Constructor injection in NestJS

Introduction

Constructor injection helps you give a class the things it needs to work by passing them in when you create it. This makes your code cleaner and easier to manage.

When you want to give a service or controller access to other services it depends on.

When you want to keep your code organized and easy to test.

When you want NestJS to automatically create and provide the needed parts for your class.

When you want to avoid manually creating instances inside your classes.

When you want to clearly show what a class needs to work.

Syntax

NestJS

constructor(private readonly serviceName: ServiceType) {}

The constructor is a special method that runs when the class is created.

Using private readonly automatically creates and stores the service as a class property.

Examples

This injects the UsersService into the class so you can use it inside.

NestJS

constructor(private readonly usersService: UsersService) {}

You can inject multiple services by adding them as parameters.

NestJS

constructor(private readonly logger: LoggerService, private readonly config: ConfigService) {}

You can use @Inject() to inject a custom provider by its token.

NestJS

constructor(@Inject('CUSTOM_SERVICE') private readonly customService: CustomService) {}

Sample Program

This example shows a HelloService that returns a greeting. The HelloController uses constructor injection to get the service and call its method when handling a GET request.

NestJS

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

@Injectable()
export class HelloService {
  getHello(): string {
    return 'Hello, NestJS!';
  }
}

import { Controller, Get } from '@nestjs/common';
import { HelloService } from './hello.service';

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

  @Get()
  getHello(): string {
    return this.helloService.getHello();
  }
}

Important Notes

Always mark injected services with private readonly to keep them safe inside the class.

Constructor injection works only if the class is decorated with @Injectable() or a similar decorator.

Use constructor injection to make your classes easier to test by replacing dependencies with mocks.

Summary

Constructor injection passes needed services into a class when it is created.

This keeps code clean, organized, and easy to test.

In NestJS, use constructor(private readonly service: ServiceType) inside classes decorated with @Injectable() or @Controller().

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