// javascript document
var backtop = function(btnid){
var btn = document.getelementbyid(btnid);
var d = document.documentelement || document.body;
window.onscroll = set;
btn.onclick = function () {
btn.style.display = "none";
window.onscroll = null;
this.timer = setinterval(function () {
//console.log(document.body.scrolltop)
//console.log(document.documentelement.scrolltop)
if( document.body.scrolltop ){
document.body.scrolltop -= math.ceil(document.body.scrolltop * 0.1);
}else{
document.documentelement.scrolltop -= math.ceil(document.documentelement.scrolltop * 0.1);
}
if (document.body.scrolltop == 1) clearinterval(btn.timer), window.onscroll = set, document.body.scrolltop = 0;
if (document.documentelement.scrolltop == 1) clearinterval(btn.timer), window.onscroll = set, document.documentelement.scrolltop = 0;
}, 10);
};
function set() { btn.style.display = (document.documentelement.scrolltop || document.body.scrolltop) ? 'block' : "none" }
};
backtop('gotopbtn');