functions.php

/**
 * Function to display variations sale prices only when a variation on sale is selected
 */

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 .nm-single-product-summary-col .onsale").css("display", "block");
                    $(".product-type-variable .onsale").css("display", "block");
                }
                //Otherwise hide the sale badge
                else {
                    // $(".product-type-variable .nm-single-product-summary-col .onsale").css("display", "none");
                    $(".product-type-variable .onsale").css("display", "none");
                }
            });
        })(jQuery);
    </script>
    <?php
}
add_action( 'woocommerce_before_single_variation', 'action_wc_before_single_variation' );