This JavaScript will redirect a webpage after alerting the user and giving them the option of resetting the counter. I use this to automatically logout website users that have been idel for a set amount of time. It uses a CSS popup to give them the option of resetting the timer.
Read my blog post explaining this JS: https://jimmelanson.ca/programming/2017/10/11/javascript-auto-logout-or-redirect-with-a-warning/