/**
 * display variations sale badge only when a variation on sale is selected
 */

add_action( 'woocommerce_before_single_variation', 'action_wc_before_single_variation' );
function action_wc_before_single_variation() {
    ?>
    <script type="text/javascript">
    (function($){
        // Check for the show_variation event that triggers when a variation is selected.
        $('form.variations_form').on('show_variation', function(event, data){
            //If variation is on sale, display the sale badge
            if(data.display_price < data.display_regular_price){
                $(".product-type-variable .onsale").css("display", "block");
            }
            //Otherwise hide the sale badge
            else {
                $(".product-type-variable .onsale").css("display", "none");
            }
        });
    })(jQuery);
    </script>
    <?php
}