Digital root is the recursive sum of all the digits in a number. Given n, take the sum of the digits of n. If that value has more than one digit, continue reducing in this way until a single-digit number is produced. This is only applicable to the natural numbers. https://www.codewars.com/kata/541c8630095125aba6000c00

Sum of Digits - Digital Root

function digital_root(n) {
        let values = convertIntToArray(n);
        let sum = 0;
        sum = values.reduce(doSum);

        if((convertIntToArray(n)).length > 1) {
          return digital_root(sum);
        } else {
          return sum;
        }
      }

      function convertIntToArray(int){
        return Array.from(String(int), Number);
      }
      function doSum(total, num){
        return (total + num);
      }