main.dart

void main() {
  print(fullName(null, null));
  print(fullName('John', null));
  print(fullName(null, 'Doe'));
  print(fullName("John", 'Doe'));

}

String fullName(
  String? firstName,
  String? lastName,
) =>
    '${firstName.orDefault} ${lastName.orDefault}';

extension DefaultValuesString on String? {
  String get orDefault {
    return this ?? '';    
  }
}

extension DefaultValuesInt on int? {
  int get orDefault {
    return this ?? 0;    
  }
}

extension DefaultValuesDouble on double? {
  double get orDefault {
    return this ?? 0.0;    
  }
}