[adblockpluschrome] Issue 3596 - Added support for CSS property filters to devtools panel

include.preload.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-var typeMap = {
+"use strict";
+
+const typeMap = {
   "img": "IMAGE",
   "input": "IMAGE",
   "picture": "IMAGE",
   "audio": "MEDIA",
   "video": "MEDIA",
   "frame": "SUBDOCUMENT",
   "iframe": "SUBDOCUMENT",
   "object": "OBJECT",
   "embed": "OBJECT"
 };
 
 function getURLsFromObjectElement(element)
 {
-  var url = element.getAttribute("data");
+  let url = element.getAttribute("data");
   if (url)
     return [url];
 
-  for (var i = 0; i < element.children.length; i++)
-  {
-    var child = element.children[i];
+  for (let child of element.children)
+  {
     if (child.localName != "param")
       continue;
 
-    var name = child.getAttribute("name");
+    let name = child.getAttribute("name");
     if (name != "movie"  && // Adobe Flash
         name != "source" && // Silverlight
         name != "src"    && // Real Media + Quicktime
         name != "FileName") // Windows Media
       continue;
 
-    var value = child.getAttribute("value");
+    let value = child.getAttribute("value");
     if (!value)
       continue;
 
     return [value];
   }
 
   return [];
 }
 
 function getURLsFromAttributes(element)
 {
-  var urls = [];
+  let urls = [];
 
   if (element.src)
     urls.push(element.src);
 
   if (element.srcset)
   {
-    var candidates = element.srcset.split(",");
-    for (var i = 0; i < candidates.length; i++)
-    {
-      var url = candidates[i].trim().replace(/\s+\S+$/, "");
+    for (let candidate of element.srcset.split(","))
+    {
+      let url = candidate.trim().replace(/\s+\S+$/, "");
       if (url)
         urls.push(url);
     }
   }
 
   return urls;
 }
 
 function getURLsFromMediaElement(element)
 {
-  var urls = getURLsFromAttributes(element);
-
-  for (var i = 0; i < element.children.length; i++)
-  {
-    var child = element.children[i];
+  let urls = getURLsFromAttributes(element);
+
+  for (let child of element.children)
+  {
     if (child.localName == "source" || child.localName == "track")
       urls.push.apply(urls, getURLsFromAttributes(child));
   }
 
   if (element.poster)
     urls.push(element.poster);
 
   return urls;
 }
 
 function getURLsFromElement(element)
 {
-  var urls;
+  let urls;
   switch (element.localName)
   {
     case "object":
       urls = getURLsFromObjectElement(element);
       break;
 
     case "video":
     case "audio":
     case "picture":
       urls = getURLsFromMediaElement(element);
       break;
 
     default:
       urls = getURLsFromAttributes(element);
       break;
   }
 
-  for (var i = 0; i < urls.length; i++)
+  for (let i = 0; i < urls.length; i++)
   {
     if (/^(?!https?:)[\w-]+:/i.test(urls[i]))
       urls.splice(i--, 1);
   }
 
   return urls;
 }
 
 function checkCollapse(element)
 {
-  var mediatype = typeMap[element.localName];
+  let mediatype = typeMap[element.localName];
   if (!mediatype)
     return;
 
-  var urls = getURLsFromElement(element);
+  let urls = getURLsFromElement(element);
   if (urls.length == 0)
     return;
 
   ext.backgroundPage.sendMessage(
     {
       type: "filters.collapse",
       urls: urls,
       mediatype: mediatype,
       baseURL: document.location.href
     },
 
-    function(collapse)
+    collapse =>
     {
       function collapseElement()
       {
-        var propertyName = "display";
-        var propertyValue = "none";
+        let propertyName = "display";
+        let propertyValue = "none";
         if (element.localName == "frame")
         {
           propertyName = "visibility";
           propertyValue = "hidden";
         }
 
         if (element.style.getPropertyValue(propertyName) != propertyValue ||
             element.style.getPropertyPriority(propertyName) != "important")
           element.style.setProperty(propertyName, propertyValue, "important");
       }
 
       if (collapse)
       {
         collapseElement();
 
         new MutationObserver(collapseElement).observe(
           element, {
             attributes: true,
             attributeFilter: ["style"]
           }
         );
       }
     }
   );
 }
 
 function checkSitekey()
 {
-  var attr = document.documentElement.getAttribute("data-adblockkey");
+  let attr = document.documentElement.getAttribute("data-adblockkey");
   if (attr)
     ext.backgroundPage.sendMessage({type: "filters.addKey", token: attr});
 }
 
 function getContentDocument(element)
 {
   try
   {
     return element.contentDocument;
   }
   catch (e)
   {
     return null;
   }
 }
 
