I need a JS function to get the last date of the month. as an example, I need to get the last Wednesday of this month.
I will pass Year, Month and day as a arguments.
getLastDayOfMonth('2022', '02', 'Wednesday');
function getLastDayOfMonth(year, month, day){
// Output should be like this
2022-02-23
(this is last Wednesday of the month, according to the arguments- Year, month and Day)
}
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.
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.