document.location.href = "...login page...";

var userInput = confirm('Session timed out, nt login again?');
document.location.href = (userInput === true ? 'loginpage' : 'index/main.html');

var userInput = confirm('Session timed out, nt login again?');
document.location.href = (userInput ? 'loginpage' : 'HOME');

if (userInput)
{
    document.getElementById('loginNameInputId').focus();//fill in actual id, of course
}
else
{
    window.location.href = 'anyPage or url';
}

if (userInput)
{
    document.getElementById('loginNameInputId').value = varContainingUserName;
    document.getElementById('passwordInput').focus();
}