Angular 5 Http Interceptors error when injecting service

Question

I am receiving the following strange dependency injection behavior when using custom HttpInterceptors in angular 5+.

The following simplified code works fine:

    export class AuthInterceptor implements HttpInterceptor {
        constructor(private auth: AuthService) {}

        intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
            const token = this.auth.getToken();
            return next.handle(req);
        }
    }
    export class AuthService {
        token: string;
        constructor() {
          console.log('AuthService.constructor');
        }
    }

HOWEVER....

When the AuthService has 1 or more dependencies on its own e.g.

   export class AuthService {
      token: string;
      constructor(private api: APIService) {
         console.log('AuthService.constructor');
      }
   }

angular is trying to repeatedly create new instances of AuthService until I receive the following errors:

The log is displaying the AuthService.constructor message ~400 times

and

Cannot instantiate cyclic dependency! HTTP_INTERCEPTORS ("[ERROR ->]"): in NgModule AppModule

and

app.component.html:44 ERROR RangeError: Maximum call stack size exceeded

I then tried injecting the service using the Injector class -

 export class AuthService {
      token: string;
      api: APIService;
      constructor(private injector: Injector) {
         this.api = this.injector.get(APIService);
         console.log('AuthService.constructor');
      }
   }

but getting the same error (maximum call stack size).

The APIService is a simple service that only injects the HttpClient in its constructor.

@Injectable()
export class APIService {
    constructor(private http: HttpClient) {}
}

Lastly, when I inject the AuthService into the Interceptor using the Injector, the error disappears but the AuthService is being instantiated 200+ times:

export class AuthInterceptor implements HttpInterceptor {
    auth: AuthService;
    constructor(private injector: Injector) {}
    intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
           this.auth = this.auth || this.injector.get(AuthService);
           const token = this.auth.getToken();
           return next.handle(req);
        }
    }

Looking at the official documentation and other example it seems as it is technically possible to inject services into the Http Interceptors. Is there any limitation or any other setup that might be missing?

Answer 1

So it turns out that if the service you inject into the Http Interceptor has a dependency on HttpClient, this leads to a cyclic dependency.

Since my AuthService was a mix of all different logics (login/out, routing the user, saving/loading tokens, making api calls), I separated the part needed for the interceptors into its own service (just the user credentials & tokens) and now injecting it successfully into the Interceptor.

export class AuthInterceptor implements HttpInterceptor {
    constructor(private credentials: CredentialsService) {}
    intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        const token = this.credentials.getToken();
        const api_key = this.credentials.getApiKey();
    }
}

export class CredentialsService {
    token: string;
    user: IUser;
    constructor(private http: HttpClient) {
        this.loadCredentialsFromStorage();
    }
}

This seems to work fine. Hope this helps someone.