Problem - Add Digits

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

For example, if $n = 38$, add $3 + 8 = 11$. Then add $1 + 1 = 2$. Return $2$.

There's a simple loop solution, but also a mathematical shortcut using modulo. Try the loop first.