JSFiddle - React, Tailwind, and code Playground

JavaScript

"use strict";

const addDigits = (num) => {
  let numCounter = +num;
  while (numCounter > 9) {
    numCounter = digitsCount(num);
  } 
  return numCounter
};

const digitsCount = (str) => {
  let count = 0;
  let countAssist = '' + str;
  for (let i = 0; i < countAssist.length; i++) {
      count += +countAssist[i];
    }  
    return count;
  };

//export default addDigits;

console.log(addDigits(38));
console.log(addDigits('38'));

// console.log(digitsCount(38));
// console.log(digitsCount('38'));