Third-party middleware installation in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when using this middleware setup?

Consider this Express app code snippet. What will be the response when a GET request is made to '/'?

Express

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

const app = express();

app.use(helmet());

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

app.listen(3000);

A The response will be 'Hello World' with added security headers from helmet.

B The response will be a 404 error because helmet blocks all routes by default.

C The server will crash because helmet is not configured properly.

D The response will be 'Hello World' but without any security headers.

💡 Hint

Helmet adds security headers but does not block routes.

Helmet is a middleware that adds security headers to responses. It does not block routes or crash the server if used with default settings. The GET '/' route sends 'Hello World' as response, enhanced by helmet headers.

📝 Syntax

intermediate

Which option correctly installs and uses the 'cors' middleware?

You want to enable Cross-Origin Resource Sharing (CORS) in your Express app using the 'cors' package. Which code snippet correctly installs and applies the middleware?

A

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

B

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

C

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

D

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

💡 Hint

Use ES module import and call the middleware function.

Option B correctly imports 'cors' using ES module syntax and calls it as a function in app.use. Options A and B use CommonJS require which is invalid if using ES modules. Option B forgets to call cors as a function.

🔧 Debug

advanced

Why does this middleware not run as expected?

Given this Express app code, why does the custom middleware not log requests?

Express

import express from 'express';

const app = express();

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

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

app.listen(3000);

A The middleware function is not called because app.use is incorrect.

B The middleware is missing the 'next' parameter, so it blocks the request.

C The app.listen call is missing a callback, so middleware is skipped.

D The middleware is placed after the route, so it never runs for '/' requests.

💡 Hint

Middleware order matters in Express.

Express runs middleware and routes in the order they are defined. Since the middleware is defined after the '/' route, requests to '/' are handled before the middleware runs, so the log never appears.

❓ state_output

advanced

What is the output of this middleware chain?

Analyze this Express app code. What will be the response body when a GET request is made to '/'?

Express

import express from 'express';

const app = express();

app.use((req, res, next) => {
  req.customValue = 5;
  next();
});

app.use((req, res, next) => {
  req.customValue += 10;
  next();
});

app.get('/', (req, res) => {
  res.send(`Value is ${req.customValue}`);
});

app.listen(3000);

A "Value is 10"

B "Value is 5"

C "Value is 15"

D Undefined, because req.customValue is not set

💡 Hint

Middleware can add and modify request properties.

The first middleware sets req.customValue to 5. The second adds 10, making it 15. The route sends 'Value is 15'.

🧠 Conceptual

expert

Which statement about third-party middleware installation is true?

Select the correct statement about installing and using third-party middleware in Express.

A Third-party middleware must be installed via npm and imported before use in the app.

B Third-party middleware can be used without installation if the package name is known.

C Middleware functions must always be placed after route handlers to work correctly.

D Express automatically installs popular middleware packages when detected in code.

💡 Hint

Think about how Node.js packages work.

Third-party middleware must be installed with npm or yarn and imported before use. Middleware order matters and usually middleware is placed before routes. Express does not auto-install packages.

Practice

1. What is the main purpose of third-party middleware in an Express app?

easy

A. To add extra features like parsing or logging easily

B. To replace the Express framework completely

C. To write HTML pages automatically

D. To create a database connection

Solution

1. Step 1: Understand middleware role

   Middleware adds extra functions to Express apps, like parsing data or logging requests.

2. Step 2: Compare options

   Replacing Express or creating databases are not middleware tasks; middleware enhances features.

3. Final Answer:

   To add extra features like parsing or logging easily -> Option A

4. Quick Check:

   Middleware adds features = A [OK]

Hint: Middleware adds features, not replaces core framework [OK]

Common Mistakes:

• Thinking middleware replaces Express
• Confusing middleware with database tools
• Assuming middleware generates HTML pages

2. Which code snippet correctly installs and uses the cors middleware in an Express app?

easy

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

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

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

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

Solution

1. Step 1: Check import style

   Modern Express apps use ES modules, so import cors from 'cors'; is correct.

2. Step 2: Check middleware usage

   Middleware must be called as a function: app.use(cors()); not just cors.

3. Final Answer:

   import cors from 'cors'; app.use(cors()); -> Option C

4. Quick Check:

   Import + call middleware function = C [OK]

Hint: Import middleware and call it with () in app.use [OK]

Common Mistakes:

• Forgetting parentheses after middleware name
• Mixing require and import syntax
• Using app.use(cors) without calling it

3. What will be the output when this Express app code runs and receives a JSON POST request?

import express from 'express';
import bodyParser from 'body-parser';
const app = express();
app.use(bodyParser.json());
app.post('/data', (req, res) => {
  res.send(req.body.name);
});
app.listen(3000);

medium

A. The server responds with an empty string

B. The server responds with '[object Object]'

C. The server crashes with a syntax error

D. The server responds with the value of 'name' from the JSON body

Solution

1. Step 1: Understand bodyParser.json()

   This middleware parses JSON request bodies and sets req.body to the parsed object.

2. Step 2: Check response code

   The route sends back req.body.name, so the response is the 'name' value from JSON.

3. Final Answer:

   The server responds with the value of 'name' from the JSON body -> Option D

4. Quick Check:

   bodyParser parses JSON, response sends req.body.name = B [OK]

Hint: bodyParser.json() parses JSON, so req.body has data [OK]

Common Mistakes:

• Not using bodyParser.json(), so req.body is undefined
• Expecting raw JSON string instead of parsed object
• Sending req.body instead of req.body.name

4. Identify the error in this Express app code using helmet middleware:

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

medium

A. helmet is imported incorrectly

B. helmet middleware is not called as a function

C. app.listen is missing a callback

D. app.get route is missing a response

Solution

1. Step 1: Check middleware usage

   Middleware must be called as a function: app.use(helmet()); not app.use(helmet);.

2. Step 2: Verify other code parts

   Import is correct, app.listen works without callback, and route sends response properly.

3. Final Answer:

   helmet middleware is not called as a function -> Option B

4. Quick Check:

   Middleware must be called with () = A [OK]

Hint: Always call middleware with () in app.use [OK]

Common Mistakes:

• Using app.use(helmet) instead of app.use(helmet())
• Thinking app.listen needs a callback
• Forgetting to send response in route

5. You want to add morgan logging middleware only for POST requests in your Express app. Which code correctly applies this?

hard

A. app.use((req, res, next) => { if(req.method === 'POST') morgan('dev')(req, res, next); else next(); });

B. app.post('*', morgan('dev'));

C. app.use(morgan('dev')); if(req.method === 'POST') next();

D. app.use('/post', morgan('dev'));

Solution

1. Step 1: Understand middleware conditional use

   To run middleware only on POST, wrap it in a function checking req.method.

2. Step 2: Analyze options

   app.use((req, res, next) => { if(req.method === 'POST') morgan('dev')(req, res, next); else next(); }); calls morgan only if method is POST, else calls next(). Others either apply to wrong paths or misuse middleware.

3. Final Answer:

   app.use((req, res, next) => { if(req.method === 'POST') morgan('dev')(req, res, next); else next(); }); -> Option A

4. Quick Check:

   Conditional middleware call for POST = D [OK]

Hint: Wrap middleware in function checking req.method for selective use [OK]

Common Mistakes:

• Using app.use with path that doesn't match POST method
• Calling middleware without next() in else branch
• Trying to use app.post with middleware incorrectly