JSFiddle - React, Tailwind, and code Playground

HTML

<p class="className">has a class</p>
<p>has no class</p>
<p class="className">has a class</p>
<p class="className">has a class</p>

CSS

.className { font-weight: bold; }

JavaScript

$(document).ready(function(){
    $('p').not('[class]').wrapInner('<em />');
});