s

var str = 123456789+ '';
var a = str.split('').reverse().reduce((prev,next,index) => {
  return ((index % 3) ? next : (next + ',')) + prev;
})
console.log(a)//123,456,789

[1, 2, 3, 1, 2, 3, 4].reduce((items, item) => {
   if(item in items){
        items[item]++;
    }else{
        items[item] = 1;
    }
    return items;
},{}) //{1: 2, 2: 2, 3: 2, 4: 1}