JSFiddle - React, Tailwind, and code Playground

HTML

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

JavaScript

console.log($('input[name="locationthemes"]:checked').serialize());

$('input[name="locationthemes"]:checked').each(function() {
   console.log(this.value); 
});