SOURCE

function flatten(arr) {
    const res = [];

    for (let i = 0; i < arr.length; i++) {
        if (arr[i] instanceof Array) {
            res[i] = flatten(arr[i]);
        } else {
            res.push(arr[i]);
        }
    }

    return res;
}

console.log(flatten([1,2,3,4,5, [2,4, [3, 56]]]));