router('/api/v1'));

💡 Hint

The app.use method takes the path prefix first, then the router.

The correct syntax is app.use('/api/v1', router);. Other options are invalid syntax or incorrect usage.

🔧 Debug

advanced

Why does this virtual path router not respond as expected?

Given the code below, why does a request to /api/products return 404 Not Found?

Express

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

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

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

app.listen(3000);

A The router.get method requires a second argument to be an array.

B The router is not mounted correctly; it should be app.use(router, '/api').

C The app.listen call is missing a callback function.

D The route path is missing a leading slash; it should be '/products'.

💡 Hint

Check the route path string inside router.get.

Route paths must start with a slash. Without it, the route does not match requests correctly.

🧠 Conceptual

expert

How does Express handle nested virtual path prefixes?

If you mount a router routerA at /api and inside it mount another router routerB at /v2, what is the full path to access a route /items defined inside routerB?

Express

const express = require('express');
const app = express();
const routerA = express.Router();
const routerB = express.Router();

routerB.get('/items', (req, res) => {
  res.send('Items v2');
});

routerA.use('/v2', routerB);
app.use('/api', routerA);

app.listen(3000);

A /api/v2/items

B /v2/api/items

C /api/items/v2

D /items/api/v2

💡 Hint

Think about how mounting routers adds path prefixes in order.

Mounting routerB at '/v2' inside routerA mounted at '/api' creates the combined path '/api/v2'. Adding '/items' gives '/api/v2/items'.