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.