I was working on form submit where I wanted to prompt with a simple Javascript confirm dialog when the user clicks the submit button on the form.  Naturally, all the form fields (name, adress, etc.) I setup using required field validators.  My custom javascript confirm originally looked like this.

I called the javascript from the code behind.

The big problem I found was that the javascript confirm function was being called regardless of if the page completely validated or not.  I wanted the functionality to first validate the page, then if the page was valid, run the custom code.   I came up with this code using Page_IsValid.  It uses Page_ClientValidate() to validate the page, then I check if the page is valid.  Simple yet effective! 

Good luck!