conditional validator for Angular reactive forms

requiredIf.ts

const requiredIf(condition: (control: AbstractControl) => boolean): (control: FormControl) => ValidationResult {
  return (control: FormControl): ValidationResult => {
    // Important: control.parent might not be defined, depending on your form structure
    return !condition(control.parent) || !!control.value ? null : { [`required_if`]: true };
  };
}