Third-party middleware usage in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when using this middleware setup?

Consider this Express.js server code using a third-party middleware. What will the server respond with when a GET request is made to /hello?

Node.js

import express from 'express';
import helmet from 'helmet';

const app = express();

app.use(helmet());

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

app.listen(3000);

A The server responds with 'Hello World!' and includes security headers added by helmet.

B The server responds with 'Hello World!' but no additional headers are added.

C The server throws an error because helmet is not compatible with Express.

D The server responds with a 404 Not Found error.

💡 Hint

Helmet adds security headers automatically when used as middleware.

The helmet() middleware adds security-related HTTP headers to all responses. Since it is used before the route handler, the response to /hello will include these headers along with the 'Hello World!' message.

📝 Syntax

intermediate

Which option correctly uses the 'cors' middleware in Express?

You want to enable CORS for all routes in your Express app using the 'cors' package. Which code snippet correctly applies the middleware?

A

import cors from 'cors';
app.use(cors);

B

import cors from 'cors';
app.use(cors());

C

const cors = require('cors');
app.use(cors);

D

const cors = require('cors');
app.use(cors());

💡 Hint

Use the middleware function returned by calling cors()

When using ES modules, you import the 'cors' package and call it as a function to get the middleware. Then you pass that middleware to app.use(). Option B correctly does this.

🔧 Debug

advanced

Why does this middleware not run for the POST request?

Given this Express app code, why does the middleware not log the message when a POST request is made to /submit?

Node.js

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

app.use('/get', (req, res, next) => {
  console.log('Middleware triggered');
  next();
});

app.post('/submit', (req, res) => {
  res.send('Submitted');
});

app.listen(3000);

A The middleware is not attached to POST requests, only GET requests.

B The middleware is missing the next() call, so it never passes control.

C The middleware only runs for routes starting with '/get', so it does not run for '/submit'.

D The app.listen call is missing a callback, so middleware is not registered.

💡 Hint

Check the path prefix used in app.use for the middleware.

The middleware is registered only for paths starting with '/get'. Since the POST request is to '/submit', the middleware does not run.

❓ state_output

advanced

What is the order of console logs when using multiple middleware?

Consider this Express app code with two middleware functions and one route handler. What is the order of console logs when a GET request is made to /test?

Node.js

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

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

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

app.get('/test', (req, res) => {
  console.log('Route handler');
  res.send('Done');
});

app.listen(3000);

A First middleware -> Second middleware -> Route handler

B Route handler -> First middleware -> Second middleware

C Second middleware -> First middleware -> Route handler

D First middleware -> Route handler -> Second middleware

💡 Hint

Middleware runs in the order they are added before the route handler.

Express executes middleware in the order they are registered. Both middleware call next() to pass control. Finally, the route handler runs.

🧠 Conceptual

expert

Which statement about third-party middleware in Express is TRUE?

Select the correct statement about using third-party middleware in Express.js applications.

A Middleware functions cannot be asynchronous and must always complete synchronously.

B Third-party middleware must always be added after all route handlers to work properly.

C Once a middleware sends a response, the next middleware in the chain will still execute automatically.

D Middleware functions can modify the request and response objects before passing control to the next middleware or route handler.

💡 Hint

Think about how middleware can change request or response data.

Middleware can modify req and res objects and decide whether to pass control to the next middleware or end the response. They can be asynchronous. Once a response is sent, next middleware does not run automatically.

Practice

1. What is the main purpose of using third-party middleware in a Node.js Express app?

easy

A. To replace the Express framework completely

B. To add extra features without writing them from scratch

C. To write all server code manually

D. To make the app run without Node.js

Solution

1. Step 1: Understand middleware role

   Middleware adds features like logging or parsing to an app without extra coding.

2. Step 2: Compare options

   Only To add extra features without writing them from scratch correctly describes middleware's purpose; others are incorrect or unrelated.

3. Final Answer:

   To add extra features without writing them from scratch -> Option B

4. Quick Check:

   Middleware adds features [OK]

Hint: Middleware adds features without extra coding [OK]

Common Mistakes:

• Thinking middleware replaces Express
• Believing middleware runs without Node.js
• Confusing middleware with full app code

2. Which of the following is the correct way to add a third-party middleware called helmet to an Express app?

easy

A. use.app(helmet);

B. app.helmet();

C. helmet(app);

D. app.use(helmet());

Solution

1. Step 1: Recall Express middleware syntax

   Middleware is added using app.use() with the middleware function called.

2. Step 2: Check options

   Only app.use(helmet()); uses app.use(helmet()); correctly; others have wrong syntax.

3. Final Answer:

   app.use(helmet()); -> Option D

4. Quick Check:

   Middleware added with app.use() [OK]

Hint: Use app.use() with middleware function call [OK]

Common Mistakes:

• Forgetting parentheses after middleware name
• Using incorrect method names like app.helmet()
• Passing middleware without calling it

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

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

medium

A. Throws an error because morgan is not used correctly

B. Responds with 'Hello World' but no logs appear

C. Logs request details in console and responds with 'Hello World'

D. Server crashes on start

Solution

1. Step 1: Understand morgan middleware

   Morgan logs HTTP request info to the console when used with app.use().

2. Step 2: Analyze code behavior

   GET '/' triggers morgan logging and sends 'Hello World' response.

3. Final Answer:

   Logs request details in console and responds with 'Hello World' -> Option C

4. Quick Check:

   morgan logs + response sent [OK]

Hint: Morgan logs requests when used with app.use() [OK]

Common Mistakes:

• Assuming morgan changes response content
• Thinking morgan causes errors if used like this
• Ignoring middleware effect on logging

4. Identify the error in this code that tries to use the cors middleware:

const express = require('express');
const cors = require('cors');
const app = express();
app.use(cors);
app.get('/', (req, res) => res.send('OK'));
app.listen(3000);

medium

A. Missing parentheses when calling cors middleware

B. cors middleware is not installed

C. app.get() is missing a callback function

D. app.listen() is called without a port

Solution

1. Step 1: Check middleware usage

   Middleware must be called as a function: cors(), not just cors.

2. Step 2: Verify code

   Code uses app.use(cors); missing parentheses, causing error.

3. Final Answer:

   Missing parentheses when calling cors middleware -> Option A

4. Quick Check:

   Middleware must be called with () [OK]

Hint: Always call middleware as a function with () [OK]

Common Mistakes:

• Passing middleware reference without ()
• Assuming app.get() lacks callback
• Forgetting to specify port in listen

5. You want to add both helmet for security and express.json() for parsing JSON bodies in your Express app. Which code correctly adds both middlewares in the right order?

hard

A. app.use(helmet()); app.use(express.json());

B. app.use(express.json); app.use(helmet);

C. app.use(helmet); app.use(express.json);

D. app.use(express.json()); app.use(helmet());

Solution

1. Step 1: Understand middleware order importance

   Security middleware like helmet should run before body parsing for best protection.

2. Step 2: Check middleware calls

   Both must be called as functions with parentheses; options B and C miss these, A has them but wrong order.

3. Step 3: Confirm correct order and syntax

   app.use(helmet()); app.use(express.json()); calls helmet first, then express.json(), both correctly called.

4. Final Answer:

   app.use(helmet()); app.use(express.json()); -> Option A

5. Quick Check:

   Helmet before json, both called [OK]

Hint: Call middleware with () and order security before parsing [OK]

Common Mistakes:

• Not calling middleware as functions
• Placing parsing before security middleware
• Mixing up middleware order