営業日ベースでn日後の日付を計算する

addBusinessDays.js

function addBusinessDays(date, daysToAdd, holidays) {
  // weekends: 0 = Sunday, 6 = Saturday
  const weekendDays = [0, 6];
  // create a set of holidays for O(1) lookup time
  const holidaySet = new Set(holidays.map((holiday) => holiday.getTime()));
  let businessDaysAdded = 0;
  let currentDate = new Date(date.getTime());
  
  while (businessDaysAdded < daysToAdd) {
    // add one day to the current date
    currentDate.setDate(currentDate.getDate() + 1);
    // check if the current date is a weekend or holiday
    const currentDay = currentDate.getDay();
    const isWeekend = weekendDays.includes(currentDay);
    const isHoliday = holidaySet.has(currentDate.getTime());
    // if it's not a weekend or holiday, count it as a business day added
    if (!isWeekend && !isHoliday) {
      businessDaysAdded++;
    }
  }
  
  return currentDate;
}