1分でできるJavaScriptで国民の祝日を判定する方法

function isHoliday(date) {
  const req = new XMLHttpRequest();
  req.open('get', 'syukujitsu.csv', true);
  req.send(null);
  req.onload = function() {
    checkDate(req.responseText, date, function(result) {
      console.log(date, result);
    });
  };
}

function checkDate(str, date, callback) {
  const data = [];
  const nDate = new Date(date);
  let dataArr = [];
  let checkDate =
    nDate.getFullYear() +
    '/' + (nDate.getMonth() + 1) +
    '/' + nDate.getDate();
  let isHoliday = false;
  let tmp = str.split('\n');
  for (let i=0; i< tmp.length; i++) {
    dataArr = tmp[i].split(',');
    if (dataArr[0] === checkDate) {
      isHoliday = true;
      break;
    }
  };
  callback(isHoliday);
}
isHoliday('2018-01-01');
// => 2018-01-01 true

isHoliday('2018-01-02');
// => 2018-01-02 false

isHoliday('2018/08/05 12:34');
// => 2018/08/05 12:34 false

isHoliday('2019-05-05 00:00:00');
// => 2019-05-05 00:00:00 true