function getArr(arr){
    if(Object.prototype.toString.call([])==="[object Array]"&&arr.length>0){
        var ar = arr.pop();
        return Array.prototype.concat.call(getArr(arr),getArr(ar));
    }
    return arr;
}

getArr([1,2,3,4,[5,6,7,[8]]])
[1,2,3,4,5,6,7,8]