u is not defined javascript code example

Example 1: reference error $ is not defined jquery

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>

Example 2: Uncaught ReferenceError: $ is not defined

You should put the references to the jquery scripts first.
<script src="jquery-3.5.1.js"></script>
<script type="text/javascript" src="script.js"></script>