回到顶端js实现

 function goTop(){  
            var _btn = document.getElementById("goTop");  
            if (document.documentElement && document.documentElement.scrollTop) {  
                var _con = document.documentElement;
            } 
            else if (document.body) {  
                var _con = document.body;  
            }  
            window.onscroll = set;  
            _btn.style.left = "125px";
            _btn.onclick = function() {
                _btn.style.display = "none";
                window.onscroll = null;
                this.timer = setInterval(function() {
                    _con.scrollTop -= Math.ceil(_con.scrollTop * 0.3);
                    if (_con.scrollTop == 0) {
                        clearInterval(_btn.timer, window.onscroll = set);
                    }
                }, 10);
            };  
            function set() {  
                _btn.style.display = _con.scrollTop ? 'block': "none";  
            }  
        };  
        document.write("<div id=\"goTop\" style=\"font-size:12px; width:20px; height:55px; background:#0950A2; color:#fff; line-height:14px; position:fixed; bottom:30px; display:none; cursor:pointer; text-align:center; padding:5px 0; z-index:1; _position:absolute; _top:expression(eval(document.compatMode && document.compatMode == \'CSS1Compat\') ? documentElement.scrollTop + (documentElement.clientHeight - this.clientHeight) - 30 : document.body.scrollTop + (document.body.clientHeight - this.clientHeight) - 30);\">返回顶部</div>");  
        window.onscroll = goTop;