How does Angular Fire store offline data?

Question

I am building an Ionic app using AngularFire2, which needs to work offline as well as online. It will be built for iOS and Android (not web).

AngularFirestore allows persistence locally using AngularFirestoreModule.enablePersistence() which is great.

But how does Angular Fire store data offline - by which I mean, is it localStorage, IndexDB or what? I need to be really sure that the data can't get wiped either on device restart, app update etc.

Local storage can get wiped from devices by such events AFAIK, so if that is the answer, then I need to find a different solution for offline.

Thanks for any help.