javascript - Sum up a number until it becomes 1 digit JS

Question

Before you mark duplicate note that it isn't. Others didn't look for the exact same thing as me.

What is the most compact possible way to sum up a number in javascript until there is only one digit left. For example: You input 5678 then the script adds it together (5+6+7+8) and gets 26, but since its more than 1 digit it adds it again and gets 2+6=8.

Is there anyway to do this with a number of any size? And how compact can the script get?

Answer 1

If you're looking for short, it's hard to beat:

var n = 5678;
sum  = n % 9 || 9;

console.log(sum)