Local strategy (username/password) in NestJS - Mini Project: Build & Apply

Local Strategy (username/password) Authentication in NestJS

📖 Scenario: You are building a simple NestJS backend that allows users to log in using a username and password. This is a common way websites check who you are before letting you access private information.We will create a local authentication strategy that checks the username and password against a small list of users stored in the code.

🎯 Goal: Build a NestJS local strategy that verifies a username and password from a predefined list of users.When a user tries to log in, the system will check if the username and password match one of the users and allow access if correct.

📋 What You'll Learn

Create a list of users with usernames and passwords

Add a configuration variable for password minimum length

Implement the local strategy to validate username and password

Complete the strategy class with necessary decorators and methods

💡 Why This Matters

🌍 Real World

Local username/password authentication is a common way to secure web applications by verifying user identity before granting access.

💼 Career

Understanding how to implement authentication strategies in NestJS is essential for backend developers building secure APIs and services.

1

Create the users list

Create a constant array called users with these exact user objects: { username: 'john', password: 'changeme' } and { username: 'maria', password: 'guess' }.

NestJS

# Create the users array with two user objects
# Your code here

Hint

Use const users = [ ... ] and include the exact user objects inside the array.

2

Add password minimum length config

Create a constant called MIN_PASSWORD_LENGTH and set it to 6.

NestJS

const users = [
  { username: 'john', password: 'changeme' },
  { username: 'maria', password: 'guess' }
];

# Define the minimum password length
# Your code here

Hint

Use const MIN_PASSWORD_LENGTH = 6; to set the minimum password length.

3

Implement validate method in LocalStrategy

Create a class called LocalStrategy with a method validate(username: string, password: string) that returns the user object if the username and password match a user in users. Return null if no match is found or if the password length is less than MIN_PASSWORD_LENGTH.

NestJS

const users = [
  { username: 'john', password: 'changeme' },
  { username: 'maria', password: 'guess' }
];

const MIN_PASSWORD_LENGTH = 6;

# Create LocalStrategy class with validate method
# Your code here

Hint

Use users.find to locate the user and check password length before returning.

4

Complete LocalStrategy with decorator and export

Add the @Injectable() decorator above the LocalStrategy class and export the class with export class LocalStrategy.

NestJS

const users = [
  { username: 'john', password: 'changeme' },
  { username: 'maria', password: 'guess' }
];

const MIN_PASSWORD_LENGTH = 6;

# Add @Injectable() decorator and export the LocalStrategy class
# Your code here

Hint

Import Injectable from @nestjs/common and add @Injectable() above the class. Use export class LocalStrategy.

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