NestJSIntermediate

How NestJS Dependency Injection Works

Master DI patterns in 5 minutes with interactive examples

⏱️ 5 min read💡 Intermediate

🎯 What You'll Learn:

How NestJS DI container works internally
3 types of injection patterns
Common DI pitfalls and how to avoid them
Production-ready best practices

NestJS Dependency Injection Flow

How the magic happens ✨

Module declares providers

@Module({ providers: [UserService] })

NestJS scans for @Injectable

Finds decorated classes

Container creates instances

Automatic instantiation

Interactive Flow Diagram

🧠 Understanding the Concept

Dependency Injection (DI) is a design pattern where dependencies are provided to a class rather than the class creating them. In NestJS, the IoC (Inversion of Control) container automatically manages this for you.

Why use DI?

Testability: Mock dependencies easily in tests
Loose Coupling: Change implementations without breaking code
Scalability: Share instances across application (singleton pattern)
Maintainability: Clear dependency graph

💻 Code Example: Basic DI
user.service.ts
import { Injectable } from '@nestjs/common';

@Injectable()
export class UserService {
  private users = [];

  findAll() {
    return this.users;
  }

  create(user: any) {
    this.users.push(user);
    return user;
  }
}
user.controller.ts
import { Controller, Get } from '@nestjs/common';
import { UserService } from './user.service';

@Controller('users')
export class UserController {
  // Constructor injection
  constructor(private readonly userService: UserService) {}

  @Get()
  findAll() {
    return this.userService.findAll();
  }
}
user.module.ts
import { Module } from '@nestjs/common';
import { UserController } from './user.controller';
import { UserService } from './user.service';

@Module({
  providers: [UserService],
  controllers: [UserController],
  exports: [UserService], // Export if other modules need it
})
export class UserModule {}

🚀 Production Pattern: Custom Providers

useFactory pattern for dynamic providers

1import { Module } from '@nestjs/common';
2import { ConfigService } from '@nestjs/config';
3
4@Module({
5  providers: [
6    {
7      provide: 'DATABASE_CONNECTION',
8      useFactory: (configService: ConfigService) => {
9        const host = configService.get('DB_HOST');
10        const port = configService.get('DB_PORT');
11        return createConnection({ host, port });
12      },
13      inject: [ConfigService],
14    },
15  ],
16})
17export class DatabaseModule {}

When to use useFactory?

Configuration-based initialization
Async initialization
Third-party library integration
Different instances per request (REQUEST scope)

⚠️ Common Pitfalls

1. Circular Dependencies

When Service A depends on Service B, and Service B depends on Service A.

solution.ts

1import { forwardRef, Inject } from '@nestjs/common';
2
3constructor(@Inject(forwardRef(() => ServiceB)) private serviceB: ServiceB) {}

2. Forgetting @Injectable()

Every provider must have the @Injectable() decorator.

3. Missing Provider Registration

Provider must be in module's providers array or imported from another module.

✅ Key Takeaways

✓DI makes code testable, maintainable, and scalable
✓Use @Injectable() decorator on all provider classes
✓Register providers in @Module() decorator
✓Inject via constructor parameters for cleaner code
✓Use custom providers with useFactory for dynamic scenarios
✓Avoid circular dependencies with forwardRef()

🎯 Interactive Challenge: Arrange the DI Flow

Drag and drop the steps below to arrange them in the correct order of NestJS Dependency Injection flow

Attempts: 0

How to play:

• Drag and drop the steps to arrange them in the correct order
• The flow should represent the logical sequence of events
• Click “Check Order” when you think you've got it right
• Get feedback and hints if needed

1
🎯
Controller requests service
constructor(private service: UserService)

2
✅
NestJS injects instance
Automatic dependency resolution

3
🔍
NestJS scans @Injectable
Finds decorated classes

4
📦
Module declares providers
@Module({ providers: [UserService] })

Interactive Flow Diagram

💡 Learn by Doing: Implement DI

Practice implementing Dependency Injection in NestJS. Complete the challenge below!

Your Challenge:

Create a PostService and inject it into a PostController using constructor injection. The PostService should have a findAll() method that returns an array of posts.

Interactive Editor

Attempts: 0

📇 Memorize Key Concepts

Master NestJS DI through multiple learning modes. Choose your preferred learning style below!

Flashcards: Click to flip cards and review concepts. Self-assess your knowledge with “I Know This” or “Need Review” buttons.

📇 Flashcards

Test your knowledge with these flashcards

Progress: 0/10 cards reviewedMastery: 0%

Known: 0Review: 0

Card 1 of 10Core Concepteasy

Question

What is Dependency Injection?

Click to reveal answer

🧪 Test Your Knowledge

Question 1 of 2Score: 0/2

What is the main benefit of Dependency Injection?

Download PDF Version

Get this guide as a PDF + receive daily backend tips

Daily tips

100% secure

Free forever

📚 Related Posts

Express vs NestJS

Compare the two popular Node.js frameworks

Want more NestJS content?

Subscribe to get weekly tutorials and best practices

Subscribe to DailyNest