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/async-form-validation-in-firebase-enforce-uniqueness/
– Reactive Forms – https://angular.io/guide/reactive-forms
– Firestore – https://stackoverflow.com/questions/46549766/whats-the-difference-between-cloud-firestore-and-the-firebase-realtime-database
– AngularFire2 – https://github.com/angular/angularfire2