ExistingProvider - Angular

Configures the Injector to return a value of another useExisting token.

API

interface ExistingProvider extends ExistingSansProvider {
  provide: any;
  multi?: boolean | undefined;
  override useExisting: any;
}

provide

any

An injection token. Typically an instance of Type or InjectionToken, but can be any.

multi

boolean | undefined

When true, injector returns an array of instances. This is useful to allow multiple providers spread across many files to provide configuration information to a common token.

useExisting

any

Existing token to return. (Equivalent to injector.get(useExisting))

Usage Notes

class Greeting {
          salutation = 'Hello';
        }

        class FormalGreeting extends Greeting {
          override salutation = 'Greetings';
        }

        const injector = Injector.create({
          providers: [
            {provide: FormalGreeting, deps: []},
            {provide: Greeting, useExisting: FormalGreeting},
          ],
        });

        expect(injector.get(Greeting).salutation).toEqual('Greetings');
        expect(injector.get(FormalGreeting).salutation).toEqual('Greetings');
        expect(injector.get(FormalGreeting)).toBe(injector.get(Greeting));

Multi-value example

const locale = new InjectionToken<string[]>('locale');
        const injector = Injector.create({
          providers: [
            {provide: locale, multi: true, useValue: 'en'},
            {provide: locale, multi: true, useValue: 'sk'},
          ],
        });

        const locales: string[] = injector.get(locale);
        expect(locales).toEqual(['en', 'sk']);