2023-07-06.js

// https://www.codewars.com/kata/5a0d6d8c6975982b5b000383/train/javascript

function lastDayIsFriday( initialYear, endYear ) {
	if ( !endYear ) endYear = initialYear;

	let lastFridays = 0;

	for ( let year = initialYear; year <= endYear; year++ ) {		// iterate years range
		for ( let month = 1; month <= 12; month++ ) {				// iterate 12 month
			const lastDayOfMonth = new Date( year, month, 0 );		// construct new Date, while "0" represents last day in month (28/29/30/31): 1991-11-31
			if ( lastDayOfMonth.getDay() === 5 ) lastFridays++;		// .getDay() returns 0-6, representing weekdays: sunday - saturday
		}
	}

	return lastFridays;
}

console.log( lastDayIsFriday( 1901, 2000 ) );	// :>> 171
console.log( lastDayIsFriday( 1901, 2017 ) );	// :>> 200
console.log( lastDayIsFriday( 1991 ) );			// :>> 1
console.log( lastDayIsFriday( 2017 ) );			// :>> 2
console.log( lastDayIsFriday( 1997, 2042 ) );	// :>> 81
console.log( lastDayIsFriday( 1906, 1912 ) );	// :>> 11


// TESTING: 
const weekday = [ 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday' ];
const lastDayOfMonth = new Date( 2018, 11, 0 );
lastDayOfMonth; // :>> Fri Nov 30 2018 00:00:00 GMT+0100 (CEST)
console.log( weekday[lastDayOfMonth.getDay()] ); // :>> Friday
if ( weekday[lastDayOfMonth.getDay()] === 'Friday' ) console.log( true ); // :>> true