jQuery validator and a custom rule that uses AJAX

For anyone else who stumbles upon this, validate supports ‘remote’ method, which may not have existed in 2010:

https://jqueryvalidation.org/remote-method/

$("#myform").validate({
  rules: {
    email: {
      required: true,
      email: true,
      remote: {
        url: "check-email.php",
        type: "post",
        data: {
          username: function() {
            return $("#username").val();
          }
        }
      }
    }
  }
});