How to get the value of the id in this function?

I have a function that retrieve data of one user. Now I want to get in this function the user_id like that:

    this.storage.get(USER_ID).then(val => {
             this.id = val;
)}

so the api knows from which user it need the id. My main function I have to insert this is:

ngOnInit() {
    this.subscription = this.authService.authenticationStateSubject.pipe(
      switchMap(isAuthenticated => {
        if (isAuthenticated) {
          return this.userService.getUserDetails(this.id);
        } else {
          return of(null);
        }
      }),
    ).subscribe(
      result => {
        if (result) {
          this.information = result;
          console.log(this.information);
        } else {
        }
      },
      error => {
      }
    );
  }

I tried to put my snippet right after the if (isAuthenticated) { but somehow it doesn't work with the two last brackets. Can I actually connect these two snippets?