Load jQuery and add jQuery code

<script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
<script src="/demo.js"></script>

<div id="text"></div>

$().ready(function() {
   $("#text").html("Text added by jQuery code.");
});

When you change the JavaScript file, make sure it is reloaded! Browser cashing can cause you a bad day.