req.cookies with cookie-parser in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does req.cookies contain after using cookie-parser?

Consider an Express app using cookie-parser middleware. A client sends a request with the header Cookie: user=alice; theme=dark. What will req.cookies contain inside a route handler?

Express

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.use(cookieParser());
app.get('/', (req, res) => {
  res.json(req.cookies);
});

A {"user": "alice", "theme": "dark"}

B ["user=alice", "theme=dark"]

C undefined

D "user=alice; theme=dark"

💡 Hint

Think about how cookie-parser parses the cookie header into an object.

The cookie-parser middleware parses the cookie header string into an object where each cookie name is a key and its value is the value. So req.cookies is an object like { user: 'alice', theme: 'dark' }.

📝 Syntax

intermediate

Which code correctly uses cookie-parser to access cookies?

Which of the following Express app snippets correctly sets up cookie-parser and accesses a cookie named sessionId?

A

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.use(cookieParser);
app.get('/', (req, res) => {
  res.send(req.cookies.sessionId);
});

B

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.get('/', (req, res) => {
  res.send(req.cookies.sessionId);
});
app.use(cookieParser());

C

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.use(cookieParser());
app.get('/', (req, res) => {
  res.send(req.cookies.sessionId);
});

D

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.use(cookieParser());
app.get('/', (req, res) => {
  res.send(req.cookie.sessionId);
});

💡 Hint

Remember to call cookieParser() as a function and use req.cookies (plural).

Option C correctly calls cookieParser() middleware before routes and accesses req.cookies.sessionId. Option C applies middleware after routes, so cookies won't be parsed. Option C passes the middleware function without calling it. Option C uses req.cookie instead of req.cookies.

🔧 Debug

advanced

Why does req.cookies remain empty despite sending cookies?

An Express app uses cookie-parser middleware. The client sends cookies, but req.cookies is always empty. What is the most likely cause?

Express

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.get('/', (req, res) => {
  res.json(req.cookies);
});
app.use(cookieParser());

A The client did not send any cookies in the request headers.

B The middleware <code>cookie-parser</code> is used after the route, so cookies are not parsed before the route runs.

C The <code>cookie-parser</code> middleware is missing the secret key argument.

D The <code>req.cookies</code> property is deprecated and replaced by <code>req.signedCookies</code>.

💡 Hint

Middleware order matters in Express.

Middleware must be registered before routes to affect them. Here, cookie-parser is registered after the route, so req.cookies is empty inside the route handler.

❓ state_output

advanced

What is the value of req.signedCookies after using cookie-parser('secret')?

Given the Express app below, what will req.signedCookies contain if the client sends a cookie token=abc123 signed with the secret secret?

Express

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();
app.use(cookieParser('secret'));
app.get('/', (req, res) => {
  res.json({
    cookies: req.cookies,
    signedCookies: req.signedCookies
  });
});

A undefined

B {}

C null

D {"token": "abc123"}

💡 Hint

Signed cookies are verified and available in req.signedCookies.

When cookie-parser is initialized with a secret, it verifies signed cookies. If the signature matches, the cookie appears in req.signedCookies. So req.signedCookies contains the verified cookie { token: 'abc123' }.

🧠 Conceptual

expert

Which statement about cookie-parser and req.cookies is TRUE?

Choose the correct statement about how cookie-parser works with req.cookies in Express.

A <code>cookie-parser</code> parses the <code>Cookie</code> header and populates <code>req.cookies</code> as an object with cookie names and values.

B <code>req.cookies</code> is automatically available in Express without any middleware.

C <code>cookie-parser</code> encrypts cookies before sending them to the client.

D <code>req.cookies</code> contains only signed cookies when <code>cookie-parser</code> is used with a secret.

💡 Hint

Think about what cookie-parser does and what req.cookies holds.

cookie-parser reads the Cookie header and creates an object req.cookies with cookie names and values. It does not encrypt cookies; encryption is done elsewhere. req.cookies is not available without middleware. Signed cookies appear in req.signedCookies, not req.cookies.

Practice

1. What does req.cookies represent in an Express app when using cookie-parser middleware?

easy

A. A function to set cookies on the response

B. An object containing cookies sent by the browser

C. A string of all cookies concatenated

D. An array of cookie names only

Solution

1. Step 1: Understand the role of cookie-parser

   The cookie-parser middleware parses cookies from the HTTP request headers.

2. Step 2: Identify what req.cookies holds

   After parsing, req.cookies is an object where each cookie name is a key and its value is the cookie's value.

