A utility function that prints out an ordered calendar based on the passed in date or present day date. It was inspired by the Facebook page publishing tools date picker functionality.

calendar.js

function isLeapYear(year){
	return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
}

function calendar(inputDate){
	const presentDate = inputDate ? (new Date(inputDate)) : (new Date());

	console.log("date is ", presentDate)

	if(Boolean(presentDate) === false){
		console.log("invalid date provided.");
		return;
	}

	const months = {
		'0':31,
		'1':isLeapYear(presentDate.getFullYear()) ? 29 : 28,
		'2':31,
		'3':30,
		'4':31,
		'5':30,
		'6':31,
		'7':31,
		'8':30,
		'9':31,
		'10':30,
		'11':31,
	}

	const monthNames = ['JAN','FEB', 'MAR', 'APR', 'MAY', 'JUN', 'JUL', 'AUG', 'SEP', 'OCT', 'NOV', 'DEC']

	const month = presentDate.getMonth();

	console.log("Month is ", monthNames[month])

	const daysOfTheWeek = ['SUN', 'MON', 'TUE', 'WED', 'THU', 'FRI', 'SAT'];

	const firstDayOfMonth = (new Date(`${presentDate.getFullYear()}-${month+1}-01`)).getDay();

	const daysInMonth = months[month.toString()];

	const numberOfDaysInOneWeek = 7;

	const numberOfWeeksInMonth = Math.ceil((daysInMonth+firstDayOfMonth)/numberOfDaysInOneWeek);

	const calendarRows = [];

	let calendarRow = (new Array(7).fill('   '));

	for(let u = 0; u < daysInMonth; u++){
		let dayPosition = (firstDayOfMonth + u) % numberOfDaysInOneWeek;

		// check if we have gone to a new row and it's not empty
		if(dayPosition == 0 && Boolean(calendarRow[calendarRow.length -1].trim()) == true){
			calendarRows.push(calendarRow);
			calendarRow = (new Array(7)).fill('   ');
		}
		
		calendarRow[dayPosition] = u+1 < 10 ? `  ${u+1}` : ` ${u+1}`

	}

	// add the last calendar row
	if((numberOfWeeksInMonth - calendarRows.length) == 1){
		calendarRows.push(calendarRow);
	}

	console.log(daysOfTheWeek.join(' '));
	calendarRows.forEach((row) => {
		console.log(row.join(' '))
	})

}

calendar('2021-12');