Simple Log In Error

is it possible to write some javascript within my ASP page, which will determine:

1. are either the username or password text box empty
2. is the username syntactically invalid
3. is the password syntactically invalid

The idea is that when a user clicks on the Log In 'submit' button, the javascript will detect if any of the above conditions are true, and if so, will raise a window error box informing the user of the error. If none of the conditions are true, then the info will simply be sent to the next page which determines if the username - password combination match with one in my DB.

if this is possible, how would i go about enforcing the script to be run, when the user clicks on the 'Log In' button.