JAVASCRIPT

Sum of Digits / Digital Root

// codewars:Sum of Digits / Digital Root
function digitalRoot(n) {
  
   let result = 0;
  String(n).split('').map(num => {
    result += Number(num);
  });
  return result >= 10 ? digitalRoot(result) : result;
  
}

Sum of Digits / Digital Root

function digital_root(n) {
  return (n - 1) % 9 + 1;
}