Reactive Form Username check

Hello everyone, I’m new in the forum. This is the first time I have used custom validators. Basically I want to check if the username is already in use and if so the Update Profile button is not enabled. But it doesn’t work, if I try to enter in the form a username that already exists in the Real time database of firebase the button is enabled, but in the console I get the message “username già in uso”. Am I doing something wrong with the UsernameValidator class?

import { FormControl } from '@angular/forms';
import * as firebase from 'firebase';
export class UsernameValidator {
    static validUsername(fc: FormControl){
        .once("value",snapshot => {
          if (snapshot.exists()){
            console.log("username già in uso")
            return ({validUsername: false});         
            return (null);           

This is where I define the form group and validators:

constructor(private route: ActivatedRoute, private router: Router, 
              public pfService: ProfileService, public fb: FormBuilder,
              public authService: AuthenticationService) 
  { = this.authService.userData.uid;
    //Underscore and dot can't be next to each other (e.g
    //Underscore or dot can't be used multiple times in a row (e.g user__name /
    this.validPattern = "^(?=.{6,20}$)(?!.*[_.]{2})[a-z0-9._]+$"; 
    this.validPatternName = "^[a-z]{3,10}$";
    this.userForm ={
      txtUsername:  ["",[Validators.required,Validators.pattern(this.validPattern),
      txtName:     ["",[Validators.required,Validators.pattern(this.validPatternName)]],
    this.userForm .valueChanges.subscribe(()=> {

This is the HTML code:

  <ion-list lines="full">
    <form [formGroup]="userForm" (ngSubmit)="updateForm()" >
        <ion-label class="form-control" position="floating">Userame</ion-label>
        <ion-input formControlName="txtUsername" type="text" ></ion-input>
        <span [hidden]="userForm.controls.txtUsername.valid || userForm.controls.txtUsername.pristine">Lunghezza tra 6 e 20 caratteri</span>
        <ion-label class="form-control" position="floating">Name</ion-label>
        <ion-input formControlName="txtName" type="text" > </ion-input>
        <span [hidden]="userForm.controls.txtName.valid || userForm.controls.txtName.pristine">Lunghezza tra 6 e 20 caratteri</span>   
        <p *ngIf="userForm.get('txtUsername').hasError('validUsername')" color="danger">Username GIà IN USO</p>

          <ion-button type="submit" *ngIf="userForm.controls.txtUsername.valid && userForm.controls.txtName.valid " color="primary" shape="full" expand="block">Update Profile</ion-button>

The validator won’t return anything as it going to get stucked inside the snapshot. It return always null. How can I modify the code?

static validUsername(fc: FormControl) {

It always helps readability to declare return types for every function you write. If it doesn’t return anything, use void. In this case, you want this function to return something. It doesn’t, though.