Require one from two fields using Angular 2

Question

I\'m trying to create a contact form. The form looks like this:

Answer 1

Slightly refactored Florian's answer for the lazy as it was making ts-sonar angry (cognitive-complexity rule):

const isFieldEmpty = (fieldName: string, fg: FormGroup) => {
    const field = fg.get(fieldName).value;
    if (typeof field === 'number') { return field && field >= 0 ? true : false; }
    if (typeof field === 'string') { return field && field.length > 0 ? true : false; }
};

export function atLeastOne(...fields: string[]) {
  return (fg: FormGroup): ValidationErrors | null => {
    return fields.some(fieldName => isFieldEmpty(fieldName, fg))
      ? null
      : ({ atLeastOne: 'At least one field has to be provided.' } as ValidationErrors);
  };
}