-function ElementHidingTracer(selectors)
-{
-  this.selectors = selectors || [];

[Sebastian Noack, 2017/01/11 16:20:40]

It seems you removed the only code that initialized the ElementHidingTracer
without arguments. So unless I missed something, why not just removing support
for the "selectors" argument?

[wspee, 2017/01/12 13:39:58]

No longer relevant.

[wspee, 2017/01/19 16:20:14]

Done.

+function ElementHidingTracer()
+{
+  this.selectors = [];
+  this.filters = [];
 
   this.changedNodes = [];
   this.timeout = null;
 
   this.observer = new MutationObserver(this.observe.bind(this));
   this.trace = this.trace.bind(this);
 
   if (document.readyState == "loading")
     document.addEventListener("DOMContentLoaded", this.trace);
   else
     this.trace();
 }
 ElementHidingTracer.prototype = {
-  checkNodes: function(nodes)
-  {
-    var matchedSelectors = [];
-
-    // Find all selectors that match any hidden element inside the given nodes.
-    for (var i = 0; i < this.selectors.length; i++)
-    {
-      var selector = this.selectors[i];
-
-      for (var j = 0; j < nodes.length; j++)
-      {
-        var elements = nodes[j].querySelectorAll(selector.selector);
-        var matched = false;
-
-        for (var k = 0; k < elements.length; k++)
+  addSelectors(selectors, filters)
+  {
+    if (document.readyState != "loading")
+      this.checkNodes([document], selectors, filters);
+
+    this.selectors.push(...selectors);
+    this.filters.push(...filters);
+  },
+
+  checkNodes(nodes, selectors, filters)
+  {
+    let matchedSelectors = [];
+
+    for (let i = 0; i < selectors.length; i++)
+    {
+      nodes: for (let node of nodes)
+      {
+        let elements = node.querySelectorAll(selectors[i]);
+
+        for (let element of elements)
         {
           // Only consider selectors that actually have an effect on the
           // computed styles, and aren't overridden by rules with higher
           // priority, or haven't been circumvented in a different way.
-          if (getComputedStyle(elements[k]).display == "none")
+          if (getComputedStyle(element).display == "none")
           {
-            if (selector.filter)
-              matchedSelectors.push(selector.filter.replace(/^.*?##/, ""));
-            else
-              matchedSelectors.push(selector.selector);
-            matched = true;
-            break;
+            matchedSelectors.push(filters[i].replace(/^.*?##/, ""));
+            break nodes;
           }
         }
-
-        if (matched)
-          break;
       }
     }
 
     if (matchedSelectors.length > 0)
       ext.backgroundPage.sendMessage({
         type: "devtools.traceElemHide",
         selectors: matchedSelectors
       });
   },
 
-  onTimeout: function()
-  {
-    this.checkNodes(this.changedNodes);
+  onTimeout()
+  {
+    this.checkNodes(this.changedNodes, this.selectors, this.filters);
     this.changedNodes = [];
     this.timeout = null;
   },
 
-  observe: function(mutations)
+  observe(mutations)
   {
     // Forget previously changed nodes that are no longer in the DOM.
-    for (var i = 0; i < this.changedNodes.length; i++)
+    for (let i = 0; i < this.changedNodes.length; i++)
     {
       if (!document.contains(this.changedNodes[i]))
         this.changedNodes.splice(i--, 1);
     }
 
-    for (var j = 0; j < mutations.length; j++)
-    {
-      var mutation = mutations[j];
-      var node = mutation.target;
+    for (let mutation of mutations)
+    {
+      let node = mutation.target;
 
       // Ignore mutations of nodes that aren't in the DOM anymore.
       if (!document.contains(node))
         continue;
 
