Why use AbstractFormControl Example?

Answer : AbstractFormControl is abstract class that is used for custom form Control 
and validation. Example :

emailDomain(domainName: string) {

return (params: AbstractControl): { [key: string]: any } | null => {

const email: string = params.value;

const domain = email.substring(email.lastIndexOf('@' + 1));

if (email === '' || domain.toLowerCase() === domainName.toLowerCase()) {

return null;

} else {

return { 'emailDomain': true };

}

};