Token generation and validation in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the JWT module in a NestJS service.

NestJS

import { [1] } from '@nestjs/jwt';

Drag options to blanks, or click blank then click option'

AJwtStrategy

BJwtModule

CJwtGuard

DJwtService

Attempts:

3 left

💡 Hint

Common Mistakes

Importing JwtModule instead of JwtService

Using JwtGuard which is for authorization

Using JwtStrategy which is for passport strategies

The JwtService is imported from @nestjs/jwt to generate and validate tokens.

2fill in blank

medium

Complete the code to generate a JWT token with a payload.

NestJS

const token = this.jwtService.[1](payload);

Drag options to blanks, or click blank then click option'

AsignAsync

Bsign

Cdecode

Dverify

Attempts:

3 left

💡 Hint

Common Mistakes

Using verify or decode instead of sign

Using signAsync when synchronous sign is intended

The sign method creates a JWT token synchronously from the payload.

3fill in blank

hard

Fix the error in the code to verify a JWT token.

NestJS

const payload = this.jwtService.[1](token);

Drag options to blanks, or click blank then click option'

Adecode

Bsign

Cverify

DsignAsync

Attempts:

3 left

💡 Hint

Common Mistakes

Using decode which does not validate

Using sign which creates tokens

Using signAsync which is for signing

The verify method checks the token's validity and returns the decoded payload.

4fill in blank

hard

Fill both blanks to create a JWT token with expiration and secret options.

NestJS

const token = this.jwtService.sign(payload, { [1]: '1h', [2]: 'mySecretKey' });

Drag options to blanks, or click blank then click option'

AexpiresIn

Bsecret

Calgorithm

Daudience

Attempts:

3 left

💡 Hint

Common Mistakes

Using algorithm or audience instead of expiresIn or secret

Mixing up option names

The expiresIn option sets token expiry time, and secret defines the signing key.

5fill in blank

hard

Fill all three blanks to decode a token safely with error handling.

NestJS

try {
  const payload = this.jwtService.[1](token, { [2]: 'mySecretKey' });
  return payload;
} catch (error) {
  throw new [3]('Invalid token');
}

Drag options to blanks, or click blank then click option'

Averify

Bsecret

CUnauthorizedException

Ddecode

Attempts:

3 left

💡 Hint

Common Mistakes

Using decode which does not validate

Not providing the secret key

Throwing generic errors instead of UnauthorizedException

Use verify to check the token with the secret. If invalid, throw UnauthorizedException to signal access denial.

Practice

1. What is the main purpose of generating a token in a NestJS application?

easy

A. To prove a user's identity without sharing their password

B. To store user passwords securely

C. To encrypt the entire database

D. To create a backup of user data

Solution

1. Step 1: Understand token purpose

   Tokens are used to prove who a user is without sharing sensitive information like passwords.

2. Step 2: Compare options

   Only To prove a user's identity without sharing their password correctly describes token use; others describe unrelated tasks.

3. Final Answer:

   To prove a user's identity without sharing their password -> Option A

4. Quick Check:

   Token purpose = Identity proof without password [OK]

Hint: Tokens prove identity safely without passwords [OK]

Common Mistakes:

• Thinking tokens store passwords
• Confusing tokens with database encryption
• Assuming tokens backup data

2. Which of the following is the correct way to generate a JWT token using the jsonwebtoken package in NestJS?

easy

A. jwt.sign(payload, secretKey, { expiresIn: '1h' })

B. jwt.create(payload, secretKey)

C. jwt.generateToken(secretKey, payload)

D. jwt.verify(payload, secretKey)

Solution

1. Step 1: Recall JWT token creation syntax

   The sign method creates a token with payload, secret key, and options.

2. Step 2: Check options

   Only jwt.sign(payload, secretKey, { expiresIn: '1h' }) uses sign correctly with payload, secret, and expiration.

3. Final Answer:

   jwt.sign(payload, secretKey, { expiresIn: '1h' }) -> Option A

4. Quick Check:

   Token creation uses sign() method [OK]

Hint: Use jwt.sign() to create tokens with payload and secret [OK]

Common Mistakes:

• Using jwt.create() which does not exist
• Mixing order of arguments
• Using jwt.verify() to create tokens

3. Given the following code snippet in a NestJS service, what will be the output if the token is valid?

const token = jwt.sign({ userId: 123 }, 'secretKey', { expiresIn: '1h' });
const decoded = jwt.verify(token, 'secretKey');
console.log(decoded.userId);

medium

A. undefined

B. null

C. 123

D. Throws an error

Solution

1. Step 1: Understand token creation and verification

   The token is created with payload { userId: 123 } and signed with 'secretKey'.

2. Step 2: Verify token with the same secret

   Verification returns the decoded payload, so decoded.userId is 123.

3. Final Answer:

   123 -> Option C

4. Quick Check:

   Valid token returns payload userId = 123 [OK]

Hint: Verify returns original payload if token and secret match [OK]

Common Mistakes:

• Expecting undefined or null instead of payload
• Using wrong secret causing error
• Confusing verify with sign

4. Identify the error in this NestJS token validation code:

try {
  const payload = jwt.verify(token, secret);
  return payload.userId;
} catch (error) {
  return 'Invalid token';
}

medium

A. Secret key is not defined

B. Missing async keyword for jwt.verify

C. jwt.verify should be jwt.sign

D. No error, code works correctly

Solution

1. Step 1: Analyze jwt.verify usage

   jwt.verify is synchronous and used correctly inside try-catch to handle errors.

2. Step 2: Check for missing elements

   Assuming secret and token are defined, no syntax or logic errors exist.

3. Final Answer:

   No error, code works correctly -> Option D

4. Quick Check:

   jwt.verify with try-catch is correct pattern [OK]

Hint: jwt.verify is synchronous; use try-catch for errors [OK]

Common Mistakes:

• Thinking jwt.verify is async
• Confusing verify with sign
• Assuming secret is undefined without context

5. You want to generate a JWT token in NestJS that expires in 30 minutes and includes the user's email and role. Which code snippet correctly achieves this?

hard

A. jwt.sign({ email, role }, secretKey, { expiresAt: '30m' })

B. jwt.sign({ email, role }, secretKey, { expiresIn: '30m' })

C. jwt.sign({ email, role }, secretKey, { expiration: 1800 })

D. jwt.sign({ email, role }, secretKey, { expireIn: 30 })

Solution

1. Step 1: Recall correct expiration option for jwt.sign

   The option to set token lifetime is expiresIn with string or number format.

2. Step 2: Validate option values

   Only jwt.sign({ email, role }, secretKey, { expiresIn: '30m' }) uses expiresIn: '30m' correctly; others use invalid keys or formats.

3. Final Answer:

   jwt.sign({ email, role }, secretKey, { expiresIn: '30m' }) -> Option B

4. Quick Check:

   Use expiresIn with '30m' for 30 minutes expiry [OK]

Hint: Use expiresIn: '30m' to set 30-minute token expiry [OK]

Common Mistakes:

• Using expiresAt or expiration instead of expiresIn
• Passing number without units as string
• Misspelling expiresIn option