What is Express - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is Express used for in Node.js?

Express is a popular framework in Node.js. What is its main purpose?

A To style web pages with CSS

B To manage databases and perform SQL queries

C To create web servers and handle HTTP requests easily

D To compile JavaScript into machine code

💡 Hint

Think about what web servers do and how Express helps with that.

Express is a minimal and flexible Node.js web application framework that provides tools to build web servers and APIs. It simplifies handling HTTP requests and responses.

❓ component_behavior

intermediate

What does this Express code output when accessed at '/'?

Consider this Express app code. What will a user see when they visit the root URL '/'?

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 browser shows a 404 Not Found error

C The server crashes with an error

D The browser shows a blank page

💡 Hint

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

The app listens for GET requests at '/' and responds with the text 'Hello from Express!'. So visiting '/' shows that text.

📝 Syntax

advanced

Which option correctly creates an Express app and starts listening on port 4000?

Choose the code snippet that correctly creates an Express app and starts the server on port 4000 without errors.

A

const express = require('express')
const app = express
app.listen(4000)

B

import express from 'express'
const app = express()
app.listen(4000, () => console.log('Server running'))

C

import express from 'express'
const app = express
app.listen(4000, () => console.log('Server running'))

D

const express = require('express')
const app = express()
app.listen(4000, () => console.log('Server running'))

💡 Hint

Remember to call the express function to create the app and use correct import syntax for ES modules.

Option B uses ES module import syntax and calls express() to create the app. It also provides a callback to listen(). Options C and D forget to call express() as a function. Option B uses CommonJS require which may not work if ES modules are expected.

🔧 Debug

advanced

Why does this Express app crash with 'TypeError: app.get is not a function'?

Look at this code snippet. Why does it crash with the error 'TypeError: app.get is not a function'?

Express

import express from 'express';
const app = express();
app.get('/', (req, res) => res.send('Hi'));
app.listen(3000);

A Because express is not called as a function to create the app instance

B Because the route '/' is missing a callback function

C Because app.listen is missing a port number

D Because import syntax is incorrect

💡 Hint

Check how the app variable is assigned from express.

The code assigns 'app = express' instead of 'app = express()'. Without calling express(), app is just a reference to the function, not an Express app instance. So app.get is undefined.

❓ state_output

expert

What is the output after these Express middleware run in order?

Given this Express app with middleware, what will the client see when requesting '/'?

Express

import express from 'express';
const app = express();

app.use((req, res, next) => {
  req.message = 'Hello';
  next();
});

app.use((req, res, next) => {
  req.message += ' from middleware';
  next();
});

app.get('/', (req, res) => {
  res.send(req.message);
});

app.listen(3000);

A Error: req.message is undefined

B Hello

C from middleware

D Hello from middleware

💡 Hint

Think about how middleware modifies the request object before the route handler runs.

The first middleware sets req.message to 'Hello'. The second appends ' from middleware'. The route sends req.message, so the output is 'Hello from middleware'.