String.prototype.asCurrency = function() {
	var f1 = this;
	var f2 = (Math.round((f1-0) * 100)) / 100;
	f2 = Math.floor(f2) == f2 ? f2 + ".00" : (Math.floor(f2 * 10) == f2 * 10) ? f2 + '0' : f2;
	f2 = String(f2);
	r = /(\d+)(\d{3})/;
	fs = String(f2);
	while (r.test(f2)) {
	      f2 = f2.replace(r, '$1' + ',' + '$2');
	}
	return ('￥' + f2); // TODO 沒考慮金額為負的情況
}

var s = "3.1415926";
alert(s.asCurrency());