这里的一个问题是你调用你的回调，而不传递函数本身：

One problem here is you're invoking your callback, not passing the function itself:

$.post("http://www.example.com/ajax/validate.php",{ // async validation
        type: 'valid_old_change_pass', 
        pass_old: pass_old,
        pass_new: pass_new
    }, valid_pass_combo_callback); // Remove (data) so the callback
                                   // isn't invoked immediately.