Local strategy (username/password) in NestJS - Step-by-Step Execution

Concept Flow - Local strategy (username/password)

User submits username & password

↓

LocalStrategy validate() called

↓

Check username & password in DB

↓

Return user object

↓

Passport attaches user to request

↓

Request proceeds to controller

User sends credentials, LocalStrategy checks them, returns user if valid, else rejects.

Execution Sample

NestJS

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

This function checks username and password, returns user if valid, else throws error.

Execution Table

Step	Action	Input	Check Result	Outcome
1	Call validate()	username='alice', password='secret'	N/A	Proceed to check user
2	Call authService.validateUser()	username='alice', password='secret'	User found and password matches	Returns user object
3	Check if user exists	user object	User exists	Return user
4	Attach user to request	user object	N/A	User attached, request proceeds
5	Call validate() with wrong password	username='alice', password='wrong'	User not found or password mismatch	Throw UnauthorizedException

💡 Execution stops when user is returned or UnauthorizedException is thrown.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5
username	undefined	'alice'	'alice'	'alice'
password	undefined	'secret' or 'wrong'	'secret' or 'wrong'	'wrong'
user	undefined	{id:1, name:'alice'} or null	{id:1, name:'alice'} or null	null

Key Moments - 2 Insights

Why does validate() throw UnauthorizedException sometimes?

Because authService.validateUser() returned null, meaning username or password was wrong (see execution_table step 5).

What happens if validate() returns a user?

Passport attaches the user to the request object so controllers can access it (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the outcome at step 3 when user exists?

A Throw UnauthorizedException

B Return user object

C Call authService.validateUser() again

D Attach null to request

💡 Hint

Check the 'Outcome' column in execution_table row 3.

At which step does the UnauthorizedException get thrown?

A Step 2

B Step 3

C Step 5

D Step 4

💡 Hint

Look for 'Throw UnauthorizedException' in the 'Outcome' column.

If the password is correct, what variable changes after step 2?

A user becomes user object

B password becomes undefined

C username becomes null

D user becomes null

💡 Hint

Check variable_tracker for 'user' after step 2.

Concept Snapshot

Local strategy checks username and password.
validate() calls authService.validateUser().
If valid, returns user object.
If invalid, throws UnauthorizedException.
User attached to request for later use.

Full Transcript

In NestJS, the Local strategy uses a validate() method to check username and password. When a user submits credentials, validate() calls authService.validateUser() to verify them. If the user is found and password matches, validate() returns the user object. Passport then attaches this user to the request so controllers can access it. If credentials are wrong, validate() throws an UnauthorizedException, stopping the request. This flow ensures only valid users proceed.

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