var thousandBitSeparator = function(numStr){
     var b = /([-+]?\d{3})(?=\d)/g;
     return numStr.replace(b, function($0, $1){
         return $1 + ',';
     });
 }

<script language="JavaScript" type="text/javascript"> 
function formatNumber(num){ 
 if(!/^(\+|-)?(\d+)(\.\d+)?$/.test(num)){ 
  return num; 
 } 
 var a = RegExp.$1,b = RegExp.$2,c = RegExp.$3; 
 var re = new RegExp().compile("(\\d)(\\d{3})(,|$)"); 
 while(re.test(b)){ 
  b = b.replace(re,"$1,$2$3"); 
 } 
 return a +""+ b +""+ c; 
} 
var num=1234567/3; 
alert("num="+num+"，四舍五入："+Math.round(num)+"，两位有效数字："+num.toFixed(2)+"，添加千位分隔符："+formatNumber(num)); 
</script>