Add Digits.js

/*Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?*/

/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    return num==0 ? 0 : (num%9 == 0 ? 9 : (num%9))
};