MapService.ts

//Use the InjectionToken for the mapsConfig, this can be later used to inject it with DI
export const MapsConfig = new InjectionToken<IMapsConfig>('MAPS_CONFIG');

 @Injectable({
    providedIn: MapModule
})
export class MapService {
    private _key: string;

    constructor(
        @Optional() @Inject(MapsConfig) config: IMapsConfig = null
    ) {
        if (config) {
            this._key = config.key;
        }
    }
}