easyWay.js

// digit sum
// easy way 

function digitSum(number) {
   number = Array
      .from(number.toString())
      .reduce(function(acc, n){
         return acc + parseInt(n);
       },0);
  if (number > 9) number = digitSum(number);
  return number;
}

console.log(digitSum(3219));

numberAddtoDigit.js

function addToDigit(number) {
  var acc = 0;
  while(number > 0) {
    var reminder = number % 10;
    number = (number - reminder) / 10;
    acc += reminder;
  }
  if (acc > 9) acc = addToDigit(acc);
  
  return acc;
}

console.log(addToDigit(3251)); // => 6