Dependency Injection

TL;DR

• NestJS uses constructor injection. Declare type in constructor → Nest resolves it.
• Register providers in module's providers array. Export to share across modules.
• @Injectable() marks a class as a provider.

Basic Pattern

// service
@Injectable()
export class UsersService {
  private users: User[] = [];

  findAll(): User[] { return this.users; }
  create(dto: CreateUserDto): User { /* ... */ }
}

// controller — just declare in constructor
@Controller('users')
export class UsersController {
  constructor(private readonly usersService: UsersService) {}

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

// module — register everything
@Module({
  controllers: [UsersController],
  providers: [UsersService],
  exports: [UsersService], // share with other modules
})
export class UsersModule {}

Provider Types

Injection Scopes

@Injectable({ scope: Scope.DEFAULT })   // Singleton (default)
@Injectable({ scope: Scope.REQUEST })   // New instance per request
@Injectable({ scope: Scope.TRANSIENT }) // New instance per injection

Custom Injection Token

// token
export const CONFIG = Symbol('CONFIG');

// register
{ provide: CONFIG, useValue: { port: 3000 } }

// inject
constructor(@Inject(CONFIG) private config: AppConfig) {}