Why Constructor injection in NestJS? - Purpose & Use Cases

The Big Idea

Discover how constructor injection frees you from messy manual setup and makes your NestJS code shine!

The Scenario

Imagine building a NestJS service that needs to use a database and a logger. You have to create and manage these dependencies yourself inside the service.

The Problem

Manually creating and managing dependencies inside your service leads to messy code, tight coupling, and makes testing very hard. You must remember to create and pass every dependency everywhere.

The Solution

Constructor injection lets NestJS automatically provide the needed dependencies when creating your service. You just declare what you need in the constructor, and NestJS handles the rest.

Before vs After

✗ Before

class UserService {
  constructor() {
    this.db = new Database();
    this.logger = new Logger();
  }
}

✓ After

class UserService {
  constructor(private db: Database, private logger: Logger) {}
}

What It Enables

This makes your code cleaner, easier to test, and lets NestJS manage dependencies for you automatically.

Real Life Example

When building a user service, constructor injection lets you swap the database or logger easily without changing the service code, perfect for testing or changing implementations.

Key Takeaways

Manual dependency management is error-prone and messy.

Constructor injection lets NestJS provide dependencies automatically.

This leads to cleaner, more maintainable, and testable code.

Practice

1. What is the main purpose of constructor injection in NestJS?

easy

A. To provide required services to a class when it is created

B. To manually create instances of services inside methods

C. To define routes for HTTP requests

D. To write database queries inside the constructor

Solution

1. Step 1: Understand constructor injection concept

   Constructor injection is used to pass dependencies (services) into a class automatically when it is created.

2. Step 2: Identify the purpose in NestJS context

   In NestJS, constructor injection helps keep code clean by providing needed services without manual instantiation.

3. Final Answer:

   To provide required services to a class when it is created -> Option A

4. Quick Check:

   Constructor injection = provide services automatically [OK]

Hint: Constructor injection means passing services via constructor [OK]

Common Mistakes:

• Thinking constructor injection creates routes
• Manually creating service instances inside methods
• Confusing injection with database queries

2. Which of the following is the correct syntax for constructor injection of a service named UsersService in a NestJS class?

easy

A. constructor(private readonly usersService: UsersService) {}

B. constructor(usersService: UsersService) {}

C. constructor(private usersService) {}

D. constructor(public readonly usersService) {}

Solution

1. Step 1: Check for correct access modifier and typing

   In NestJS, constructor injection requires private readonly with the service type for proper injection and immutability.

2. Step 2: Compare options

   constructor(private readonly usersService: UsersService) {} (A) is correct. B misses access modifier, C misses type, D misses type annotation.

3. Final Answer:

   constructor(private readonly usersService: UsersService) {} -> Option A

4. Quick Check:

   Use private readonly with type for injection [OK]

Hint: Use 'private readonly serviceName: ServiceType' in constructor [OK]

Common Mistakes:

• Omitting access modifier like private or public
• Not specifying the service type
• Using public readonly without specifying the service type

3. Given the following code, what will be the output when appController.getHello() is called?

import { Injectable, Controller } from '@nestjs/common';

@Injectable()
class HelloService {
  getMessage() {
    return 'Hello, NestJS!';
  }
}

@Controller()
class AppController {
  constructor(private readonly helloService: HelloService) {}

  getHello() {
    return this.helloService.getMessage();
  }
}

medium

A. Error: helloService is not defined

B. undefined

C. 'Hello, NestJS!'

D. 'Hello, World!'

Solution

1. Step 1: Understand constructor injection usage

   The AppController receives HelloService via constructor injection and stores it as helloService.

2. Step 2: Trace method call

   Calling getHello() calls helloService.getMessage() which returns 'Hello, NestJS!'.

3. Final Answer:

   'Hello, NestJS!' -> Option C

4. Quick Check:

   Injected service method returns 'Hello, NestJS!' [OK]

Hint: Injected service methods return expected values [OK]

Common Mistakes:

• Assuming service is undefined without injection
• Confusing method return values
• Expecting default or unrelated strings

4. Identify the error in the following NestJS service injection code:

@Injectable()
class CatsService {}

@Controller('cats')
class CatsController {
  constructor(catsService: CatsService) {}

  findAll() {
    return 'All cats';
  }
}

medium

A. CatsService is not decorated with @Injectable()

B. Missing access modifier (private/public) in constructor parameter

C. Controller decorator is missing path

D. findAll method should be async

Solution

1. Step 1: Check constructor injection syntax

   The constructor parameter catsService: CatsService lacks an access modifier like private or public, so it won't be assigned as a class property.

2. Step 2: Verify other parts

   @Injectable() is present on service, @Controller('cats') has path, and findAll can be sync. So no other errors.

3. Final Answer:

   Missing access modifier (private/public) in constructor parameter -> Option B

4. Quick Check:

   Constructor injection needs access modifier [OK]

Hint: Always add private/public to constructor params for injection [OK]

Common Mistakes:

• Omitting private/public in constructor injection
• Thinking @Injectable is missing when it is present
• Assuming controller path is mandatory for injection

5. You want to inject two services, AuthService and UsersService, into a NestJS controller. Which constructor correctly injects both services following best practices?

hard

A. constructor(public authService: AuthService, public usersService: UsersService) {}

B. constructor(authService: AuthService, usersService: UsersService) {}

C. constructor(private authService: AuthService, usersService: UsersService) {}

D. constructor(private readonly authService: AuthService, private readonly usersService: UsersService) {}

Solution

1. Step 1: Check for access modifiers and readonly usage

   Best practice is to use private readonly for injected services to keep them immutable and private.

2. Step 2: Verify both services are injected with correct syntax

   constructor(private readonly authService: AuthService, private readonly usersService: UsersService) {} injects both services with private readonly, while others miss modifiers or use public.

3. Final Answer:

   constructor(private readonly authService: AuthService, private readonly usersService: UsersService) {} -> Option D

4. Quick Check:

   Use private readonly for multiple injected services [OK]

Hint: Use private readonly for all injected services in constructor [OK]

Common Mistakes:

• Omitting access modifiers on one or both services
• Using public instead of private readonly
• Mixing access modifiers inconsistently