DI Container Internals — Series 2

Preview — 3 of 10 questions

design:paramtypes for this constructor is [ConfigService, Object]. How does Nest still resolve the second argument?

javascript

@Injectable()
export class ReportService {
  constructor(
    private readonly config: ConfigService,
    @Inject('PDF_ENGINE') private readonly engine: unknown,
  ) {}
}

A@Inject() stores the token under a separate metadata key, keyed by parameter index; the injector merges those self-declared dependencies over the emitted design:paramtypes, so index 1 resolves to 'PDF_ENGINE' instead of Object

BNest scans the module's providers for one whose instance is assignable to unknown and picks it

C@Inject() rewrites the class's design:paramtypes array in place at decoration time

DThe parameter name engine is read from the function source and matched against provider tokens

What is true of this form compared with constructor injection?

javascript

@Injectable()
export class ReportService {
  @Inject(ConfigService) private readonly config: ConfigService;
}

AIt is resolved lazily, on first property access

BIt is the only form that works inside abstract base classes

CIt works — the injector collects property dependencies from their own metadata key and assigns them after instantiation — but the field is unset during the constructor, and the dependency is easier to overlook when reading or testing the class

DIt is rejected at startup; Nest supports constructor injection only

Why does the injector see nothing usable?

javascript

@Injectable()
export class NotifierService {
  constructor(private readonly channel: EmailChannel | SmsChannel) {}
}
// Nest can't resolve dependencies of the NotifierService (?).

ABecause union types are forbidden in a Nest provider's constructor

BBecause TypeScript cannot emit a single runtime value for a union type, so design:paramtypes records Object — a token nothing is registered under, which surfaces as the unresolvable ?

CBecause both classes are registered, and the injector refuses to choose between two candidates

DBecause emitDecoratorMetadata skips any parameter whose type is declared with private readonly

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