>I've tried originally form.action = form.action + '&btnRemoveOverdue=Remove+Overdue'; with the same result.

With that approach, can you show the entire form.action value in the message?

>I am not sure about your other suggestion. When I was having an error in the script (unclosed }), I did get the message from the browser about an error. But I am not getting any alerts and I think it may be due to the fact I'm not using browser directly, I'm using C++ application. Although I am also going to try to check generated HTML directly in the browser, thanks.

As soon as you get the error, the Javascript will end. So, you will not get the alert you have in the code.