js将数字转化为带有千分号的字符串

<script type="text/javascript">

window.onload=function(){

var str="123333333333";

var len = str.length;

var step=3;

var splitor=",";

if(len > step) {

var l1 = len%step,l2 = parseInt(len/step),arr = [],first = str.substr(0, l1);

if(first != '') {

arr.push(first);

};

for(var i=0; i<l2 ; i++) {

arr.push(str.substr(l1 + i*step, step));

};

str = arr.join(splitor);

};

　　alert(str);

}

</script>