# Dependency Injection — NestJS: Building Scalable Node.js APIs

Source: https://www.skillbyai.com/en/nestjs/nestjs-dependency-injection

> Understand Nest's IoC container, constructor injection and provider scopes so you never instantiate services by hand.

## Inject via constructor

NestJS automatically instantiates and injects services via the constructor. No manual `new` needed.

```typescript
@Injectable()
export class UsersService {
  // service logic
}

@Controller('users')
export class UsersController {
  constructor(private usersService: UsersService) {}

  @Get()
  findAll() {
    return this.usersService.findAll();
  }
}
```

## Lazy loading & scopes

By default, services are singletons (one instance for the app). You can also use `REQUEST` or `TRANSIENT` scopes for per-request or per-use instances.

## Constructor injection in action

Nest reads the constructor parameter types and passes the matching provider.

- `@Injectable()` marks the class as a provider.
- `private readonly users: UsersService` both declares and stores the dependency.
- You never write `new UsersService()` yourself.

```ts
import { Controller, Get, Injectable } from '@nestjs/common';

@Injectable()
export class UsersService {
  findAll() {
    return ['asha', 'ravi'];
  }
}

@Controller('users')
export class UsersController {
  constructor(private readonly users: UsersService) {}

  @Get()
  list() {
    return this.users.findAll();
  }
}
```

## Why DI makes tests easy

Because the controller receives its service instead of creating it, a test can hand it a fake service. No database is needed to test the controller.

Quick check

**Quiz:** A provider is listed in `providers` but you inject it in another module and get an error. What is missing?

- [x] It must be exported by its module and that module imported
- [ ] The class needs `@Controller()`
- [ ] Nest only injects providers named `Service`

*Answer:* It must be exported by its module and that module imported. Providers are private to their module until exported, and the consumer must import that module.