const input = "123123";
let i = 0;
let sum = 0;

while (i < input.length) {
  sum += parseInt(input[i], 10);
  i++;
}

console.log("sum", sum); // 12

function sum(x) {
  if (x.length === 1) {
    return parseInt(x, 10);
  }
  return parseInt(x[x.length - 1], 10) + sum(x.slice(0, x.length - 1));
}

console.log("sum", sum("1231233")); // 12