How Express builds on Node.js HTTP - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Express middleware do?

Consider this Express middleware function:

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

What will be printed when a GET request is made to the server?

Express

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

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

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

app.listen(3000);

A undefined

B POST

C GET

D Error

💡 Hint

Look at the request method property logged.

The middleware logs the HTTP method of the incoming request. For a GET request, it prints 'GET'.

📝 Syntax

intermediate

Identify the correct way to create an Express server using Node.js HTTP module

Which option correctly creates an Express app and starts a server using Node.js HTTP module?

Express

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

A http.createServer(app.listen(3000));

B

const server = http.createServer(app);
server.listen(3000);

C

app.createServer(http);
app.listen(3000);

D

const server = http.createServer();
server.listen(3000, app);

💡 Hint

Express apps can be passed as request listeners to Node.js HTTP servers.

Option B correctly passes the Express app as the request handler to http.createServer and then listens on port 3000.

🔧 Debug

advanced

Why does this Express app not respond to requests?

Given this code, why does the server not respond to requests?

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

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

// Missing app.listen call

Express

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

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

A Express requires a callback in app.get to be async

B The route handler is incorrect and causes an error

C The app needs to import http module to work

D The server never starts listening because app.listen is missing

💡 Hint

Check if the server is actually listening for requests.

Without calling app.listen, the Express app does not start a server and cannot respond to requests.

❓ state_output

advanced

What is the output of this Express app using Node.js HTTP server?

What will be the response body when a client requests '/'?

Express

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

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

const server = http.createServer(app);
server.listen(3000);

A Welcome

B Cannot GET /

C Error: app is not a function

D Empty response

💡 Hint

Express app can be used as a request handler for Node.js HTTP server.

The HTTP server forwards requests to the Express app which sends 'Welcome' for '/' route.

🧠 Conceptual

expert

How does Express extend Node.js HTTP request and response objects?

Which statement best describes how Express builds on Node.js HTTP module?

A Express wraps Node.js HTTP request and response objects to add helper methods like res.send and req.params

B Express replaces Node.js HTTP module with its own implementation

C Express uses a different protocol than HTTP for requests and responses

D Express requires manual parsing of request body unlike Node.js HTTP

💡 Hint

Think about how Express adds convenience methods to the request and response.

Express extends the native Node.js HTTP objects by adding useful methods and properties to simplify handling requests and responses.

Practice

1. What is the main advantage of using Express over the basic Node.js HTTP module?

easy

A. It replaces the HTTP protocol with a new one.

B. It simplifies routing and middleware handling.

C. It automatically creates databases for your app.

D. It runs Node.js code faster by compiling it.

Solution

1. Step 1: Understand Node.js HTTP module

   The Node.js HTTP module provides basic tools to create servers but requires manual handling of routing and requests.

2. Step 2: Identify Express features

   Express adds easy routing and middleware support, making it simpler to write web apps without dealing with low-level HTTP details.

3. Final Answer:

   It simplifies routing and middleware handling. -> Option B

4. Quick Check:

   Express adds routing and middleware = A [OK]

Hint: Express adds routing and middleware on top of HTTP [OK]

Common Mistakes:

• Thinking Express changes the HTTP protocol
• Believing Express creates databases automatically
• Assuming Express speeds up Node.js runtime

2. Which of the following is the correct way to create an Express app instance?

easy

A. const app = express();

B. const app = express.createApp();

C. const app = new express();

D. const app = express.app();

Solution

1. Step 1: Recall Express app creation syntax

   Express exports a function that when called returns an app instance, so you use express() directly.

2. Step 2: Check each option

   const app = express(); uses express() correctly. The other options use invalid methods or constructors.

3. Final Answer:

   const app = express(); -> Option A

4. Quick Check:

   Express app created by calling express() [OK]

Hint: Create app by calling express() function [OK]

Common Mistakes:

• Using new express() instead of express()
• Trying express.createApp() which doesn't exist
• Calling express.app() which is invalid

3. Given this Express code snippet, what will be the response when a GET request is made to '/'?

const express = require('express');
const app = express();
app.get('/', (req, res) => {
  res.send('Hello World');
});
app.listen(3000);

medium

A. The server throws an error because listen is missing a callback.

B. The server responds with an empty page.

C. The server responds with 'Hello World'.

D. The server crashes due to missing middleware.

Solution

1. Step 1: Analyze the route handler

   The app defines a GET route for '/' that sends 'Hello World' as the response.

2. Step 2: Check server listen and response

   Calling app.listen(3000) starts the server; no callback is required for it to work. The response will be 'Hello World'.

3. Final Answer:

   The server responds with 'Hello World'. -> Option C

4. Quick Check:

   GET '/' sends 'Hello World' [OK]

Hint: GET '/' route sends 'Hello World' response [OK]

Common Mistakes:

• Thinking listen needs a callback to work
• Assuming missing middleware causes crash
• Expecting empty response without send()

4. What is wrong with this Express code snippet?

const express = require('express');
const app = express();
app.get('/hello', (req, res) => {
  res.send('Hi there!');
});
app.listen(3000)
console.log('Server started');

medium

A. Missing semicolon after app.listen(3000) causes syntax error.

B. The server will not respond because app.listen lacks a callback.

C. The route handler uses res.send incorrectly.

D. The code works fine; no errors present.

Solution

1. Step 1: Check syntax and semicolons

   JavaScript does not require semicolons strictly; missing one here does not cause an error.

2. Step 2: Verify app.listen and route handler

   app.listen without a callback works fine. res.send is used correctly to send a response.

3. Final Answer:

   The code works fine; no errors present. -> Option D

4. Quick Check:

   Missing semicolon is allowed; code runs [OK]

Hint: Missing semicolons often don't cause errors in Node.js [OK]

Common Mistakes:

• Assuming missing semicolon causes syntax error
• Thinking app.listen needs a callback to work
• Believing res.send usage is incorrect here

5. How does Express simplify handling multiple routes compared to using Node.js HTTP module directly?

hard

A. Express lets you define routes with app.get/post methods instead of manual URL checks.

B. Express automatically generates HTML pages for each route.

C. Express replaces HTTP methods with custom ones for easier coding.

D. Express requires you to write more code to handle each route.

Solution

1. Step 1: Understand Node.js HTTP routing

   Using Node.js HTTP module, you must manually check request URLs and methods to handle routes.

2. Step 2: Understand Express routing

   Express provides app.get(), app.post(), etc., to define routes clearly and simply without manual URL parsing.

3. Final Answer:

   Express lets you define routes with app.get/post methods instead of manual URL checks. -> Option A

4. Quick Check:

   Express routing uses app.get/post for easy route handling [OK]

Hint: Express uses app.get/post for easy route setup [OK]

Common Mistakes:

• Thinking Express auto-generates HTML pages
• Believing Express changes HTTP methods
• Assuming Express requires more code for routes