Property 'authState' does not exist on type 'AngularFireAuth' in ionic 2 app

I solve this problem re-installing Firebase and AngularFire2:

npm uninstall angularfire2 --save
npm uninstall firebase --save
npm install angularfire2 firebase --save

Update:

npm install firebase @angular/fire --save

The corrected answer last line is now deprecated

npm install firebase @angular/fire --save