text: function digital_root(n) {
    let digitalRoot = n;
  
    while (digitalRoot > 9)
      digitalRoot = [...digitalRoot.toString()].reduce(
        (a, b) => Number(a) + Number(b),
        0
      );
  
    return digitalRoot;
  }
  
  console.log(digital_root(123));
  console.log(digital_root(1));
  console.log(digital_root(16));
  console.log(digital_root(456));
  
  // Digital root is the recursive sum of all the digits in a number.
  
  // Given n, take the sum of the digits of n. If that value has more than one digit,
  // continue reducing in this way until a single-digit number is produced. The input will be a non-negative integer.
  
  // Examples
  //     16  -->  1 + 6 = 7
  //    942  -->  9 + 4 + 2 = 15  -->  1 + 5 = 6
  // 132189  -->  1 + 3 + 2 + 1 + 8 + 9 = 24  -->  2 + 4 = 6
  // 493193  -->  4 + 9 + 3 + 1 + 9 + 3 = 29  -->  2 + 9 = 11  -->  1 + 1 = 2
  