Me vs. A Minor Edit in Squarespace – Episode 1

I was recently tasked with creating one of those ‘back to top’ scrolly buttons in a Squarespace site. I built a vanilla HTML prototype, which worked across all browsers, within like an hour.
However, upon migrating my little script to Squarespace, I found myself cursing a blue streak when I could get it to work in Chrome, but nowhere else. For anyone who encounters the same fate – it turns out the problem was how I was calling the scrollTop function – Firefox & IE do not like $(‘body’), document.body or document.documentElement on Squarespace (please tweet at me if you have found exceptions to this, or know why it happened).
In the end, here’s what worked for me, cross-browser. First, use the header code injection block to include a jQuery CDN:
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

JS (injected into the footer):

window.onload = function(){ 
  window.onscroll = function() {scrollFunction()}; //fires the animation on scroll
    function scrollFunction() {
       if (document.body.scrollTop &gt; 200) { //button appears once you scroll down 200px. To alter the appearance point, change the #
       $('#backToTop').addClass('button-visible'); //toggles the class that holds the CSS animations
       } else {
         $('#backToTop').removeClass('button-visible');
       }
     }
     var backToTop = $('#backToTop'); //button
     backToTop.on('click', function(){
        //on click, animates to the top of the page
         $('html').animate({ scrollTop: 0 }, 'fast'); //so the scroll works in firefox
         $('body').animate({ scrollTop: 0 }, 'fast'); //so the scroll works in webkit
          return false;
      });
  };  

CSS(injected into the header):

.backToTop{ 
        opacity: 0;
        -webkit-transition: opacity 0.25s ease-in;
        position: fixed;
        right: 0px;
        bottom: 30px;
        font-size: 2em;  
    }
    .backToTop::before {
        position: fixed;
        content:"\2191";
        opacity: 1;
        color: #9b1408;
        margin-left: 20px;
    }
    .backToTop::after {
        content:"";
        background-color: #aaa;
        opacity: 0.4;
        z-index:-999999;
        padding: 10px 30px;
    }
    .button-visible{
        opacity: 1;
        -webkit-transition: opacity 0.25s ease-in;
        transition: opacity 0.25s ease-in;
    }

HTML (which I put in a code block in the footer):

<span id="backToTop" class="backToTop"></span>