formatCurrency( 123 | 123.321 ) -> ( "$0" | "<b class="neg">-$123<i class="cents c123">.123</i></b>" )

formatCurrency.js

function formatCurrency(num){
	num = (num||'0').toString().replace(/\$|\,/g,'');
	if(isNaN(num)) num = "0";
	if(num+0 == 0) return '$0';
	
	var sign = (num == (num = Math.abs(num)));
	num = Math.floor(num*100+0.50000000001);
	
	var cents = num%100;
	num = Math.floor(num/100).toString();
	
	if(cents<10) cents = "0" + cents;
	
	numlength = num.length;
	
	for (var i = 0; i < Math.floor((num.length-(1+i))/3); i++)
		num = num.substring(0,num.length-(4*i+3))+','+ num.substring(num.length-(4*i+3));
	
	return [
		'<span class="',
			sign?' pos':' neg',
			' l', numlength,
		'">',
			sign ? '' : '-',
			'$',
			num,
			'<i class="cents c'+cents+'">.', cents, '</i>',
		'</span>'
	].join('');
}

formatCurrency_test.js

document.write(formatCurrency(0) +'\n')
document.write(formatCurrency(1) +'\n')
document.write(formatCurrency(-1) +'\n')
document.write(formatCurrency(0.5) +'\n')
document.write(formatCurrency(1.5) +'\n')
document.write(formatCurrency(-1.5) +'\n')
document.write(formatCurrency(999999999999) +'\n')
document.write(formatCurrency(5555.5555) +'\n')