       // Since querySelectorAll() doesn't consider the root itself
       // and since CSS selectors can also match siblings, we have
       // to consider the parent node for attribute mutations.
       if (mutation.type == "attributes")
         node = node.parentNode;
 
-      var addNode = true;
-      for (var k = 0; k < this.changedNodes.length; k++)
-      {
-        var previouslyChangedNode = this.changedNodes[k];
+      let addNode = true;
+      for (let i = 0; i < this.changedNodes.length; i++)
+      {
+        let previouslyChangedNode = this.changedNodes[i];
 
         // If we are already going to check an ancestor of this node,
         // we can ignore this node, since it will be considered anyway
         // when checking one of its ancestors.
         if (previouslyChangedNode.contains(node))
         {
           addNode = false;
           break;
         }
 
         // If this node is an ancestor of a node that previously changed,
         // we can ignore that node, since it will be considered anyway
         // when checking one of its ancestors.
         if (node.contains(previouslyChangedNode))
-          this.changedNodes.splice(k--, 1);
+          this.changedNodes.splice(i--, 1);
       }
 
       if (addNode)
         this.changedNodes.push(node);
     }
 
     // Check only nodes whose descendants have changed, and not more often
     // than once a second. Otherwise large pages with a lot of DOM mutations
     // (like YouTube) freeze when the devtools panel is active.
     if (this.timeout == null)
       this.timeout = setTimeout(this.onTimeout.bind(this), 1000);
   },
 
-  trace: function()
-  {
-    this.checkNodes([document]);
+  trace()
+  {
+    this.checkNodes([document], this.selectors, this.filters);
 
     this.observer.observe(
       document,
       {
         childList: true,
         attributes: true,
         subtree: true
       }
     );
   },
 
-  disconnect: function()
+  disconnect()
   {
     document.removeEventListener("DOMContentLoaded", this.trace);
     this.observer.disconnect();
     clearTimeout(this.timeout);
   }
 };
 
 function runInPageContext(fn, arg)
 {
-  var script = document.createElement("script");
+  let script = document.createElement("script");
   script.type = "application/javascript";
   script.async = false;
   script.textContent = "(" + fn + ")(" + JSON.stringify(arg) + ");";
   document.documentElement.appendChild(script);
   document.documentElement.removeChild(script);
 }
 
 // Chrome doesn't allow us to intercept WebSockets[1], and therefore
 // some ad networks are misusing them as a way to serve adverts and circumvent
 // us. As a workaround we wrap WebSocket, preventing blocked WebSocket
 // connections from being opened.
 // [1] - https://bugs.chromium.org/p/chromium/issues/detail?id=129353
 function wrapWebSocket()
 {
-  var eventName = "abpws-" + Math.random().toString(36).substr(2);
-
-  document.addEventListener(eventName, function(event)
+  let eventName = "abpws-" + Math.random().toString(36).substr(2);
+
+  document.addEventListener(eventName, event =>
   {
     ext.backgroundPage.sendMessage({
       type: "request.websocket",
       url: event.detail.url
-    }, function (block)
+    }, block =>
     {
       document.dispatchEvent(
         new CustomEvent(eventName + "-" + event.detail.url, {detail: block})
       );
     });
   });
 
-  runInPageContext(function(eventName)
+  runInPageContext(eventName =>
   {
     // As far as possible we must track everything we use that could be
     // sabotaged by the website later in order to circumvent us.
-    var RealWebSocket = WebSocket;
-    var closeWebSocket = Function.prototype.call.bind(RealWebSocket.prototype.close);
-    var addEventListener = document.addEventListener.bind(document);
-    var removeEventListener = document.removeEventListener.bind(document);
-    var dispatchEvent = document.dispatchEvent.bind(document);
-    var CustomEvent = window.CustomEvent;
+    let RealWebSocket = WebSocket;
+    let closeWebSocket = Function.prototype.call.bind(RealWebSocket.prototype.close);
+    let addEventListener = document.addEventListener.bind(document);
+    let removeEventListener = document.removeEventListener.bind(document);
+    let dispatchEvent = document.dispatchEvent.bind(document);
+    let CustomEvent = window.CustomEvent;
 
