diff --git a/web/pdf_thumbnail_viewer.js b/web/pdf_thumbnail_viewer.js
index 1185371bf..5c2796b58 100644
--- a/web/pdf_thumbnail_viewer.js
+++ b/web/pdf_thumbnail_viewer.js
@@ -18,7 +18,7 @@ import {
 } from './ui_utils';
 import { PDFThumbnailView } from './pdf_thumbnail_view';
 
-var THUMBNAIL_SCROLL_MARGIN = -19;
+const THUMBNAIL_SCROLL_MARGIN = -19;
 
 /**
  * @typedef {Object} PDFThumbnailViewerOptions
@@ -30,201 +30,193 @@ var THUMBNAIL_SCROLL_MARGIN = -19;
  */
 
 /**
- * Simple viewer control to display thumbnails for pages.
- * @class
+ * Viewer control to display thumbnails for pages in a PDF document.
+ *
  * @implements {IRenderableView}
  */
-var PDFThumbnailViewer = (function PDFThumbnailViewerClosure() {
+class PDFThumbnailViewer {
   /**
-   * @constructs PDFThumbnailViewer
    * @param {PDFThumbnailViewerOptions} options
    */
-  function PDFThumbnailViewer(options) {
-    this.container = options.container;
-    this.renderingQueue = options.renderingQueue;
-    this.linkService = options.linkService;
-    this.l10n = options.l10n || NullL10n;
+  constructor({ container, linkService, renderingQueue, l10n = NullL10n, }) {
+    this.container = container;
+    this.linkService = linkService;
+    this.renderingQueue = renderingQueue;
+    this.l10n = l10n;
 
     this.scroll = watchScroll(this.container, this._scrollUpdated.bind(this));
     this._resetView();
   }
 
-  PDFThumbnailViewer.prototype = {
-    /**
-     * @private
-     */
-    _scrollUpdated: function PDFThumbnailViewer_scrollUpdated() {
-      this.renderingQueue.renderHighestPriority();
-    },
+  /**
+   * @private
+   */
+  _scrollUpdated() {
+    this.renderingQueue.renderHighestPriority();
+  }
 
-    getThumbnail: function PDFThumbnailViewer_getThumbnail(index) {
-      return this.thumbnails[index];
-    },
+  getThumbnail(index) {
+    return this._thumbnails[index];
+  }
 
-    /**
-     * @private
-     */
-    _getVisibleThumbs: function PDFThumbnailViewer_getVisibleThumbs() {
-      return getVisibleElements(this.container, this.thumbnails);
-    },
+  /**
+   * @private
+   */
+  _getVisibleThumbs() {
+    return getVisibleElements(this.container, this._thumbnails);
+  }
 
-    scrollThumbnailIntoView:
-        function PDFThumbnailViewer_scrollThumbnailIntoView(page) {
-      var selected = document.querySelector('.thumbnail.selected');
-      if (selected) {
-        selected.classList.remove('selected');
+  scrollThumbnailIntoView(page) {
+    let selected = document.querySelector('.thumbnail.selected');
+    if (selected) {
+      selected.classList.remove('selected');
+    }
+    let thumbnail = document.querySelector(
+      'div.thumbnail[data-page-number="' + page + '"]');
+    if (thumbnail) {
+      thumbnail.classList.add('selected');
+    }
+    let visibleThumbs = this._getVisibleThumbs();
+    let numVisibleThumbs = visibleThumbs.views.length;
+
+    // If the thumbnail isn't currently visible, scroll it into view.
+    if (numVisibleThumbs > 0) {
+      let first = visibleThumbs.first.id;
+      // Account for only one thumbnail being visible.
+      let last = (numVisibleThumbs > 1 ? visibleThumbs.last.id : first);
+      if (page <= first || page >= last) {
+        scrollIntoView(thumbnail, { top: THUMBNAIL_SCROLL_MARGIN, });
       }
-      var thumbnail = document.querySelector(
-        'div.thumbnail[data-page-number="' + page + '"]');
-      if (thumbnail) {
-        thumbnail.classList.add('selected');
-      }
-      var visibleThumbs = this._getVisibleThumbs();
-      var numVisibleThumbs = visibleThumbs.views.length;
+    }
+  }
 
