JavaScript get last day of Month - based on Year, Month and day

Solution 1:

Create a Date instance with the last day of the month then work backwards one day at a time until you match the day of the week you're after

const normaliseDay = day => day.trim().toLowerCase()
const dayIndex = new Map([
  ["sunday",    0],
  ["monday",    1],
  ["tuesday",   2],
  ["wednesday", 3],
  ["thursday",  4],
  ["friday",    5],
  ["saturday",  6],
])

const getLastDayOfMonth = (year, month, dow) => {
  const day = dayIndex.get(normaliseDay(dow))
  
  // init as last day of month
  const date = new Date(Date.UTC(parseFloat(year), parseFloat(month), 0))
  
  // work back one-day-at-a-time until we find the day of the week
  while (date.getDay() != day) {
    date.setDate(date.getDate() - 1)
  }
  
  return date
}

console.log(getLastDayOfMonth("2022", "02", "Wednesday"))

The numeric values are parsed as numbers so we don't run into problems with zero-padded octal numbers.

Solution 2:

You can get the last day of the month then subtract the required number of days based on the difference between the end of month day and required day, e.g.

/* Return last instance of week day for given year and month
 * @param {number|string} year: 4 digit year
 * @param {number|string} month: calendar month number (1=Jan)
 * @param {string} day: English week day name, e.g. Sunday,
 *                      Sun, su, case insensitive
 * @returns {Date} date for last instance of day for given
 *                      year and month
 */
function getLastDayOfMonth(year, month, day) {
  // Convert day name to index
  let i = ['su','mo','tu','we','th','fr','sa'].indexOf(day.toLowerCase().slice(0,2));
  // Get end of month and eom day index
  let eom = new Date(year, month, 0);
  let eomDay = eom.getDay();
  // Subtract required number of days
  eom.setDate(eom.getDate() - eomDay + i - (i > eomDay? 7 : 0));
  return eom;
}

// Examples - last weekdays for Feb 2022
['Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sunday'].forEach(day => 
  console.log(day.slice(0,3) + ': ' + getLastDayOfMonth('2022', '02', day).toLocaleDateString('en-GB',{
    year:'numeric',
    month:'short',
    day:'numeric',
    weekday:'short'
  })
));

In the above:

new Date(year, month, 0)

leverages the fact that the month is calendar month whereas the date constructor months are 0 indexed, so the above sets the date to the zeroth day of March, which resolves to the last day in February.

The part:

eom.getDate() - eomDay + i - (i > eomDay? 7 : 0)

subtracts the eomDay index to get to the prior Sunday, then i adds days to get the required day. However, if that goes beyond the end of the month (i.e. i is greater than eomDay so adds more days that eomDay subtracts), it subtracts 7 days.

JavaScript get last day of Month - based on Year, Month and day

Solution 1:

Solution 2:

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