     function checkRequest(url, callback)
     {
-      var incomingEventName = eventName + "-" + url;
+      let incomingEventName = eventName + "-" + url;
       function listener(event)
       {
         callback(event.detail);
         removeEventListener(incomingEventName, listener);
       }
       addEventListener(incomingEventName, listener);
 
       dispatchEvent(new CustomEvent(eventName, {
         detail: {url: url}
       }));
     }
 
     function WrappedWebSocket(url)
     {
       // Throw correct exceptions if the constructor is used improperly.
       if (!(this instanceof WrappedWebSocket)) return RealWebSocket();
       if (arguments.length < 1) return new RealWebSocket();
 
-      var websocket;
+      let websocket;
       if (arguments.length == 1)
         websocket = new RealWebSocket(url);
       else
         websocket = new RealWebSocket(url, arguments[1]);
 
-      checkRequest(websocket.url, function(blocked)
+      checkRequest(websocket.url, blocked =>
       {
         if (blocked)
           closeWebSocket(websocket);
       });
 
       return websocket;
     }
     WrappedWebSocket.prototype = RealWebSocket.prototype;
     WebSocket = WrappedWebSocket.bind();
     Object.defineProperties(WebSocket, {
       CONNECTING: {value: RealWebSocket.CONNECTING, enumerable: true},
       OPEN: {value: RealWebSocket.OPEN, enumerable: true},
       CLOSING: {value: RealWebSocket.CLOSING, enumerable: true},
       CLOSED: {value: RealWebSocket.CLOSED, enumerable: true},
       prototype: {value: RealWebSocket.prototype}
     });
 
     RealWebSocket.prototype.constructor = WebSocket;
   }, eventName);
 }
 
 function ElemHide()
 {
   this.shadow = this.createShadowTree();
   this.style = null;
   this.tracer = null;
 
   this.elemHideEmulation = new ElemHideEmulation(
     window,
-    function(callback)
+    callback =>
     {
       ext.backgroundPage.sendMessage({
         type: "filters.get",
         what: "elemhideemulation"
       }, callback);
     },
     this.addSelectors.bind(this)
   );
 }
 ElemHide.prototype = {
   selectorGroupSize: 200,
 
-  createShadowTree: function()
+  createShadowTree()
   {
     // Use Shadow DOM if available as to not mess with with web pages that
     // rely on the order of their own <style> tags (#309). However, creating
     // a shadow root breaks running CSS transitions. So we have to create
     // the shadow root before transistions might start (#452).
     if (!("createShadowRoot" in document.documentElement))
       return null;
 
     // Using shadow DOM causes issues on some Google websites,
     // including Google Docs, Gmail and Blogger (#1770, #2602, #2687).
     if (/\.(?:google|blogger)\.com$/.test(document.domain))
       return null;
 
     // Finally since some users have both AdBlock and Adblock Plus installed we
     // have to consider how the two extensions interact. For example we want to
     // avoid creating the shadowRoot twice.
-    var shadow = document.documentElement.shadowRoot ||
+    let shadow = document.documentElement.shadowRoot ||
                  document.documentElement.createShadowRoot();
     shadow.appendChild(document.createElement("shadow"));
 
     // Stop the website from messing with our shadow root (#4191, #4298).
     if ("shadowRoot" in Element.prototype)
     {
-      runInPageContext(function()
-      {
-        var ourShadowRoot = document.documentElement.shadowRoot;
+      runInPageContext(() =>
+      {
+        let ourShadowRoot = document.documentElement.shadowRoot;
         if (!ourShadowRoot)
           return;
-        var desc = Object.getOwnPropertyDescriptor(Element.prototype, "shadowRoot");
-        var shadowRoot = Function.prototype.call.bind(desc.get);
+        let desc = Object.getOwnPropertyDescriptor(Element.prototype, "shadowRoot");
+        let shadowRoot = Function.prototype.call.bind(desc.get);
 