-      // If the thumbnail isn't currently visible, scroll it into view.
-      if (numVisibleThumbs > 0) {
-        var first = visibleThumbs.first.id;
-        // Account for only one thumbnail being visible.
-        var last = (numVisibleThumbs > 1 ? visibleThumbs.last.id : first);
-        if (page <= first || page >= last) {
-          scrollIntoView(thumbnail, { top: THUMBNAIL_SCROLL_MARGIN, });
-        }
-      }
-    },
+  get pagesRotation() {
+    return this._pagesRotation;
+  }
 
-    get pagesRotation() {
-      return this._pagesRotation;
-    },
+  set pagesRotation(rotation) {
+    this._pagesRotation = rotation;
+    for (let i = 0, l = this._thumbnails.length; i < l; i++) {
+      this._thumbnails[i].update(rotation);
+    }
+  }
 
-    set pagesRotation(rotation) {
-      this._pagesRotation = rotation;
-      for (var i = 0, l = this.thumbnails.length; i < l; i++) {
-        var thumb = this.thumbnails[i];
-        thumb.update(rotation);
-      }
-    },
+  cleanup() {
+    PDFThumbnailView.cleanup();
+  }
 
-    cleanup: function PDFThumbnailViewer_cleanup() {
-      PDFThumbnailView.cleanup();
-    },
+  /**
+   * @private
+   */
+  _resetView() {
+    this._thumbnails = [];
+    this._pageLabels = null;
+    this._pagesRotation = 0;
+    this._pagesRequests = [];
 
-    /**
-     * @private
-     */
-    _resetView: function PDFThumbnailViewer_resetView() {
-      this.thumbnails = [];
-      this._pageLabels = null;
-      this._pagesRotation = 0;
-      this._pagesRequests = [];
+    // Remove the thumbnails from the DOM.
+    this.container.textContent = '';
+  }
 
-      // Remove the thumbnails from the DOM.
-      this.container.textContent = '';
-    },
+  setDocument(pdfDocument) {
+    if (this.pdfDocument) {
+      this._cancelRendering();
+      this._resetView();
+    }
 
-    setDocument: function PDFThumbnailViewer_setDocument(pdfDocument) {
-      if (this.pdfDocument) {
-        this._cancelRendering();
-        this._resetView();
-      }
+    this.pdfDocument = pdfDocument;
+    if (!pdfDocument) {
+      return Promise.resolve();
+    }
 
-      this.pdfDocument = pdfDocument;
-      if (!pdfDocument) {
-        return Promise.resolve();
-      }
-
-      return pdfDocument.getPage(1).then((firstPage) => {
-        var pagesCount = pdfDocument.numPages;
-        var viewport = firstPage.getViewport(1.0);
-        for (let pageNum = 1; pageNum <= pagesCount; ++pageNum) {
-          var thumbnail = new PDFThumbnailView({
-            container: this.container,
-            id: pageNum,
-            defaultViewport: viewport.clone(),
-            linkService: this.linkService,
-            renderingQueue: this.renderingQueue,
-            disableCanvasToImageConversion: false,
-            l10n: this.l10n,
-          });
-          this.thumbnails.push(thumbnail);
-        }
-      });
-    },
-
-    /**
-     * @private
-     */
-    _cancelRendering: function PDFThumbnailViewer_cancelRendering() {
-      for (var i = 0, ii = this.thumbnails.length; i < ii; i++) {
-        if (this.thumbnails[i]) {
-          this.thumbnails[i].cancelRendering();
-        }
-      }
-    },
-
-    /**
-     * @param {Array|null} labels
-     */
-    setPageLabels: function PDFThumbnailViewer_setPageLabels(labels) {
-      if (!this.pdfDocument) {
-        return;
-      }
-      if (!labels) {
-        this._pageLabels = null;
-      } else if (!(labels instanceof Array &&
-                   this.pdfDocument.numPages === labels.length)) {
-        this._pageLabels = null;
-        console.error('PDFThumbnailViewer_setPageLabels: Invalid page labels.');
-      } else {
-        this._pageLabels = labels;
-      }
-      // Update all the `PDFThumbnailView` instances.
-      for (var i = 0, ii = this.thumbnails.length; i < ii; i++) {
-        var thumbnailView = this.thumbnails[i];
-        var label = this._pageLabels && this._pageLabels[i];
-        thumbnailView.setPageLabel(label);
-      }
-    },
-
-    /**
-     * @param {PDFThumbnailView} thumbView
-     * @returns {PDFPage}
-     * @private
-     */
-    _ensurePdfPageLoaded(thumbView) {
-      if (thumbView.pdfPage) {
-        return Promise.resolve(thumbView.pdfPage);
-      }
-      var pageNumber = thumbView.id;
-      if (this._pagesRequests[pageNumber]) {
-        return this._pagesRequests[pageNumber];
-      }
-      var promise = this.pdfDocument.getPage(pageNumber).then((pdfPage) => {
-        thumbView.setPdfPage(pdfPage);
-        this._pagesRequests[pageNumber] = null;
-        return pdfPage;
-      });
-      this._pagesRequests[pageNumber] = promise;
-      return promise;
-    },
-
-    forceRendering() {
-      var visibleThumbs = this._getVisibleThumbs();
-      var thumbView = this.renderingQueue.getHighestPriority(visibleThumbs,
-                                                             this.thumbnails,
-                                                             this.scroll.down);
-      if (thumbView) {
-        this._ensurePdfPageLoaded(thumbView).then(() => {
-          this.renderingQueue.renderView(thumbView);
+    return pdfDocument.getPage(1).then((firstPage) => {
+      let pagesCount = pdfDocument.numPages;
+      let viewport = firstPage.getViewport(1.0);
+      for (let pageNum = 1; pageNum <= pagesCount; ++pageNum) {
+        let thumbnail = new PDFThumbnailView({
+          container: this.container,
+          id: pageNum,
+          defaultViewport: viewport.clone(),
+          linkService: this.linkService,
+          renderingQueue: this.renderingQueue,
+          disableCanvasToImageConversion: false,
+          l10n: this.l10n,
         });
-        return true;
+        this._thumbnails.push(thumbnail);
       }
-      return false;
-    },
-  };
+    });
+  }
 
