GM script: Insert button to auto updating page with links to read pages for download

GoogleBooksImages.user.js

// ==UserScript==
// @name         Google books images
// @namespace    gist.github.com/StefRe
// @version      1.2
// @description  Insert button to auto updating page with links to read pages for download
// @include      https://books.google.*/books*
// @grant        none
// ==/UserScript==

var imgsrcL, imgWindow, viewport, link;

function updateImgList() {
  var imgL = viewport.getElementsByTagName('img'), imgsrc;
  for (var i = 0; i < imgL.length; i++) {
    imgsrc = imgL[i].src;
    if (imgsrc == undefined || imgsrc == '') {
      continue;
    }
    if (imgsrcL.indexOf(imgsrc) == - 1) {
      imgsrcL.push(imgsrc);
      //PA = regular page, PT = part, PP = preface page, PR = page without pagination, RA = volume
      var p = /.*&pg=([^&]+).*/.exec(imgsrc);
      if (p && p.length > 1) {
        // only download page images, not restricted_logo, background...
        var a = imgWindow.document.createElement('a');
        a.href = imgsrc;
        a.download = p[1] + '.png'; // TODO: title pages can be jpg
        a.innerHTML = p[1];
        var ni = imgWindow.document.createNodeIterator(imgWindow.document.body, NodeFilter.SHOW_ELEMENT,
                   function(node) {
                    return node.nodeName.toLowerCase() === 'a' && node.innerHTML > p[1] ?
                      NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_REJECT;
                   });
        var nextNode = ni.nextNode();
        imgWindow.document.body.insertBefore(a, nextNode);
        imgWindow.document.body.insertBefore(imgWindow.document.createElement('br'), nextNode);
      }
    }
  }
}

function Refresh() {
  viewport = document.getElementById('viewport')
  viewport.addEventListener('DOMSubtreeModified', updateImgList);
  imgWindow = window.open();
  imgsrcL = [];
  updateImgList();
  imgWindow.document.title = document.title;
}

link = document.createElement('a');
link.innerHTML = ' ⇩';
link.style.cursor = 'pointer';
link.onclick = Refresh;
document.getElementsByClassName('kd-appname')[0].appendChild(link);