var str = '36125813';
function test(str){
var re =/(?=(?!\b)(\d{3})+$)/g;
return str.replace(re,',');
}
console.log(test(str))
var str = '36125813';
function test(str){
var re =/(?=(?!\b)(\d{3})+$)/g;
return str.replace(re,',');
}
console.log(test(str))