         Object.defineProperty(Element.prototype, "shadowRoot", {
-          configurable: true, enumerable: true, get: function()
+          configurable: true, enumerable: true, get()
           {
-            var shadow = shadowRoot(this);
+            let shadow = shadowRoot(this);
             return shadow == ourShadowRoot ? null : shadow;
           }
         });
       }, null);
     }
 
     return shadow;
   },
 
-  addSelectors: function(selectors, filters)
+  addSelectors(selectors, filters)
   {
     if (selectors.length == 0)
       return;
-
-    if (this.tracer != null)
-    {
-      // The selector is beeing used to identify the filter that is responsible
-      // for hiding a particular element in the devtools panel. The
-      // ElementHidingTracer uses the selector to figure out if a filter is
-      // used on the current document. In case of an ElemHideEmulationFilter
-      // the selector doesn't match the selector of the filter
-      // ([-abp-properties=...] vs actual css selector depending on the sites
-      // css). The To still allow the devtools panel to find the correct filter
-      // we pass both the selector and the filter string to the
-      // ElementHidingTracer.
-      for (var i = 0; i < selectors.length; i++)
-      {
-        this.tracer.selectors.push({

[Sebastian Noack, 2017/01/11 16:20:40]

Is it guaranteed that the order of selectors match the order of filters? I have
some doubts from looking at the code in elemHideEmulation.js:

  addSelectors: function(stylesheets)
  {
    var selectors = [];
    var filters = {};
    for (var i = 0; i < stylesheets.length; i++)
      this.findSelectors(stylesheets[i], selectors, filters);
    this.addSelectorsFunc(selectors, Object.keys(filters));
  },

Note that objects in JavaScript don't guarantee properties to be retrieved in
insertion order (like dicts in Python). However, we could simply use a Set in
elemHideEmulation.js which preserves the insertion order and seems more
appropriate anyway, but wasn't supported by all supported browsers we run that
code on back then.

[Sebastian Noack, 2017/01/11 16:20:40]

Creating a new object for each selector might potentially add a significant
memory footprint. A possibly better solution would be to use two arrays, one for
the selectors and one for the filters. Perhaps you could benchmark the memory
usage with the current approach and compare it against the one I'm suggesting.

[wspee, 2017/01/12 13:39:58]

Indeed ... if you look at findSelectors there can even be more selectors than
filters.

findSelectors: function(stylesheet, selectors, filters)
{
  ...
        var subSelectors = splitSelector(rule.selectorText);
        for (var k = 0; k < subSelectors.length; k++)
          selectors.push(pattern.prefix + subSelectors[k] + pattern.suffix);
        filters[pattern.text] = true;
  ...
},

So I have switched the whole approach to something else entirely ....

ElemHideEmulationFilter are only created if there is a style that matches the
filter and document changes are also handled by ElemHideEmulation so it wasn't
really necessary to use the ElementHidingTracer for ElemHideEmulationFilter.

Instead I let addSelectors fire devtools.traceElemHide directly similar to how
it works in firefox
(adblockplus/lib/child/elemHideEmulation.js:initElemHideEmulation).

Only drawback is that devtools.traceElemHide gets fired regardless of whether
the devtools are opened or not. I could add a flag to ElemHide but that whould
be somewhat redundant with ElemHide.tracer but I cannot test ElemHide.tracer as
it gets assigned after addSelectors is called by ElemHideEmulation. But
logHiddenElements does (should have?!) check if the panel is opened anyways so
it's only a slight penalty. Let me now if you think otherwise.

[wspee, 2017/01/12 13:39:58]

No longer relevant.

[wspee, 2017/01/19 16:20:14]

According to the chrome profiler an array with 10000 objects uses less memory
than two arrays with 10000 entries, so I kept that approach.

[wspee, 2017/01/19 16:20:14]

Done.

