get date of N-th X-day in specific Year-Month

nthXdayDate.js

/*
get date of N-th X-day in specific Year-Month

logic behind first_x_day_date
                xday
            0 1 2 3 4 5 6
getDay  0 | 1 2 3 4 5 6 7
(wday)  1 | 7 1 2 3 4 5 6 
        2 | 6 7 1 2 3 4 5 
        3 | 5 6 7 1 2 3 4
        4 | 4 5 6 7 1 2 3 
        5 | 3 4 5 6 7 1 2 
        6 | 2 3 4 5 6 7 1
    
    when getDay is 0, result is just => xday+1
    as getDay increase, result is slide off by getDay => xday+1 - getDay
    but, date should not be negative so add 7 and get remain => (xday+1-getDay + 7) % 7
    but, remain makes 7 to 0, so using IF or OR to make 0 to 7 again => (xday+1-getDay+7)%7 || 7

WARN: may not same as Month's Nth week's weekday, it just consider Nth occuring weeday
WARN: input validation not included
WARN: 5th weekday may not exist or not, not concerned yet

*/


function nthXdayDate(year, month, nth, xday) {
  // year = YYYY
  // month = 1~12
  // nth = 1~5 ?
  // xday = 0~6 (SUN ~ SAT)
  const first_date_wday = new Date(year, month - 1).getDay()
  const first_xday_date = (8 + xday - first_date_wday) % 7 || 7
  const nth_xday_date = first_xday_date + (nth - 1) * 7
  return nth_xday_date
}