Token generation and validation in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS JWT token validation snippet?

Consider a NestJS service method that validates a JWT token using the JwtService. What will be the output if the token is expired?

NestJS

async validateToken(token: string) {
  try {
    const payload = this.jwtService.verify(token);
    return { valid: true, payload };
  } catch (error) {
    return { valid: false, error: error.message };
  }
}

A Throws an unhandled exception

B { valid: true, payload: <decoded_payload> }

C { valid: false, error: 'invalid token' }

D { valid: false, error: 'jwt expired' }

💡 Hint

Think about what happens when JwtService.verify encounters an expired token.

The JwtService.verify method throws an error with message 'jwt expired' when the token is expired. The catch block catches this and returns an object with valid false and the error message.

📝 Syntax

intermediate

Which option correctly generates a JWT token with a 1 hour expiration in NestJS?

Given the JwtService instance, which code snippet correctly creates a token that expires in 1 hour?

NestJS

const payload = { username: 'user1', sub: 123 };

A this.jwtService.sign(payload, { expiresIn: '1h' });

B this.jwtService.sign(payload, { expiration: 3600 });

C this.jwtService.sign(payload, { expires: 3600 });

D this.jwtService.sign(payload, { expireIn: '1h' });

💡 Hint

Check the correct option key name for expiration in JwtService.sign options.

The correct option key is expiresIn with a string like '1h' or a number in seconds. Other keys like 'expiration' or 'expireIn' are invalid.

🔧 Debug

advanced

Why does this NestJS token validation code always throw an error?

Analyze the following code snippet. Why does it always throw an error when validating tokens?

NestJS

async validateToken(token: string) {
  const payload = this.jwtService.verify(token, { secret: 'wrongSecret' });
  return payload;
}

A Because the secret used to verify the token is incorrect, causing a JsonWebTokenError.

B Because the token is expired and no catch block handles the error.

C Because the token is malformed and cannot be parsed.

D Because the method is missing async/await keywords.

💡 Hint

Check the secret key used for verification compared to the signing key.

The verify method throws a JsonWebTokenError if the secret used does not match the one used to sign the token. This causes the error.

🧠 Conceptual

advanced

What is the main purpose of the 'sub' claim in a JWT token?

In JWT tokens, the 'sub' claim is often included. What does it represent?

A It contains the token's signature algorithm.

B It specifies the token's expiration time.

C It identifies the subject or user the token is issued for.

D It indicates the token issuer's name.

💡 Hint

Think about what 'sub' stands for in identity tokens.

The 'sub' claim stands for 'subject' and identifies the principal that is the subject of the JWT, usually the user ID.

❓ state_output

expert

What is the value of 'isValid' after running this NestJS token validation code?

Given the following code, what will be the value of 'isValid' if the token is valid but the payload does not contain the 'sub' property?

NestJS

let isValid = false;
try {
  const payload = this.jwtService.verify(token);
  if (payload.sub) {
    isValid = true;
  }
} catch {
  isValid = false;
}

A true

B false

C Throws an error

D undefined

💡 Hint

Check the condition that sets isValid to true.

The code sets isValid to true only if payload.sub exists. If the token is valid but payload.sub is missing, isValid remains false.