Save Service Return Variable on Angular

I have a service that returns me a user-type object with the id of a user:

My object:

export class Usuario{
    email: string;
    password: string;
}

In my component I instantiate my object so that it can receive the return of the service:

usuario: Usuario = new Usuario();

And then my service makes the request by returning the object with the id I need:

 this.authService.fazerLogin(email,password)
     .subscribe(
          (data) => {
              this.usuario = data;
          )}

If I do a console.log (user), I get the id I need, however I'd like to save the return in a string variable instead of an object so that I can save this value in a localstorage to make requests ...

How do I extract this value into a variable?