JSFiddle - React, Tailwind, and code Playground

HTML

<iframe id="theiframe">

<form>
    <div class="pluginConnectButton">
        <div class="pluginButton pluginButtonInline pluginConnectButtonDisconnected" title="">
           
        </div>
        <div class="pluginButton pluginButtonPressed pluginButtonInline pluginButtonX pluginConnectButtonConnected hidden_elem" title="">
           
        </div>
</form>

</iframe>

JavaScript

alert($('#theiframe').hasClass('hidden_elem'));