[рони]

const arrS = [
            [3, 4, 5],
            [6, 7, 8]
        ];

        function sum(arr) {
            let s = 0;
            for (let a of arr) {
                s += Array.isArray(a) ? sum(a) : a;
            }
            return s;
        }

        console.log(sum(arrS));