I'm making a web app in PHP and want to place "Back to top" element in the footer only if user need to scroll the page, in other case it will be not shown. How can I verify that the height of the user's browser screen is too small to display the `<body>` tag without frameworks? 

Right now footer looks like:

```
    <footer class="container pt-4 my-md-5 pt-md-5 border-top">
    <p class="float-right"><a href="#">Back to top</a></p>
    <p>© 2020 · <a href="#" target="_blank">Telegram</a> · <a href="#" target="_blank">GitHub</a></p>
    </footer>

```