Express application structure - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route handler?

Consider this Express route handler code. What will the client receive when accessing /hello?

Express

const express = require('express');
const app = express();

app.get('/hello', (req, res) => {
  res.send('Hello, world!');
});

app.listen(3000);

A The client receives a JSON object { message: 'Hello, world!' }

B The client receives the text 'Hello, world!'

C The client receives an empty response

D The server crashes with an error

💡 Hint

Look at what the res.send method sends back.

The res.send method sends the string directly as the response body. So the client will see 'Hello, world!'.

📝 Syntax

intermediate

Which option correctly sets up a middleware in Express?

You want to add a middleware that logs every request method and URL. Which code snippet correctly does this?

A app.use((req, res) => { console.log(req.method, req.url); });

B app.addMiddleware((req, res, next) => { console.log(req.method, req.url); next(); });

C app.middleware((req, res, next) => { console.log(req.method, req.url); next(); });

D app.use((req, res, next) => { console.log(req.method, req.url); next(); });

💡 Hint

Express uses a specific method to add middleware functions.

The app.use method adds middleware. The middleware must call next() to continue the request cycle.

🔧 Debug

advanced

Why does this Express app crash on startup?

Examine the code below. Why does the server crash when you run it?

Express

const express = require('express');
const app = express();

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

app.listen(3000);
console.log('Server started');

A TypeError because res.send is called incorrectly

B No error; server starts normally

C SyntaxError due to missing closing parenthesis in app.listen call

D ReferenceError because express is not defined

💡 Hint

Check the parentheses in the app.listen line.

The app.listen call is missing a closing parenthesis, causing a syntax error and preventing the server from starting.

❓ state_output

advanced

What is the value of count after these requests?

Given this Express app, what is the value of count after three GET requests to /count?

Express

const express = require('express');
const app = express();
let count = 0;

app.get('/count', (req, res) => {
  count += 1;
  res.send(`Count is ${count}`);
});

app.listen(3000);

A 3

B 1

C 0

D Undefined

💡 Hint

Each request increases count by 1.

The variable count starts at 0 and increments by 1 on each request. After 3 requests, it becomes 3.

🧠 Conceptual

expert

Which statement best describes Express app structure?

Which of the following best describes the typical structure of an Express application?

A An Express app uses modular routing and middleware, separating concerns into different files.

B Express apps require a database connection in the main file to run.

C Express apps do not support middleware functions.

D An Express app is a single file with all routes and middleware defined inline.

💡 Hint

Think about how to keep code organized and maintainable.

Express apps are usually organized by separating routes and middleware into different files to keep code clean and manageable.