JSFiddle - React, Tailwind, and code Playground

HTML

<script>
    $.custom = {

    test : function() {
        alert("wtf");
    }

};
</script>

<div id="helloworld" style="border:solid; border-width:5px;">
    BLAH
</div>
<script>
    $.custom.test();
</script>

JavaScript

//$.custom.test(); // working here