Code implementation of goTop to return to the top of the page controlled by js_javascript skills

Have you ever seen a lot of floating stuff with [Back to top] in the lower right corner of the page? It doesn't exist at the beginning. It appears as soon as you move the scroll bar, and disappears again when you return to the top.
Like:

The js code controlled is as follows:

Copy code The code is as follows:

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 = document. documentElement.scrollWidth/2 500 "px";
_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("

return
Back
Top
Part

");
window.onscroll = goTop;