getWeek.js

var getWeek = function(d) {var e=new Date(d),f=e.setDate(d.getDate()-(d.getDay()+6)%7+3);e.setMonth(0,4);return((f-e)/6048e5)+1.5|0};

var testData = {
	'2005-01-01': 53,
	'2005-01-02': 53,
	'2005-12-31': 52,
	'2007-01-01': 01,
	'2007-12-30': 52,
	'2007-12-31': 01,
	'2008-01-01': 01,
	'2008-12-29': 01,
	'2008-12-31': 01,
	'2009-01-01': 01,
	'2009-12-31': 53,
	'2010-01-03': 53,
	'2009-12-31': 53,
	'2010-01-01': 53,
	'2010-01-02': 53,
	'2010-01-03': 53,
	'2008-12-28': 52,
	'2008-12-29': 01,
	'2008-12-30': 01,
	'2008-12-31': 01,
	'2009-01-01': 01,
	'1970-12-30': 53,
	'2008-09-26': 39
};

for ( var i in testData ) {
	var d = new Date( i ),
		stamp = +d,
		week;

	if ( ( week = getWeek( d ) ) == testData[ i ] ) {
		if ( +d == stamp )
			console.log( 'OK', i, testData[ i ] )
		else
			console.log( 'OK but \'d\' has changed. Was: ' + new Date( i ) + ', is: ' + d );
	}
	else
		console.log( 'Wrong: week for ' + i + '. Is: ' + week + ', Should be: ' + testData[ i ] );
}