woocomerce - add validation on checkout field blur - out of input in JS

woocomerce - add validation on checkout field blur - out of input in JS

add_action( 'wp_footer', 'ybc_javascript_validation' );
function ybc_javascript_validation(){

    if( !is_checkout() ) return;

    ?> <script>jQuery(function($){
            $('body').on('blur change', '#billing_birthday', function(){
                // alert($(this).val());
                var formRow = $(this).parents('.form-row');
                // if( /\d/.test( $(this).val() ) ) {
                    formRow.addClass('woocommerce-invalid');
                // } else {
                //     formRow.addClass('woocommerce-validated');
                // }
            });

        });</script> <?php
}