repeatedly add all of its digits until the result has only one digit

#javascript #programming #tutorial #interview

We're provided a positive integer num. Can you write a method to repeatedly add all of its digits until the result has only one digit?

Here's an example: if the input was 49, we'd go through the following steps:

Example:

input: 49
output: 4

// start with 49
4 + 9 = 13
1+3 = 4

Solution 1

function sumDigits(num) {
  function getSum(num) {
    let _num = num;
    let result = 0;
    while (_num > 0) {
      result += _num % 10;
      _num = Math.floor(_num / 10);
    }
    return result;
  }
  while (num > 9) {
    num = getSum(num);
  }
  return num
}

console.log(sumDigits(49));

Solution 2

function sumDigits(num) {
  while (num > 9) {
    num = num
      .toString()
      .split("")
      .reduce((a, b) => a + parseInt(b), 0);
  }
  return num;
}

console.log(sumDigits(49));