Ajax for Express Forms dispatches 2 events you can use to modify the form's response:
You can modify either the request argument or the error argument and return the event.
For instance, you might want to check some of the answers using the "request" argument and send back an ErrorList object through the "error" argument if you found something wrong.
Another possibility is to modify some of the answers in the "request" argument before allowing the submission.
If you modify the "message" argument and return the event, your modified message will be displayed.
For instance, you could check if the sender is logged in and customize the success message with the user's name.