digital_root_iterative.js

function digital_root(n) {
  // ...
  
  function sum_roots(m) {
    let rootArr = [];
    let currentNum = m;
  
    do {
      rootArr.push(currentNum % 10);
      currentNum = Math.trunc(currentNum / 10);
    } while (currentNum >= 1) 

    return rootArr.reduce((acc, val) => acc + val, 0);
  }
  
  
  let result = n;
  
  while(result > 10) {
    result = sum_roots(result);
  }
  
  
  return result;
  

}