JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" name="locationthemes" id="checkbox-1" value="2" checked="checked" class="custom" />
<label for="checkbox-1">Castle</label>
<input type="checkbox" name="locationthemes" id="checkbox-2" value="3" class="custom" />
<label for="checkbox-2">Barn</label>
<input type="checkbox" name="locationthemes" id="checkbox-3" value="5" class="custom" />
<label for="checkbox-3">Restaurant</label>
<input type="checkbox" name="locationthemes" id="checkbox-4" value="8" checked="checked" class="custom" />
<label for="checkbox-4">Bar</label>

JavaScript

$('input:checkbox[name=locationthemes]:checked').each(function() 
    {
       alert( $(this).val());
    });