Injectable & DI Basics — Series 2

Preview — 3 of 10 questions

What does listing a bare class in providers mean?

javascript

@Module({ providers: [CatsService] })
export class CatsModule {}

// equivalent to:
@Module({ providers: [{ provide: CatsService, useClass: CatsService }] })
export class CatsModule {}

AIt is shorthand for a class provider whose injection token is the class itself and whose implementation is that same class

BIt registers the class as a controller if it has route decorators, and as a provider otherwise

CIt creates a new instance of the class for every injection site

DIt exports the class automatically to every module that imports this one

What happens at startup?

javascript

@Module({ controllers: [CatsController] }) // CatsService not listed
export class CatsModule {}

@Controller('cats')
export class CatsController {
  constructor(private readonly catsService: CatsService) {}
}

ANest instantiates CatsService automatically, since the constructor's type annotation is enough to locate the class

BThe application fails to start with Nest can't resolve dependencies of the CatsController (?), naming the missing argument and the module context to check

CThe application starts, and this.catsService is undefined until the first request

DNest falls back to a no-op stub whose methods all return null

Is this allowed, and what does it require?

javascript

@Injectable()
export class CatsService {
  constructor(private readonly logger: LoggerService) {}
}

ANo — only controllers may inject providers; services must instantiate their own dependencies with new

BYes, but only if LoggerService is declared @Global()

CYes — providers can inject other providers exactly like controllers do, as long as LoggerService is resolvable from this module

DYes, but the injected service is a fresh instance created specifically for CatsService

Answer all 10 questions (7 more), see explanations for every answer, and track your score.