Jenishbek

let arr = [1, [2, 7, 8], [3, 4, [5, [6, 7]]]];
let arrRes = [];
function func(arr) {
         let res = [];
            for (elem of arr) {
                if (typeof elem == 'object') {
                    res.push(func2(elem));
                } else {
                    arrRes.push(elem);
                }
            }
            return arrRes;
        }
console.log(func(arr));