warn the user before to unload the page (closing) and if user agree to stay redirect him

warn.js

var warn = true;
window.onbeforeunload = function(e){
    e = e || window.event;
    if(warn){ 
        e.returnMessage = 'yop'; 
        setTimeout(function(){
            warn = false; 
            location.href='http://google.fr';
        }, 1);
        return 'Are you sure? I got a secret to show you before!';
    }
}