Auto select "In Stock" filter

Add the below script into your Footer Script (Admin panel > Storefront > Footer Script)

<script>
    (function () {
        var $ = window.jQuerySupermarket;

        $(document).ready(function () {
            $('#facetedSearch-navList--bool .navList-action').each(function (id, el) {
                var $el = $(el);

                if ($el.text().indexOf('In Stock') !== -1 && !$el.hasClass('is-selected')) {
                    setTimeout(function() {
                        $el.click();
                    }, 400);
                }
            });
        });
    })();
</script>