How to return to self page after clicking Confirm box OK button

I have designed a code of SessionFilter in jsp Spring application and i kept 5 minutes of Session timeout period. I want to display a confirmation message to user before 30 seconds of timeout saying "Your Session is going to expire in another 30 Second. Do you want to continue." If user clicks Cancel button, the flow has to navigate to Login page and if the user clicks OK button, the page should stay in same page keeping session variable alive.

Below javascript code i used to stay in same page after user clicks OK button of Confirm box but it not not working as expected.

function notifyUser()
{
setTimeout(function() {
var timeLeft = confirm('Your Session is going to expire in another 30 Second. Do you want to continue.');
if(timeLeft == true)
{
window.location = self.location.reload(true);

}
else
{
window.location = 'index.jsp';
}
}, 30000);
}

<body onLoad="notifyUser()">

</body>

Please suggest how to do this.

Thanks you,
Shwetha.

Please find the formatted code below,

useinstead of using

and does refreshing the page invalidate the session?