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/**
 * Auto update cart after quantity change
 *
 * @return  string
 **/
add_action( 'woocommerce_after_cart', 'custom_after_cart' );
function custom_after_cart() {
    echo '<script>
    jQuery(document).ready(function($) {

        var upd_cart_btn = $(".update-cart-button");
        upd_cart_btn.hide();

        $(".cart-form").find(".qty").on("change", function(){
            upd_cart_btn.trigger("click");
        });

    });
    </script>';
}