JQuery - Uncaught ReferenceError: $ is not defined at

Exception

Uncaught ReferenceError: $ is not defined
    at timer.js:14

Solution

1.  Please make sure that JQuery JS file is added in the HTML file 
2. JQuery JS file should be declared first than other JS files like

<script src='https://code.jquery.com/jquery-3.4.1.min.js'></script>     
<script src='js/app.min.js'></script>
<script src='js/timer.js'></script>