isLeapYear implementation, with a test suite

isLeapYear-testsuite.js

// See http://stackoverflow.com/questions/61088/hidden-features-of-javascript/64950#64950

function isLeapYear(year) {
	return !(year % 4 != 0 || (year % 100 == 0 && year % 400 != 0));
}

function isLeapYear_refButSlow(year) {
	return new Date(year, 1, 29, 0, 0).getMonth() != 2;
}

var range = (function() {
	var undef;
    var forEach = function(func) {
		var cur;
	    while ((cur = this()) !== undef)
			func(cur);
	};
	
	return function(from, to, step) {
		var f;
		if (to > from) {
			if (step === undef) u = 1;
			f = function() {
				if (from > to) return undef;
				var cur = from;
				from += step;
				return cur;
			};
		} else {
			if (step === undef) undef = -1;
			f = function() {
				if (from < to) return undef;
				var cur = from;
				from += step;
				return cur;
			};
		}
		f.forEach = forEach;
		return f;
	}
})();

function test(year) {
	WScript.Echo(year+": "+(isLeapYear(year) == isLeapYear_refButSlow(year) ? "OK" : "Err"));
}

range(1500, 2400, 100).forEach(test);
range(1970, 2011, 1).forEach(test);