calendar-functions.js

const MILISECONDS_IN_A_DAY = 1000 * 60 * 60 * 24;

const getDaysInMonth = (month, year = (new Date()).getFullYear()) => {
	return new Date(year, month + 1, 0).getDate();
}

const getMonday = (d) => {
	const parsedDate = new Date(d);
	const day = parsedDate.getDay();
	const difference = parsedDate.getDate() - day + (day == 0 ? -6 : 1);
	return new Date(parsedDate.setDate(difference));
}

const getDateDifference = (firstDate, secondDate) => {
	return Math.ceil(Math.abs(firstDate.getTime() - secondDate.getTime()) / MILISECONDS_IN_A_DAY);
}

const getDaysForMonthFormatted = (month, year = (new Date()).getFullYear()) => {
	const firstDateOfMonth = new Date(year, month, 1);
	const lastMonday = getMonday(firstDateOfMonth);
	const difference = getDateDifference(firstDateOfMonth, lastMonday);
	const daysInMonth = getDaysInMonth(month, year);
	const days = new Array(42).fill(0).map( (_, index) => {
		return new Date(year, month, index + 1 - difference);
	});
	return days;
}

new Array(12).fill(0).reduce( (totalDays, item, monthIndex) => {
	var daysInThisMonth = daysInMonth(monthIndex, currentYear);
	var datesInThisMonth = new Array(daysInThisMonth).fill(0).map((_, dayIndex) => {
		return new Date(currentYear, monthIndex, dayIndex + 1);
	});
	return totalDays.concat(datesInThisMonth);
}, []);