3. Final Answer:

   An object containing cookies sent by the browser -> Option B

4. Quick Check:

   req.cookies = object of cookies [OK]

Hint: Remember: req.cookies is an object, not a string or function [OK]

Common Mistakes:

• Thinking req.cookies is a function
• Expecting req.cookies to be a string
• Confusing req.cookies with response cookies

2. Which of the following is the correct way to use cookie-parser middleware in an Express app?

easy

A. app.use(cookieParser);

B. app.cookieParser();

C. app.use(cookieParser());

D. ;)(resraPeikooc.ppa

Solution

1. Step 1: Recall middleware usage in Express

   Middleware functions are added using app.use() and must be called as functions if they return middleware.

2. Step 2: Apply this to cookie-parser

   The correct syntax is app.use(cookieParser()); to invoke the function and add the middleware.

3. Final Answer:

   app.use(cookieParser()); -> Option C

4. Quick Check:

   Middleware added with app.use(cookieParser()) [OK]

Hint: Use parentheses to call middleware function inside app.use [OK]

Common Mistakes:

• Forgetting parentheses after cookieParser
• Calling cookieParser as a method on app
• Passing cookieParser without calling it

3. Given this Express route with cookie-parser middleware, what will be the output if the browser sends a cookie named user with value alice?

app.get('/profile', (req, res) => {
  res.send(`User: ${req.cookies.user}`);
});

medium

A. User: [object Object]

B. User: undefined

C. Error: req.cookies is not defined

D. User: alice

Solution

1. Step 1: Check if cookie-parser is used

   Assuming cookie-parser middleware is active, req.cookies contains parsed cookies.

2. Step 2: Access the user cookie value

   The code accesses req.cookies.user, which equals 'alice' from the browser cookie.

3. Final Answer:

   User: alice -> Option D

4. Quick Check:

   Cookie value accessed correctly = User: alice [OK]

Hint: Access cookie by name on req.cookies object [OK]

Common Mistakes:

• Not using cookie-parser so req.cookies is undefined
• Trying to access cookies as a string
• Assuming cookie names are case-insensitive

4. What is wrong with this Express code snippet if req.cookies is always undefined?

const express = require('express');
const cookieParser = require('cookie-parser');
const app = express();

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

app.use(cookieParser());

medium

A. cookie-parser middleware is added after the route, so req.cookies is undefined

B. cookie-parser is not imported correctly

C. The route handler should use res.cookies instead

D. Express does not support cookies natively

Solution

1. Step 1: Check middleware order

   Middleware must be added before routes to affect req objects in those routes.

2. Step 2: Identify the problem in code

   Here, app.use(cookieParser()) is after the route, so req.cookies is not set when the route runs.

3. Final Answer:

   cookie-parser middleware is added after the route, so req.cookies is undefined -> Option A

4. Quick Check:

   Middleware order matters = cookie-parser middleware is added after the route, so req.cookies is undefined [OK]

Hint: Add middleware before routes to access req.cookies [OK]

Common Mistakes:

• Placing middleware after routes
• Confusing req.cookies with res.cookies
• Not calling cookieParser as a function

5. You want to create an Express route that reads a cookie named theme and sends back 'Dark mode' if its value is 'dark', otherwise 'Light mode'. Which code correctly implements this using req.cookies with cookie-parser?

hard

A. app.get('/mode', (req, res) => {
if (req.cookies.theme === 'dark') res.send('Dark mode');
else res.send('Light mode');
});

B. app.get('/mode', (req, res) => {
if (req.cookies['theme'] = 'dark') res.send('Dark mode');
else res.send('Light mode');
});

C. app.get('/mode', (req, res) => {
if (req.cookies.theme == dark) res.send('Dark mode');
else res.send('Light mode');
});

D. app.get('/mode', (req, res) => {
if (req.cookies.theme === dark) res.send('Dark mode');
else res.send('Light mode');
});

Solution

1. Step 1: Check cookie access and comparison

   Use strict equality === to compare req.cookies.theme to the string 'dark'.

2. Step 2: Identify correct syntax

   app.get('/mode', (req, res) => { if (req.cookies.theme === 'dark') res.send('Dark mode'); else res.send('Light mode'); }); uses === and string 'dark' correctly. app.get('/mode', (req, res) => { if (req.cookies['theme'] = 'dark') res.send('Dark mode'); else res.send('Light mode'); }); uses assignment = instead of comparison. app.get('/mode', (req, res) => { if (req.cookies.theme == dark) res.send('Dark mode'); else res.send('Light mode'); }); uses loose equality == with unquoted dark. app.get('/mode', (req, res) => { if (req.cookies.theme === dark) res.send('Dark mode'); else res.send('Light mode'); }); uses undefined variable dark without quotes.

3. Final Answer:

   The code using strict equality req.cookies.theme === 'dark' -> Option A

4. Quick Check:

   Use === 'dark' to compare cookie value [OK]

Hint: Use === and quotes for string comparison with cookie values [OK]

Common Mistakes:

• Using = instead of === for comparison
• Not quoting string literals
• Using loose equality == instead of ===