// style
.top {
     width: 32px;
     height: 32px;
     position: fixed;
     bottom: 50px;
     right: 30px;
     cursor: pointer;
     z-index: 9;
     background: url(images/top.png)no-repeat;
     background-size: cover;
}

//  div
<div class="top"></div>

// js
$(document).ready(function() {
    $('.top').css('display','none');
    var  backButton = $('.top');  
    function  backToTop()  {      
    $('html,body').animate({scrollTop:  0 },  800); }  
    backButton.on('click',  backToTop);    
      $(window).on('scroll',  function ()  {   
      if ($(window).scrollTop() > $(window).height()) {          
          backButton.fadeIn();      
      } else {        
          backButton.fadeOut();  
      }
    });  
 });