Why Dependency injection basics in NestJS? - Purpose & Use Cases

The Big Idea

Discover how NestJS can handle your app's wiring so you don't have to!

The Scenario

Imagine building a large NestJS app where every service manually creates its own dependencies using new. You have to update many places if a dependency changes.

The Problem

Manually creating dependencies leads to tangled code, hard-to-test services, and lots of repeated setup. It's easy to make mistakes and hard to change things later.

The Solution

Dependency injection lets NestJS automatically provide the right dependencies to your classes. You just declare what you need, and NestJS handles the rest.

Before vs After

✗ Before

class UserService {
  constructor() {
    this.repo = new UserRepository();
  }
}

✓ After

@Injectable()
class UserService {
  constructor(private readonly repo: UserRepository) {}
}

What It Enables

This makes your code cleaner, easier to test, and flexible to change without rewriting everything.

Real Life Example

When switching from a mock database to a real one, you just change the provider setup once, and all services get the new database automatically.

Key Takeaways

Manual dependency creation causes tangled, hard-to-maintain code.

Dependency injection automates providing dependencies to classes.

This leads to cleaner, testable, and flexible applications.

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