Local strategy (username/password) in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Passport local strategy in NestJS.

NestJS

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

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

APassportModule

BJwtStrategy

CPassportStrategy

DAuthGuard

Attempts:

3 left

💡 Hint

Common Mistakes

Importing JwtStrategy instead of PassportStrategy

Importing AuthGuard which is not a strategy

Importing PassportModule which is a module, not a strategy

The PassportStrategy is imported from @nestjs/passport to implement username/password authentication.

2fill in blank

medium

Complete the constructor to inject the AuthService in the LocalStrategy class.

NestJS

constructor(private readonly [1]: AuthService) {}

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

AuserService

BpassportService

ClocalService

DauthService

Attempts:

3 left

💡 Hint

Common Mistakes

Using userService which is not injected here

Using localService which is not defined

Using passportService which does not exist

The authService is injected to validate user credentials in the local strategy.

3fill in blank

hard

Fix the error in the validate method signature to properly receive username and password.

NestJS

async validate([1]: string, password: string): Promise<any> {

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

Aemail

Busername

Cuser

Dlogin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'user' which is an object, not a string parameter

Using 'email' which is not the default username field

Using 'login' which is not a standard parameter name

The validate method receives username and password as parameters to check credentials.

4fill in blank

hard

Fill both blanks to correctly call the AuthService validateUser method and handle invalid user.

NestJS

const user = await this.authService.[1]([2], password);
if (!user) {
  throw new UnauthorizedException();
}

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

AvalidateUser

Busername

Cemail

DcheckUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'checkUser' which is not defined

Passing 'email' instead of 'username'

Calling a method other than 'validateUser'

The validateUser method is called with username and password to verify credentials.

5fill in blank

hard

Fill all three blanks to complete the LocalStrategy class extending PassportStrategy and calling super with the correct options.

NestJS

export class LocalStrategy extends PassportStrategy([1]) {
  constructor(private readonly authService: AuthService) {
    super({ usernameField: [2] });
  }

  async validate(username: string, [3]: string): Promise<any> {
    const user = await this.authService.validateUser(username, password);
    if (!user) {
      throw new UnauthorizedException();
    }
    return user;
  }
}

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

AStrategy

B'username'

Cpassword

DLocalStrategy

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 'LocalStrategy' instead of 'Strategy' to PassportStrategy

Using usernameField other than 'username'

Missing the password parameter in validate method

The class extends PassportStrategy(Strategy). The super call sets usernameField to 'username'. The validate method receives username and password.

Practice

1. What is the main purpose of the LocalStrategy in NestJS authentication?

easy

A. To encrypt user passwords

B. To generate JWT tokens for users

C. To handle user registration

D. To verify username and password for user login

Solution

1. Step 1: Understand the role of LocalStrategy

   The LocalStrategy is designed to check user credentials, specifically username and password, during login.

2. Step 2: Differentiate from other strategies

   Unlike JWT or OAuth strategies, LocalStrategy focuses on validating credentials, not token generation or registration.

3. Final Answer:

   To verify username and password for user login -> Option D

4. Quick Check:

   LocalStrategy = username/password check [OK]

Hint: Local strategy always checks username and password [OK]

Common Mistakes:

• Confusing LocalStrategy with JWT token generation
• Thinking LocalStrategy handles user registration
• Assuming LocalStrategy encrypts passwords

2. Which method must be implemented in a NestJS LocalStrategy class to check user credentials?

easy

A. authenticate()

B. validate()

C. login()

D. verify()

Solution

1. Step 1: Recall LocalStrategy method names

   In NestJS, the LocalStrategy requires a validate method to check username and password.

2. Step 2: Confirm method purpose

   The validate method returns the user if credentials are correct or throws an error if not.

3. Final Answer:

   validate() -> Option B

4. Quick Check:

   Credential check method = validate() [OK]

Hint: LocalStrategy uses validate() to check credentials [OK]

Common Mistakes:

• Using authenticate() which is not a LocalStrategy method
• Confusing login() with validate()
• Assuming verify() is required

3. Given this LocalStrategy snippet, what happens if the password is incorrect?

async validate(username: string, password: string) {
  const user = await this.authService.validateUser(username, password);
  if (!user) {
    throw new UnauthorizedException();
  }
  return user;
}

medium

A. The function returns null silently

B. The user object is returned anyway

C. An UnauthorizedException is thrown stopping login

D. The password is reset automatically

Solution

1. Step 1: Analyze the validate method logic

   If validateUser returns no user (password incorrect), the code throws UnauthorizedException.

2. Step 2: Understand exception effect

   Throwing UnauthorizedException stops the login process and signals failure.

3. Final Answer:

   An UnauthorizedException is thrown stopping login -> Option C

4. Quick Check:

   Wrong password = UnauthorizedException thrown [OK]

Hint: Throw UnauthorizedException if user not found [OK]

Common Mistakes:

• Assuming user is returned even if password is wrong
• Thinking function returns null without error
• Believing password resets automatically

4. Identify the error in this LocalStrategy validate method:

async validate(username: string, password: string) {
  const user = this.authService.validateUser(username, password);
  if (!user) {
    throw new UnauthorizedException();
  }
  return user;
}

medium

A. Missing await before authService.validateUser call

B. UnauthorizedException should not be thrown

C. validate method should not be async

D. username and password parameters are reversed

Solution

1. Step 1: Check async call usage

   The method calls validateUser which returns a Promise, but lacks await.

2. Step 2: Understand consequences of missing await

   Without await, user is a Promise, so if (!user) check is incorrect and login logic breaks.

3. Final Answer:

   Missing await before authService.validateUser call -> Option A

4. Quick Check:

   Async calls need await to get results [OK]

Hint: Always await async calls in validate() [OK]

Common Mistakes:

• Forgetting await on async service calls
• Throwing exceptions incorrectly
• Misordering parameters

5. You want to customize the LocalStrategy to allow login with either username or email. Which change is best to implement in the validate method?

hard

A. Modify validateUser to accept a single identifier and check both username and email

B. Add a second validate method for email login

C. Use two separate LocalStrategy classes for username and email

D. Change the password parameter to accept email instead

Solution

1. Step 1: Understand the goal

   Allow login by username or email using one validate method.

2. Step 2: Best approach

   Modify validateUser to accept one identifier and check if it matches username or email, then verify password.

3. Step 3: Why other options fail

   Adding multiple validate methods or strategies complicates logic; changing password parameter is incorrect.

4. Final Answer:

   Modify validateUser to accept a single identifier and check both username and email -> Option A

5. Quick Check:

   One validate method, flexible identifier [OK]

Hint: Use one validate method checking username or email [OK]

Common Mistakes:

• Creating multiple validate methods unnecessarily
• Using separate strategies for username and email
• Confusing password parameter with identifier