daysInMonth.js

daysInMonth.js

// An expansion of https://stackoverflow.com/a/1184359/201789
function daysInMonth (oneIndexedInputMonth, year) {
	const zeroIndexedInputMonth = oneIndexedInputMonth - 1
	
	const theNextZeroIndexedMonth = zeroIndexedInputMonth + 1
	const theFinalDayOfThePreviousMonth = 0
	
	return new Date(year, theNextZeroIndexedMonth, theFinalDayOfThePreviousMonth).getDate()
}