how to create custom validator function

Step 1 : Create the custom validator function 
function emailDomain(control: AbstractControl): { [key: string]: any } | null {
  const email: string = control.value;
  const domain = email.substring(email.lastIndexOf('@') + 1);
  if (email === '' || domain.toLowerCase() === 'gmail.com') {
    return null;
  } else {
    return { 'emailDomain': true };
  }
}

Step 2 : Attach the custom validator function to the control that we want to validate

email: ['', [Validators.required, emailDomain]]

Step 3 : Display the validation error message

<span *ngIf="employeeForm.get('email').errors.emailDomain">
  Email domian should be gmail.com
</span>

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