javascript - JavaScript 関数は予期しない未定義を返します

Question

私は愚かなことを見逃したに違いありませんが、なぜsumArray戻ってくるのですundefinedか???

<script>

    function sumArray(arr, n, sum){
        if(n == 0){
            console.log( arr[0] + sum ); // log shows 15 as expected
            return  arr[0] + sum;        // the function would return undefined
        }else{
            sum = sum + arr[n-1];
            sumArray(arr, n-1, sum); 
        }
    }

    var arr1 = [0,1,2,3,4,5];
    var result = sumArray(arr1, arr1.length, 0)

    console.log(result); // returns Undefined !!!

</script>