function minLength(min, value) {
  return ((value && value.toString().length >= min) || !value) ? undefined : 'invalid';
}


function minLengthNew(min, value) {
  return (value && value.toString().length < min) ? 'invalid' : undefined;
}

console.log(minLength(3, '12'));
console.log(minLength(3));
console.log(minLengthNew(3, '12'));
console.log(minLengthNew(3));