Ajax发布没有被点击

I am using codeigniter where i put the post backend coding under models/add_subscriber.php but the code not being hit. Any idea?

form.onsubmit = function(event) {
            validate();
            if (!isValid) {
                revalidateOnChange();
                return false;
            }
            var email = $("#signup_email").val();
            dataString = 'email=' + email;
            $.ajax(
                    {
                        type: "POST",
                        url: '<?php echo base_url()?>models/add_subscriber.php',
                        data: dataString,
                        success: function(response) {
                            alert(response);
                        },
                        error: function(xhr, textStatus, error) {

                        }
                    });
               return;

        };