Creating a basic Express server - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing the root URL?

Consider this Express server code. What will the server respond with when you visit http://localhost:3000/ in a browser?

Express

import express from 'express';
const app = express();
app.get('/', (req, res) => {
  res.send('Hello from Express!');
});
app.listen(3000);

A The browser shows the text: Hello from Express!

B The server crashes with an error

C The browser shows a blank page with status 200

D The browser shows a 404 Not Found error

💡 Hint

Look at the route defined with app.get and what it sends back.

The server listens on port 3000 and responds to GET requests at '/' with the text 'Hello from Express!'. So visiting the root URL shows that text.

📝 Syntax

intermediate

Which option correctly creates an Express server listening on port 8080?

Choose the code snippet that correctly starts an Express server on port 8080.

A

const app = express();
app.listen(8080, () => console.log('Server running on 8080'));

B

const app = express;
app.listen(8080);

C

const app = express();
app.listen(8080); console.log('Server running on 8080');

D

const app = express();
app.start(8080);

💡 Hint

Remember to call express as a function and use the correct method to start the server.

Option A correctly calls express() to create the app and uses app.listen with a callback to confirm the server is running.

🔧 Debug

advanced

Why does this Express server code crash?

This code crashes when run. What is the cause of the error?

Express

import express from 'express';
const app = express();
app.get('/', (req, res) => {
  res.send('Welcome!');
});
app.listen();

A express() must be called with a port number

B app.listen() is missing the port number argument

C res.send() must be called with an object, not a string

D app.get() requires a second argument to be a named function

💡 Hint

Check the app.listen() method usage.

app.listen() requires a port number to know where to listen. Omitting it causes a runtime error.

❓ state_output

advanced

What is the response when accessing '/user/42'?

Given this Express route, what will the server respond with when visiting /user/42?

Express

import express from 'express';
const app = express();
app.get('/user/:id', (req, res) => {
  res.send(`User ID is ${req.params.id}`);
});
app.listen(3000);

A 404 Not Found error

B User ID is undefined

C User ID is :id

D User ID is 42

💡 Hint

Look at how route parameters are accessed in Express.

The route uses a parameter :id, which is accessed via req.params.id. So the response includes the actual value '42'.

🧠 Conceptual

expert

What happens if you call app.listen() twice in the same Express app?

Consider an Express app where app.listen(3000) is called, and later app.listen(4000) is called again. What is the expected behavior?

A The app stops listening on 3000 and only listens on 4000

B The app listens on both ports 3000 and 4000 simultaneously

C The second call throws an error because the app is already listening

D The app ignores the second call and continues listening on 3000

💡 Hint

Think about how Node.js servers handle multiple listen calls on the same app instance.

Calling app.listen() twice on the same Express app instance causes an error because the underlying HTTP server is already running.