-  return PDFThumbnailViewer;
-})();
+  /**
+   * @private
+   */
+  _cancelRendering() {
+    for (let i = 0, ii = this._thumbnails.length; i < ii; i++) {
+      if (this._thumbnails[i]) {
+        this._thumbnails[i].cancelRendering();
+      }
+    }
+  }
+
+  /**
+   * @param {Array|null} labels
+   */
+  setPageLabels(labels) {
+    if (!this.pdfDocument) {
+      return;
+    }
+    if (!labels) {
+      this._pageLabels = null;
+    } else if (!(labels instanceof Array &&
+                 this.pdfDocument.numPages === labels.length)) {
+      this._pageLabels = null;
+      console.error('PDFThumbnailViewer_setPageLabels: Invalid page labels.');
+    } else {
+      this._pageLabels = labels;
+    }
+    // Update all the `PDFThumbnailView` instances.
+    for (let i = 0, ii = this._thumbnails.length; i < ii; i++) {
+      let label = this._pageLabels && this._pageLabels[i];
+      this._thumbnails[i].setPageLabel(label);
+    }
+  }
+
+  /**
+   * @param {PDFThumbnailView} thumbView
+   * @returns {PDFPage}
+   * @private
+   */
+  _ensurePdfPageLoaded(thumbView) {
+    if (thumbView.pdfPage) {
+      return Promise.resolve(thumbView.pdfPage);
+    }
+    let pageNumber = thumbView.id;
+    if (this._pagesRequests[pageNumber]) {
+      return this._pagesRequests[pageNumber];
+    }
+    let promise = this.pdfDocument.getPage(pageNumber).then((pdfPage) => {
+      thumbView.setPdfPage(pdfPage);
+      this._pagesRequests[pageNumber] = null;
+      return pdfPage;
+    });
+    this._pagesRequests[pageNumber] = promise;
+    return promise;
+  }
+
+  forceRendering() {
+    let visibleThumbs = this._getVisibleThumbs();
+    let thumbView = this.renderingQueue.getHighestPriority(visibleThumbs,
+                                                           this._thumbnails,
+                                                           this.scroll.down);
+    if (thumbView) {
+      this._ensurePdfPageLoaded(thumbView).then(() => {
+        this.renderingQueue.renderView(thumbView);
+      });
+      return true;
+    }
+    return false;
+  }
+}
 
 export {
   PDFThumbnailViewer,