实用的js函数

//返回顶部

   function backTop(btnId) {
            var btn = document.getElementById(btnId);
            var d = document.documentElement;
            var b = document.body;
            window.onscroll = set;
            btn.style.display = "none";
            btn.onclick = function() {
                btn.style.display = "none";
                window.onscroll = null;
                this.timer = setInterval(function() {
                    d.scrollTop -= Math.ceil((d.scrollTop + b.scrollTop) * 0.1);
                    b.scrollTop -= Math.ceil((d.scrollTop + b.scrollTop) * 0.1);
                    if ((d.scrollTop + b.scrollTop) == 0) clearInterval(btn.timer, window.onscroll = set);
                }, 10);
            };

            function set() {
                btn.style.display = (d.scrollTop + b.scrollTop > 100) ? 'block' : "none"
            }
        };
        backTop('btnID');