258. 各位相加

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function (num) {
  if (num < 10) return num
  let sum = 0
  while (num >= 10) {
    const numArr = num.toString().split('')
    sum = numArr.reduce((a, b) => Number(a) + Number(b))
    num = sum
  }
  return sum
}