基于Javascript实现返回顶部按钮

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body {
  height: 3000px;
}

. {
  position: absolute;
  : 120px;
  display: inline-block;
  width: 50px;
  height: 50px;
  line-height: 50px;
  text-decoration: none;
  text-align: center;
  background-color: #666666;
  color: #ffffff;
  right: 10px;
  transition: all 0.3s;
   visibility: hidden;
}
.:hover {
  background-color: #ff3300;
}

window.onscroll = function (e) {...}

var n_half_height = window.screen.height / 2;

var n_s = e.target.scrollingElement.scrollTop; //获取scrollTop的高度
var n_ = n_s + n_half_height;//得到位置

var ele_body = document.body;
var ele_ = document.getElementsByClassName("")[0];
var n_half_height = window.screen.height / 2;
ele_.style. = n_half_height + "px";
window.onscroll = function (e) {
 var n_s = e.target.scrollingElement.scrollTop;
 if (n_s === 0 ) {
  ele_.style.visibility = "hidden";
 }else {
  ele_.style.visibility = "visible";
 }
 var n_ = n_s + n_half_height ;
 ele_.style. = n_ + "px";
}