JS实现千分符并且保存两位小数

1. eg: num = '1212121.645321'
function format (num) {
return (num.toFixed(2) + '').replace(/\d{1,3}(?=(\d{3})+(\.\d*)?$)/g, '$&,');
}

2. eg: var str = "1234567890";
var newStr = str.replace(/\d/g,function(){
if((str.length-arguments[1]-1)%3===0&&str.length-1!==arguments[1]){
return arguments[0]+",";
}else{
return arguments[0];
}
});
console.log(newStr); //1,234,567,890"