req.cookies with cookie-parser in Express - Mini Project: Build & Apply

Using req.cookies with cookie-parser in Express

📖 Scenario: You are building a simple Express server that reads cookies sent by the browser. Cookies help remember user preferences or login info.

🎯 Goal: Create an Express app that uses cookie-parser middleware to read cookies from incoming requests and display them on the homepage.

📋 What You'll Learn

Create an Express app with express()

Use cookie-parser middleware

Read cookies from req.cookies

Send the cookies as a JSON response on the root route /

💡 Why This Matters

🌍 Real World

Web servers often need to read cookies to remember user sessions, preferences, or login states.

💼 Career

Understanding how to use cookie-parser and <code>req.cookies</code> is essential for backend developers working with Express to manage user data securely and efficiently.

1

Setup Express app and import cookie-parser

Write code to import express and cookie-parser modules, then create an Express app called app.

Express

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

// Your code here

Hint

Use require('express') and require('cookie-parser') to import modules. Then call express() to create the app.

2

Add cookie-parser middleware

Add the cookie-parser middleware to the Express app by calling app.use(cookieParser()).

Express

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

// Add cookie-parser middleware here
// Your code here

Hint

Use app.use(cookieParser()) to enable cookie parsing for all routes.

3

Create a route to read cookies

Create a GET route for '/' that reads cookies from req.cookies and sends them as JSON using res.json(req.cookies).

Express

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

app.use(cookieParser());

// Create GET '/' route to send req.cookies as JSON
// Your code here

Hint

Use app.get('/', (req, res) => { ... }) and inside send req.cookies as JSON.

4

Start the server on port 3000

Add code to start the Express server on port 3000 using app.listen(3000).

Express

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

app.use(cookieParser());

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

// Start server on port 3000
// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

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 ===