Nest.js DI (Dependency Injection)

This is chapter 6 in the Nest.js Udemy course found here: Nest.Js – The complete developers guide

Dependency Injection

DI... what?

DI is a design pattern that allows us to create loosely coupled code. It allows us to write code that is more modular, reusable, and testable.

How to use in Nest?

We use DI by creating a class that will be injected into another class. We then use the @Injectable() decorator to mark the class as injectable. We then use the @Inject() decorator to inject the class into another class.

Project

we're going to create a simple project to simulate a computer. First we're deleting the files (except main.ts) in the project (auto-generated using nest new nestjs-di) and starting from scratch.

Setup the project

Let's setup the project using Nest CLI:

# Generate the modules
nest g module computer
nest g module cpu
nest g module disk
nest g module power

# Generate the services
nest g service cpu
nest g service disk
nest g service power

# Generate the controllers
nest g controller computer

Share services between modules

Our disk and cpu modules require some power to work correctly. After adding a supplyPower(watts: number) method to the power service, we can inject the power service into the cpu and disk services.

1. Export the power service

We need to export the power service from the power module so that we can use it in other modules.

// power.module.ts
@Module({
  providers: [PowerService],
  exports: [PowerService], // <-- Export the power service
})

2. Import the power service

We can now import the power service into the cpu and disk modules.

// cpu.module.ts
// disk.module.ts
@Module({
  imports: [PowerModule], // <-- Import the power module
  providers: [CpuService],
})

3. Inject the power service

We can now inject the power service into the cpu and disk services.

// cpu.service.ts
// disk.service.ts
@Injectable()
export class CpuService {
  constructor(private powerService: PowerService) {} // <-- Inject the power service
}

Now we can use the service by calling e.g. this.powerService.supplyPower(100).

4. Add the disk and cpu services to the computer module

We can now add exports to the cpu and disk modules and then import them in the computer module:

// computer.module.ts
@Module({
  imports: [CpuModule, DiskModule], // <-- Import cpu & disk modules
  controllers: [ComputerController],
})