Using The Joyous Power Of Relative Dates To Calculate Days-In-Month In JavaScript

test.js

// I return the number of days in the given month. The year is required because the
// value may change in leap years.
function daysInMonth( month, year ) {

	var lastDayOfMonth = new Date(
		year,

		// Go to the "next" month. This is always safe to do; if the next month is beyond
		// the boundary of the current year, it will automatically become the appropriate
		// month of the following year. For example, ( December + 1 ) ==> January.
		( month + 1 ),

		// Go to the "zero" day. Since days range from 1-31, the "0" day will
		// automatically roll back to the last day of the previous month. And, since we
		// did ( month + 1 ) above, it will be ( month + 1 - 1 ) ... or simply, the last
		// day of "month".
		0
	);

	return( lastDayOfMonth.getDate() );

}

// I return the name (abbreviation) of the given month index.
function monthAsString( month ) {

	var months = [ "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" ];

	return( months[ month % 12 ] );

}

// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //

// Let's test the months in 2016, since we know it's a leap year, and 2017, since we
// know it is not a leap year. And, since the Date object is a beast with relative
// dates, we can just loop over a 24-month period and the latter half of the loop will
// automatically roll over to the next year.
for ( var i = 0 ; i < 24 ; i++ ) {

	console.log( `Days[ ${ monthAsString( i ) } ]: ${ daysInMonth( i, 2016 ) }` );

	// To make the console-logging easier to read, split 2016 and 2017.
	( i === 11 ) && console.log( "----" );

}