SOURCE

let str = "230323213100"

let str1 = Array.from(str).reverse()

console.log(str1)
let newArr = []
let count = 0
for (let i = 0; i < str1.length; i++) {
    if (count == 3) {
        newArr.push(",")
        count = 0
    }
    newArr.push(str1[i])
    count++
}
console.log(newArr.reverse().join(""))