console 命令行工具 X clear

console

$(document).scroll(function(){
  if(document.documentElement.scrollTop>500){
    $("div").show();
  }else{
    $("div").hide();
  }
})

$("div").click(function(){
  $("html").animate({scrollTop:0},200)
})

<h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1><h1>123</h1><h1>321</h1>

<div>eqweqwe</div>

div{
  width:80x;
  height:80px;
  background-color:red;
  position:fixed;
  bottom:20px;
  right:20px;
  display:none;
}