javascript 回到顶部

 <script type="text/javascript">
        window.onload = function(){
            var timer = null;
            //用于判断是否是用户执行onsroll事件
            var flag = true;
            var oBtn = document.getElementById("btn1");

            window.onscroll = function(){

                //如果是用户执行onscroll事件 就清除定时器
                if(!flag)
                {

                    clearInterval(timer);
                }

                flag = false;

            }

            oBtn.onclick = function(){

                timer = setInterval(function(){
                   flag = true;
                   var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
                   var speed = Math.floor(-scrollTop/6);

                   document.documentElement.scrollTop= document.body.scrollTop = scrollTop + speed;

                   if(scrollTop==0)
                   {
                        clearInterval(timer);
                   }

                },150);

            };
        }
    </script>