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