Security error

<!doctype html>
<html>
<head><meta charset="UTF-8"><title>localStorage saving</title>
    <script>
        var listen = window.addEventListener;
        (listen||window.attachEvent)((listen ?'':'on') +'message', function(ev){
            var d = typeof ev.data =='string' && ev.data[0] =='{' ? JSON.parse(ev.data) : ev.data
            if(!d.do) return;
            var tok = d.tok
                ,key = d.key;
            switch(d.do){
                case 'set':
                    var prev = localStorage[key];
                    if(d.val !==undefined)
                        localStorage[key] = JSON.stringify(d.val);
                    else
                        localStorage.removeItem(key);
                    break;
                case 'get':
                    prev = localStorage[key];
                    prev = prev === undefined || typeof prev =='string'&& prev[0] !='{'? prev : JSON.parse(prev); break
                case 'remove':
                    prev = localStorage[key];
                    if(prev !==undefined)
                        localStorage.removeItem(key);
            }
            console.log('[io2]', tok, 'prev=', prev);
            ev.source.postMessage(JSON.stringify(prev !==undefined ? {tok: tok, prev: prev} : {tok: tok, undef:1}), ev.origin);
        },!1);
    </script>
</head>
<body>
    This page supports saving of localStorage (on the domain github.io) for sites which erase it on its own domain (such as google.*, facebook.com etc.).
</body>
</html>