Error Codes in Angular using Reactive Forms

MyValidators.ts

import {FormControl} from '@angular/forms';

export class MyValidators {
  static forbiddenNames(control: FormControl): { [s: string]: boolean } {

    const forbidden = ['Mahmood', 'Reza', 'Ahmad'];

    if (forbidden.indexOf(control.value) !== -1) {
      return {'nameIsForbidden': true};
    }

    return null;
  }
}

app.component.html

<div class="container">
  <div class="row">
    <div class="col-xs-12 col-sm-10 col-md-8 col-sm-offset-1 col-md-offset-2">
      <form [formGroup]="signupForm" (ngSubmit)="onSubmit()">
        <div formGroupName="userData">
          <div class="form-group">
            <label for="username">Username</label>
            <input
              type="text"
              id="username"
              class="form-control"
              formControlName="username">
            <span
              *ngIf="!signupForm.get('userData.username').valid && signupForm.get('userData.username').touched"
              class="help-block">
              <span *ngIf="signupForm.get('userData.username').errors['nameIsForbidden']">
              This name is invalid!
              </span>
              <span *ngIf="signupForm.get('userData.username').errors['required']">
              This field is required!
            </span>
            </span>
          </div>
          <div class="form-group">
            <label for="email">email</label>
            <input
              type="text"
              id="email"
              class="form-control"
              formControlName="email">
            <span
              *ngIf="!signupForm.get('userData.email').valid && signupForm.get('userData.email').touched"
              class="help-block">Please enter a valid email</span>
          </div>
        </div>
        <div class="radio" *ngFor="let gender of genders">
          <label>
            <input
              type="radio"
              [value]="gender"
              formControlName="gender">{{ gender }}
          </label>
        </div>
        <div formArrayName="hobbies">
          <h4>Your hobbies</h4>
          <button class="btn btn-default" type="button"
                  (click)="onAddHobby()">Add Hobby
          </button>
          <div class="form-group"
               *ngFor="let hobbyControl of signupForm.get('hobbies').controls;let i=index">
            <input type="text" class="form-control" [formControlName]="i"/>
          </div>
        </div>
        <span
          *ngIf="!signupForm.valid && signupForm.touched"
          class="help-block">Please enter valid data</span>
        <button class="btn btn-primary" type="submit">Submit</button>
      </form>
    </div>
  </div>
</div>