function addTwoNumbers(num1str, num2str) {
  num1Arr = num1str.toString().split('').map(e => parseInt(e)).reverse();
  num2Arr = num2str.toString().split('').map(e => parseInt(e)).reverse();
  let index = Math.max(num1Arr.length, num2Arr.length);
  let overhead = 0;
  let result = []

  for (let i = 0; i < index; i++) {
    let num1temp = num1Arr[i] ? num1Arr[i] : 0
    let num2temp = num2Arr[i] ? num2Arr[i] : 0
    let temp = num1temp + num2temp + overhead;
    overhead = temp >= 10 ? 1 : 0;
    temp = temp % 10;
    result.push(temp);
  }
  if (overhead === 1) result.push(1);
  return result.reverse().join("")
}


let num1 = "123"
let num2 = "39"


let res = addTwoNumbers(num1, num2)

console.log(res)