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>