JSFiddle - React, Tailwind, and code Playground

HTML

<select name="county" id="county">
      <option>All Countys</option>
      <option>Middlesex</option>
      <option>Berkshire</option>
      <option>Bedfordshire</option>
    </select>

    <select name="town" id="town">
      <option>All Towns</option>
      <option class="Middlesex">Ruislip</option>
      <option class="Middlesex">Hillindon</option>
      <option class="Uxbridge">Uxbridge</option>
    </select>

JavaScript

$(document).ready(function() {
    // get a copy of all the towns
    var $town = $("#town"),
        $towns = $("#town > option").clone();

    $("#county").change(function() {
        var selCounty = $(this).val();
        
        $town.children().remove();
        if (selCounty=="All Countys")
           $town.append($towns);
        else
           $town.append($towns.filter("." + selCounty));
    });
});