app.component.ts

export class AppComponent implements OnInit {
 user: User; 
 constructor(private afAuth: AngularFireAuth,
   private router: Router,
   private ngZone: NgZone){}
   
 ngOnInit(){
   this.afAuth.user.onAuthStateChanged(user => {
     if (user){
       this.user = user;
     } else{
       this.user = null;
       this.ngZone.run(() => {
         this.router.navigate(['']);
       })
   });
}