Cannot Check If An User Is Loged In Firebase + Angular

Currently I login my user and then I want to implement some route Guards in order for them to not be accessible if the user is not logged in.

SO this is my guard.

    import { Injectable } from '@angular/core';
import { CanActivate, ActivatedRouteSnapshot, RouterStateSnapshot, UrlTree } from '@angular/router';
import { Observable } from 'rxjs';
import { AngularFireAuth } from '@angular/fire/auth';
@Injectable({
  providedIn: 'root'
})
export class AuthguardGuard implements CanActivate {
constructor(public af: AngularFireAuth){

}

  canActivate(
    next: ActivatedRouteSnapshot,
    state: RouterStateSnapshot): Observable<boolean | UrlTree> | Promise<boolean | UrlTree> | boolean | UrlTree {
      var user = this.af.auth().currentUser;

      if (user) {
        return true;
      } else {
        return false;
      }
  }

}

Problem is that --->var user = this.af.auth().currentUser; is giving an error

Cannot invoke an expression whose type lacks a call signature. Type 'Auth' has no compatible call signatures.ts(2349)
(property) AngularFireAuth.auth: firebase.auth.Auth

oO how am I supposed to check if the user is logged in? I don't want to use localSTorage for that.

I ended up using the following canActivate

    canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<boolean> | boolean {
    return this.auth.authState.pipe(map(User => {
      if (User) {
        return true;
      } else {
        this.router.navigate(['/login']);
        return false;
      }
    }));
  }