function superDigit(n, k) {
  if(n.length == 1){
    return n;
  }
  else {
    let sum = getSum(n) * k;
    sum = sum.toString();
    return superDigit(sum, 1);
  }
}

function getSum(n) {
  return n.split('').map(el => parseInt(el)).reduce((acc, curr) => acc + curr, 0);
}