JavaScript 获取月的最后一天 - 基于年、月和日

Question

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)
}

Answer 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.

Answer 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, eg

 /* 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, eg 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 (ie i is greater than eomDay so adds more days that eomDay subtracts), it subtracts 7 days.