Dependency injection basics in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject the service into the constructor.

NestJS

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

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

Aservice

BCatsService

CcatsService

DCats

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of a variable name.

Using a variable name that does not match the service.

The injected service variable name should start with a lowercase letter and match the service type.

2fill in blank

medium

Complete the code to mark the service as injectable.

NestJS

@[1]()
export class CatsService {}

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

AInjectable

BController

CModule

DComponent

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Controller or @Module instead of @Injectable.

Forgetting to add the decorator.

The @Injectable() decorator marks a class as available for dependency injection.

3fill in blank

hard

Fix the error in the provider registration.

NestJS

@Module({
  providers: [[1]],
})
export class CatsModule {}

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

ACatsService()

BCatsService

CcatsService

Dnew CatsService()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the service as a function in providers.

Using an instance instead of the class.

Providers are registered by class reference, not by calling or instantiating them.

4fill in blank

hard

Fill both blanks to inject and use the service in the controller.

NestJS

constructor(private readonly [1]: [2]) {}

getCats() {
  return this.catsService.findAll();
}

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

AcatsService

BCatsService

Ccats

DCatService

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching variable name and type.

Using incorrect service class name.

The constructor parameter name should be catsService and its type CatsService to inject properly.

5fill in blank

hard

Fill all three blanks to create a provider with a custom value and inject it.

NestJS

const customProvider = {
  provide: '[1]',
  useValue: [2],
};

constructor(@Inject('[3]') private readonly config: any) {}

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

ACONFIG_OPTIONS

B{ apiKey: '12345' }

DCONFIG

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching the provide and inject tokens.

Using incorrect object syntax for useValue.

The provider token and the injection token must match exactly. The value is the custom object provided.

Practice

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

easy

A. To automatically provide required services to classes

B. To manually create instances of services inside components

C. To write all code inside a single file

D. To avoid using classes and use only functions

Solution

1. Step 1: Understand dependency injection concept

   Dependency injection means NestJS automatically gives the needed services to classes without manual creation.

2. Step 2: Compare options with this concept

   Only To automatically provide required services to classes describes automatic service provision, others describe manual or unrelated actions.

3. Final Answer:

   To automatically provide required services to classes -> Option A

4. Quick Check:

   Dependency injection = automatic service provision [OK]

Hint: Dependency injection means NestJS gives services automatically [OK]

Common Mistakes:

• Thinking you must create service instances manually
• Confusing dependency injection with code organization only
• Believing dependency injection removes classes

2. Which decorator marks a class as injectable for NestJS dependency injection?

easy

A. @Module()

B. @Controller()

C. @Injectable()

D. @Service()

Solution

1. Step 1: Recall NestJS decorators for DI

   Classes that can be injected must be marked with @Injectable() decorator.

2. Step 2: Check each option

   @Controller is for controllers, @Module for modules, @Service is not a NestJS decorator.

3. Final Answer:

   @Injectable() -> Option C

4. Quick Check:

   @Injectable() marks injectable classes [OK]

Hint: Use @Injectable() to mark services for injection [OK]

Common Mistakes:

• Using @Controller or @Module instead of @Injectable
• Assuming @Service is a valid NestJS decorator
• Forgetting to add any decorator

3. Given this code snippet, what will be the output when appService.getMessage() is called?

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

@Injectable()
export class AppService {
  getMessage() {
    return 'Hello from AppService';
  }
}

export class AppController {
  constructor(private readonly appService: AppService) {}

  showMessage() {
    return this.appService.getMessage();
  }
}

medium

A. Error: AppService not provided

B. 'Hello from AppService'

C. undefined

D. 'Hello from AppController'

Solution

1. Step 1: Understand injection in constructor

   AppController receives AppService instance via constructor injection because AppService is marked @Injectable.

2. Step 2: Trace method call

   showMessage calls appService.getMessage(), which returns 'Hello from AppService'.

3. Final Answer:

   'Hello from AppService' -> Option B

4. Quick Check:

   Injected service method returns message [OK]

Hint: Injected service methods return their defined values [OK]

Common Mistakes:

• Assuming injection fails without @Module setup
• Expecting AppController message instead
• Confusing method return values

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

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

export class UserService {
  getUser() {
    return 'User data';
  }
}

@Injectable()
export class UserController {
  constructor(private readonly userService: UserService) {}

  fetchUser() {
    return this.userService.getUser();
  }
}

medium

A. UserService is missing @Injectable() decorator

B. UserController should not have a constructor

C. fetchUser method should be async

D. userService property should be public

Solution

1. Step 1: Check decorators on injectable classes

   UserService is injected into UserController but lacks @Injectable(), so NestJS cannot inject it.

2. Step 2: Verify other options

   Constructor in controller is valid, fetchUser need not be async, and private readonly is correct for injection.

3. Final Answer:

   UserService is missing @Injectable() decorator -> Option A

4. Quick Check:

   Injectable classes must have @Injectable() [OK]

Hint: Injectable services must have @Injectable() decorator [OK]

Common Mistakes:

• Forgetting @Injectable on services
• Thinking constructor is invalid in controllers
• Assuming async is required for injected methods

5. You want to inject two different services, LoggerService and ConfigService, into a controller. Which is the correct way to do this in NestJS?

hard

A. Inject only one service and import the other directly

B. Create instances manually inside the controller constructor

C. Use @Injectable() on the controller and call services statically

D. Use constructor injection with both services as parameters marked private readonly

Solution

1. Step 1: Recall how to inject multiple services

   In NestJS, multiple services are injected by listing them as constructor parameters with access modifiers like private readonly.

2. Step 2: Evaluate options

   Use constructor injection with both services as parameters marked private readonly correctly uses constructor injection. Create instances manually inside the controller constructor manually creates instances, breaking DI. Use @Injectable() on the controller and call services statically misuses @Injectable on controller and static calls. Inject only one service and import the other directly injects only one service, which is incomplete.

3. Final Answer:

   Use constructor injection with both services as parameters marked private readonly -> Option D

4. Quick Check:

   Multiple services injected via constructor parameters [OK]

Hint: Inject multiple services by listing them in constructor [OK]

Common Mistakes:

• Manually creating service instances inside controllers
• Using static calls instead of injection
• Injecting only one service when multiple are needed