How to validate Email if it is already used / or not in database?

Ionic Framework ionic-v3
1

Hello ,
Please share some knowledge on “How can we validate email with dynamic data coming from firebase?”

Here is my code I stuck with this…

import { FormControl } from '@angular/forms';
import { UserProvider } from '../providers/database/user/user';

export class EmailValidator {
  public users: any;
  public email: any;
  constructor(
    public _DBUSER: UserProvider
  ) {
    this.loadAndParseUsers();
  }
  loadAndParseUsers() {
    this.users = this._DBUSER.renderAllUsers();
    var allEmails = [];
    this.users.forEach(element => {
      console.log(element);
      element.forEach(user => {
        allEmails.push(user.email);
      });
    });
    console.log(allEmails);

  }
  static checkEmail(control: FormControl): any {

    return new Promise(resolve => {

      //Fake a slow response from server

      setTimeout(() => {
        if (control.value == "adminh@gmail.com") {
          console.log(control.value);

          resolve({
            "Email taken": true
          });

        } else {
          resolve(null);
        }
      }, 2000);

    });
  }
}

Static checking is working fine. But I am not able to check with dynamic email coming from database. I am quite newbie to Ionic please help. thanks