javascript数字补零

<script type="text/javascript"> 
var pad = function(tbl) {
            return function(num, n){
                return (0 >= (n - ''+num.length)) ? num : (tbl[n] || (tbl[n] = Array(n + 1).join(0)))+num;
        }    
}([]);
alert(pad(233, 6));
</script>