在提交表单之前获取jquery来检查userid是否有效

I have a form that I want to check if the userID is already used and then display the answer to the user before submitting the form. I found a way to do it through another site, but the code comes back with e.nodename is undefined. I am using jQuery 1.6. Any thoughts on how to do this? I am very new to jQuery. Thanks.I have a form that I want to check if the userI