Userscript: Show a QR code in order to easily open up the same page on your mobile device.

Google_Play_QR_Code.user.js

// ==UserScript==
// @name           Google Play QR Code
// @namespace      Google Play
// @description    Show a QR code in order to easily open up the same page on your mobile device.
// @include        https://play.google.com/store/apps/details?id=*
// ==/UserScript==
(function() {
    findxpath = function(root, xpath) {
        return document.evaluate(xpath, root, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
    }
    
    findxpath_singleNode = function(root, xpath) {
        return document.evaluate(xpath, root, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    };
    
    let path = findxpath_singleNode(document, "//link[@rel='canonical']").href;
    let blocks = findxpath(document, "//td[contains(@class, 'doc-banner-title-container')]");
    
    var newImg = document.createElement("img");
    newImg.src = "http://chart.googleapis.com/chart?chs=100x100&cht=qr&chld=L|1&chl=" + encodeURIComponent(path);
    newImg.alt = "QR code link to this page";
    
    for (i = 0; block = blocks.snapshotItem(i); i++) {
        // insert QR code as first child
        block.insertBefore(newImg, block.firstChild);
    }
})();