https://www.codewars.com/kata/541c8630095125aba6000c00

sum-of-digits.js

function digitalRoot(n) {
  if (n < 10) {
      return n;
  }
    
  let nu = n
  let s = 0
  
   while(nu > 0) {
    const r = nu % 10
    s += r
    nu = (nu - r) / 10
  }
  
  return digitalRoot(s)
}