如何在GridView中验证文本框为空状态

我有一个gridview，其中我输入了一个文本框来输入数量.该gridview中有超过5行.现在，我应该如何为该文本框编写javascript的空状态，并且应该只输入NUMERIC.

谢谢，
等待答复..

i have a gridview in that i m putted a textbox to enter the Quantities. There are more than 5 rows in that gridview. Now i how i should write the javascript for that textbox for EMPTY status and should enter only NUMERIC.

thanks,
waiting for replies..