Hierarchial Mixitup Select Filter

snippet.php

    <div class="filter-controls">
        <div class="filter-controls__drop-down">
            <select id="filterSelect">
                <?php $hiterms = get_terms("dealer_locations", array("orderby" => "slug", "parent" => 0)); ?>
                <?php foreach ($hiterms as $key => $hiterm) : ?>
                    <optgroup label="<?php echo $hiterm->name; ?>">
                        <?php $loterms = get_terms("dealer_locations", array("orderby" => "slug", "parent" => $hiterm->term_id)); ?>
                        <?php if ($loterms) : ?>
                            <?php foreach ($loterms as $key => $loterm) : ?>
                                <option data-filter=".<?php echo $loterm->slug; ?>"><?php echo $loterm->name; ?></option>
                            <?php endforeach; ?>
                        <?php endif; ?>
                    </optgroup>
                <?php endforeach; ?>
            </select>
        </div>
    </div>