Sum all the digits of a number Javascript

Question

I am newbie.

I want to make small app which will calculate the sum of all the digits of a number.

For example, if I have the number 2568, the app will calcul

Answer 1

How about this simple approach using modulo 9 arithmetic?

function sumDigits(n) {
  return (n - 1) % 9 + 1;
}