Check year is leap year

is-leap-year.js

'use strict';

function isLeapYear(year) {

	// result
	var result = undefined;

	// set as today
	if (typeof year === 'undefined') {

		year = new Date().getYear();
	}

	// is date object
	else if (typeof year === 'object' && year instanceof Date) {

		// get year
		year = year.getYear();
	}

	// year is number
	if (typeof year === 'number' && year !== NaN) {

		// assign result
		result = ((year % 4) === 0 && (year % 100) !== 0) || (year % 400) === 0;
	}

	// valid result
	if (typeof result === 'number') {

		// return result
		return result;
	}

	// invalid result
	else {

		// invalid input value
		throw new Error('invalid input value');
	}
}