// javascript document
function gotopex() {
    var obj = document.getelementbyid("gotopbtn");

    function getscrolltop() {
        var scrolltop = document.documentelement.scrolltop || window.pageyoffset || document.body.scrolltop;
        return scrolltop;
    }

    function setscrolltop(value) {
        if (window.pageyoffset !== undefined) {
            document.body.scrolltop = document.documentelement.scrolltop = value;
        } else {   // ie9 及更早版本
            if (document.documentelement.scrolltop !== undefined){
                document.documentelement.scrolltop = value;
            }else{
                document.body.scrolltop = value;
            }
        }
    }
    window.onscroll = function() {

        getscrolltop() > 0 ? obj.style.display = "" : obj.style.display = "none";
    }
    obj.onclick = function() {
        var gotop = setinterval(scrollmove, 10);
        function scrollmove() {
            setscrolltop(getscrolltop() / 1.1);
            if (getscrolltop() < 1) clearinterval(gotop);
        }
    }
}
gotopex();