Add Back To The Top Button In Shopify

The Shopify theme I am using is dawn v 15.2.0, first you need to go to Edit code, then go to Add a new snippet, add back-to-the-top.liquid and copy the following code.

Code snippet 1 :back-to-the-top

{% comment %}
  Reduce below value if you need the back to the top button to appear higher up on the page.
  That value is in pixels.
{% endcomment %}
{% assign vertical_offset_for_trigger = 300 %}

{% comment %}
  Vertical offset from bottom of browser for the position of the button.
{% endcomment %}
{% assign position_from_bottom = '1.4em' %}

<a id="BackToTop" href="#" title="Back to the top" class="back-to-top hide">
  <span>Back to top</span> <i class="fa fa-arrow-circle-o-up fa-2x"></i>
</a>
{{ '//netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.min.css' | stylesheet_tag }}
<style>
  .back-to-top {
    position: fixed;
    bottom: {{ position_from_bottom }};
    right: 0px;
    text-decoration: none;
    color: #7f7f7f;
    background-color: #eee;
    font-size: 19px;
    padding: 0.3em;
    -webkit-border-top-left-radius: 3px;
    -webkit-border-bottom-left-radius: 3px;
    -moz-border-radius-topleft: 3px;
    -moz-border-radius-bottomleft: 3px;
    border-top-left-radius: 8px;
    border-bottom-left-radius: 8px;
    z-index: 60000;
  }
  .back-to-top i {
    vertical-align: middle;
  }
  .back-to-top span {
    padding-left: 0.5em;
  }
  .back-to-top i + span {
    padding-left: 0;
  }
  .back-to-top:hover {
    text-decoration: none;
    color: #555;
  }
  .hide {
    display: none!important;
  }
</style>
<script>
    (function() {
    function trackScroll() {
        var scrolled = window.pageYOffset;
        var coords = {{ vertical_offset_for_trigger }};

        if (scrolled > coords) {
        goTopBtn.classList.remove('hide');
        }
        if (scrolled < coords) {
        goTopBtn.classList.add('hide');
        }
    }

    function backToTop() {
        if (window.pageYOffset > 0) {
        window.scrollBy(0, -80);
        setTimeout(backToTop, 0);
        }
    }

    var goTopBtn = document.querySelector('.back-to-top');

    window.addEventListener('scroll', trackScroll);
    goTopBtn.addEventListener('click', backToTop);
    })();
</script>

Go to layout/theme.liquid, add the following code just before </body>

Code snippet 2

    {% render 'back-to-the-top' %}

I am a software developer over 20 years, if you need help to customize your shopify website, you can contact me at feng.zhang114@gmali.com or wechat: qq20070821