Number.isInteger is not a function in nodejs

module.exports.is_valid_integer_list = async (list, callback) => {
  var s = list.split(',');
  for (var i = 0; i < s.length; i++) {
      if (!this.is_valid_integer(s[i])) {
          return false;
      }
  }
  return true;
}

module.exports.is_valid_integer = async (s, callback) => {
  if (Number.isInteger(s)) {
      return true;
  } else {
      return false;
  }
}

When I run this code it through the error.
“Number.isInteger is not a function”