const number = [1,2,3,4];

const sum = (number, digit = 1) => {
  const length = number.length;

  if (length == 0) {
    return [];
  }

  if (length == 1) {
    return [number[0] == 9 ? 0 : number[0] + 1]
  }

  const last = number[length-1];

  if (last == 9) {
    return [...sum(number.slice(0, length-1)), 0];
  }

  return [...number.slice(0, length-1), last + 1];
}

console.log(sum(number))