Hi,
I came to know that you are passing the field as an argument to the validate function.
For this purpose, we can use the "field.value" to get the content of that field. It is evident that you have used the field as such in the statement. If you call the "value" property for that control, you will get this solved.
Hope this reply is used in solving this problem of yours...
For any further javascript related concepts, you may visit my blog at :
http://www.dsaravanan.wordpress.com/javascript.
Regards
d_s