;$(function()
   {
       'use strict';

       var sidebar = $('#sidebar'),
        mask = $('.mask'),
        backButton = $('.back-to-top'),
        sidebar_trigger = $('#sidebar_trigger');

       function showSideBar()
      {
        mask.fadeIn();
        sidebar.css( 'right', 0 );
       }
      function  hideSideBar()
      {
        mask.fadeOut();
        sidebar.css('right', -sidebar.width());
      }
       sidebar_trigger.on('click', showSideBar);
        mask.on('click', hideSideBar);
        backButton.on('click',function()
       {
           $('html,body').animate({
               scrollTop: 0
           },800);
        })
      $(window).on ('scroll',function()
       {
           if($(window).scrollTop() > $(window).height())
           {
               backButton.fadeIn();
           }
           else {
               backButton.fadeOut();
           }
       })
       $(window).trigger('scroll');
 })