super_digit_solution.js

function super_digit(n, k) {
  const p = find_p(n, k);
  const superDigit = find_super_digit(p);
  return superDigit;
}

function find_p(n, k) {
  let concatString = "";
  for (let i = 0; i < k; i++) {
    concatString = concatString + n.toString();
  }
  return concatString;
}

function find_super_digit(p) {
  if (p.length === 1) return +p;
  let arrayOfDigits = p.split("");
  let sum = arrayOfDigits.reduce((total, current) => total + (+current), 0);
  sum = sum.toString();
  return find_super_digit(sum.toString());
}

const superDigit = super_digit(148, 3);

console.log(superDigit);