Very Important !!

use subscribe

constructor(public af: AngularFire) {
    af.auth.subscribe((authstate) => {
        console.log(authstate);
        authstate == null ? logout() : login(authstate.auth.displayName);
    });
  }

And  Think change to Callback mode ………….

==========
First :
   everyone use af.auth.getauth() , then check,  have value is login, null is not login.

But it’s mistack. Reload page, Change page  AngularFire miss…… af.auth be free.
Only login page can get af.auth

Second:
  af.auth.subscribe ( ……. )   When auth check finish, run subscribe ( ….. )
This is real get auth. But Think login different First way……..