Is this considered recursion?

Question

JavaScript

function mainFunc(res) {
    function getDigitsSum(num) {
        num = String(num);
        let result = 0;
            for (var i = 0; i < num.length; i++){
                result += Number(num[i]);
            }
        return result;
    }
    if ( getDigitsSum(res) < 10){
        return getDigitsSum(res);
    }
    return mainFunc(getDigitsSum(res));
}
//для примера число взял 7190
console.log(mainFunc(7190));

This was the question. Write a recursive function that will add the digits in a number until there is, in fact, a one-digit number. I wrote it like this, it works and even correct :0
But the question is, is this function recursive?
The most stupid question, I understand, but if you remove the second return, then everything stops working and the code gives the value 'undefined' if the sum of digits less than 9 was not obtained during the first scroll. Why is that?

Answer 1

Formally, yes, it is: after all, it mainFunc()calls itself inside.
With each call mainFunc(), the f-th is determined again and again getDigitSum()- why, if it does not change?