Collapse Digits

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

Example:

Input: 38
Output: 2
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2.
             Since 2 has only one digit, return it.

Write your solution:

function main (input) {
    return;
}

function main (input) { return; }

Test Case 1

Input: 38
Output: 2

Test Case 2

Input: 103
Output: 4

Test Case 3

Input: 987
Output: 6

Solution

function collapseDigits(num) {
    return 1 + ((num - 1) % 9);
}

Solution

function collapseDigits(num) {
    const result = String(num)
        .split('')
        .reduce((acc, cur) => acc + Number(cur), 0);

    if (result < 10) {
        return result;
    }
    return collapseDigits(result);
}