将animate函数绑定在#next按钮上：
next.onclick = function () {

           if (index == 5) {
                index = 1;
            }
            else {
                index += 1;
            }
            animate(-600);
            showButton();
        }

animate函数如下设计：
function animate(offset){

            var newLeft = parseInt(list.style.left) + offset + 'px';//设置目标值
            var interval = 10;
            var time = 300;
            var speed = offset/(time/interval);

            function go(){
                if( (speed < 0 && parseInt(list.style.left) > newLeft)  || (speed > 0 && parseInt(list.style.left) < newLeft)){
                    list.style.left = parseInt(list.style.left) + speed + 'px';
                    setTimeout(go,interval);//多次位移
                }else{
                    list.style.left = newLeft + 'px';
                    if(parseInt(list.style.left) > -600){list.style.left = '-3000px'}
                    if(parseInt(list.style.left) < -3000){list.style.left = '-600px'}    
                }
                }
            go();    
            }
            
 我的问题是：为什么点击按钮，图片无法动起来？