Angular optional multi interceptors - why is it injected as null?

Question

Given a multi injectable defined like below, without any providers, I would expect the injected list to be empty, but in reality it is null.

Is there a way to inject the value as [] instead of null if there are no providers for this injectable? I can't override the value in the constructor, because the code in the constructor runs AFTER the field initializers, so the error happens before I can correct the value.


const MY_INTERCEPTORS: InjectionToken<MyInterceptor> = new InjectionToken('MyInterceptors');


@Injectable()
export class MyService {
  constructor(
    @Optional() @Inject(MY_INTERCEPTORS) private readonly interceptors: MyInterceptor[]
  ) {
    super();
  }

  public myField$(input: Foo) = this.interceptors.reduce((agg, e) => e.intercept(agg), input);  // Expect this.interceptors to be [], but is null
}

Angular doesn't support default values with @Optional decorator. Proposal is here github.com/angular/angular/issues/25395 — leonmain
– leonmain, Commented Apr 7, 2023 at 7:40

leonmain · Accepted Answer · 2023-04-07 07:37:49Z

1

You can provide a factory to your InjectionToken. It will be used as default value

const MY_INTERCEPTORS: InjectionToken<Array<MyInterceptor>> = new InjectionToken('MyInterceptors', {
    factory: () => [],
});

And by the way, the type of InjectionToken should be array

const MY_INTERCEPTORS: InjectionToken<Array<MyInterceptor>>

answered Apr 7, 2023 at 7:37

leonmain

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2 Comments

Ákos Vandra-Meyer Over a year ago

Thanks, I'll try this later today. It sounds like the correct answer, thank you

Ákos Vandra-Meyer Over a year ago

Confirmed working.

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