Problem in confirm method in windows.onbeforeunload event

Hi

I used the follwoing code in windows.onbeforeunload

 if (isDirty.value == '') {

                return true;

            }

            else {

               confirm(msg);

                return false;

            }

It shows me an alert message when the back button in the browser is cliicked.

But the issue is that, even though i pressed the cancel button , it takes me to the