Add filter to Shopify colections page

theme.liquid

<script>
  jQuery('select[name=custom-filter-by]').on('change', function() {
    var thisfilter = jQuery('select[name=custom-filter-by]').val();
    jQuery('select[name=custom-filter-by] option[value="'+thisfilter+'"]').attr('selected','selected');
	});
  jQuery('select[name=custom-sort-by]').on('change', function() {
    var thissort = jQuery('select[name=custom-sort-by]').val();
    jQuery('select[name=custom-sort-by] option[value="'+thissort+'"]').attr('selected','selected');
	});
  /* Product Tag Filters - Good for any number of filters on any type of collection pages */
  jQuery('button#apply-sort-filter').click(function(e) {
    e.preventDefault();
    var filterby = jQuery('select[name=custom-filter-by]').val();
    var sortby = jQuery('select#custom-sort-by').val();
    var query = '';
    if (filterby != '') {
      query = filterby + '+' + filterby;
    }
      if (sortby != '') {
        query = query + '?sort-by=' + sortby;
      }
      
      var newurl = "/collections/all/" + query;
      //console.log(newurl);

      window.location.href = newurl;
  });
</script>