返回顶部的按钮设计

<a href="javascript:;" id="btn" title="回到顶部"></a>

#btn{position:fixed;display:none;}

window.onload = function(){
    var obtn = document.getElementById(‘btn‘);
    //获取页面可视区的高度
    var clientHeight = document.documentElement.clientHeight;
    var timer = null;
    var isTop = true;
    window.onscroll = function(){
        var osTop = document.documentElement.scrollTop || document.body.scrollTop;
        if (osTop >= clientHeight){
        //显示按钮
            obtn.style.display = ‘block‘;
        }else {
        //隐藏按钮
            obtn.style.display = ‘none‘;
        }
        if (!isTop){
            clearInterval(timer);
        }
        isTop = false;
    };
    obtn.onclick = function(){        
        //设置定时器
        timer = setInterval(function(){
            //获取滚动条距离顶部的高度
            var osTop = document.documentElement.scrollTop || document.body.scrollTop;
            var ispeed = Math.floor(-osTop / 6);
            document.documentElement.scrollTop = document.body.scrollTop = osTop +ispeed;
            
            isTop = true;
            if (osTop === 0){
                clearInterval(timer);
            }
        },30);
    };
};