Destructively Sum all the digits of a number in JavaScript

We are required to write a JavaScript function that takes in a number as the only argument. The function should sum the digits of the number while the sum converses to a single digit number.

For example − 

If the number is −

const num = 54564567;

Then the function should sum it like this −

5+4+5+6+4+5+6+7 = 42
4+2 = 6

Therefore, the final output should be 6

Example

const num = 54564567;
const sumDigits = (num, sum = 0) => {
   if(num){
      return sumDigits(Math.floor(num / 10), sum + (num % 10));
   };
   return sum;
}
const sumDestructively = (num) => {
   let sum = num; while(sum > 9){
      sum = sumDigits(sum);
   };
   return sum;
}
console.log(sumDestructively(num));

Output

And the output in the console will be −

6