Async Validation with Angular Reactive Forms + Firebase

Описание: Learn how to validate username uniqueness in Firebase with Angular Reactive Forms. We build a custom validator that checks the the Firestore DB for existing user records. https://angularfirebase.com/lessons/a...

Reactive Forms - https://angular.io/guide/reactive-forms
Firestore - https://stackoverflow.com/questions/4...
AngularFire2 - https://github.com/angular/angularfire2