(Algorithms in Javascript) Leetcode 258. Add Digits - Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

258-sum-digits.js

const sumDigits = (x) => {
  if(x === '') {
  	return 0;
  } else if(x < 10) {
  	return x;
  } else {
  	let newX = x + '';
    newX = newX.split('');
    let sumX = parseInt(newX.pop()) + sumDigits(parseInt(newX.join('')));
    return sumX < 10 ? sumX : sumX%10 + Math.floor(sumX/10);
  }
}