-          selector: selectors[i],
-          filter: filters ? filters[i] : null
-        });
-      }
-    }
 
     if (!this.style)
     {
       // Create <style> element lazily, only if we add styles. Add it to
       // the shadow DOM if possible. Otherwise fallback to the <head> or
       // <html> element. If we have injected a style element before that
       // has been removed (the sheet property is null), create a new one.
       this.style = document.createElement("style");
       (this.shadow || document.head
                    || document.documentElement).appendChild(this.style);
 
       // It can happen that the frame already navigated to a different
       // document while we were waiting for the background page to respond.
       // In that case the sheet property will stay null, after addind the
       // <style> element to the shadow DOM.
       if (!this.style.sheet)
         return;
     }
 
     // If using shadow DOM, we have to add the ::content pseudo-element
     // before each selector, in order to match elements within the
     // insertion point.
+    let preparedSelectors = [];
     if (this.shadow)
     {
-      var preparedSelectors = [];
-      for (var i = 0; i < selectors.length; i++)
-      {
-        var subSelectors = splitSelector(selectors[i]);
-        for (var j = 0; j < subSelectors.length; j++)
-          preparedSelectors.push("::content " + subSelectors[j]);
-      }
-      selectors = preparedSelectors;
+      for (let selector of selectors)
+      {
+        let subSelectors = splitSelector(selector);
+        for (let subSelector of subSelectors)
+          preparedSelectors.push("::content " + subSelector);
+      }
+    }
+    else
+    {
+      preparedSelectors = selectors;
     }
 
     // Safari only allows 8192 primitive selectors to be injected at once[1], we
     // therefore chunk the inserted selectors into groups of 200 to be safe.
     // (Chrome also has a limit, larger... but we're not certain exactly what it
     //  is! Edge apparently has no such limit.)
     // [1] - https://github.com/WebKit/webkit/blob/1cb2227f6b2a1035f7bdc46e5ab69debb75fc1de/Source/WebCore/css/RuleSet.h#L68
-    for (var i = 0; i < selectors.length; i += this.selectorGroupSize)
-    {
-      var selector = selectors.slice(i, i + this.selectorGroupSize).join(", ");
+    for (let i = 0; i < preparedSelectors.length; i += this.selectorGroupSize)
+    {
+      let selector = preparedSelectors.slice(i, i + this.selectorGroupSize).join(", ");
       this.style.sheet.insertRule(selector + "{display: none !important;}",
                                   this.style.sheet.cssRules.length);
     }
-  },
-
-  apply: function()
-  {
-    var selectors = null;
-
-    var checkLoaded = function()
+
+    if (this.tracer)
+      this.tracer.addSelectors(selectors, filters || selectors);
+  },
+
+  apply()
+  {
+    ext.backgroundPage.sendMessage({type: "get-selectors"}, response =>
     {
       if (this.tracer)
         this.tracer.disconnect();
       this.tracer = null;
 
-      if (selectors.trace)
-        this.tracer = new ElementHidingTracer();
-
       if (this.style && this.style.parentElement)
         this.style.parentElement.removeChild(this.style);
       this.style = null;
 
-      if (selectors.selectors)
-        this.addSelectors(selectors.selectors);
+      if (response.trace)
+        this.tracer = new ElementHidingTracer();
+
+      this.addSelectors(response.selectors);
       this.elemHideEmulation.apply();
-
-      if (this.tracer && this.tracer.selectors.length)

[Sebastian Noack, 2017/01/11 16:20:40]

Any particular reason (except for a hypothetical micro-optimization) to check
for this.tracer.selectors.length?

[wspee, 2017/01/12 13:39:58]

No longer relevant.

[wspee, 2017/01/19 16:20:15]

Done.

-        this.tracer.checkNodes([document]);
-    }.bind(this);
-
-    ext.backgroundPage.sendMessage({type: "get-selectors"}, (response) =>

[Sebastian Noack, 2017/01/11 16:20:40]

The parentheses around the "response" argument are redundant.

[wspee, 2017/01/12 13:39:58]

No longer relevant.

[wspee, 2017/01/19 16:20:14]

Done.

-    {
-      selectors = response;
-      this.elemHideEmulation.load(() => { checkLoaded(); });

[Sebastian Noack, 2017/01/11 16:20:40]

The arrow function seems redundant. Why not just passing checkLoaded without
wrapping to the load() method?

[wspee, 2017/01/12 13:39:58]

No longer relevant.

[wspee, 2017/01/19 16:20:14]

Done.

     });
   }
 };
 
 if (document instanceof HTMLDocument)
 {
   checkSitekey();
   wrapWebSocket();
 
+  // This variable is also used by our other content scripts, outside of the
+  // current scope.
   var elemhide = new ElemHide();
   elemhide.apply();
 
-  document.addEventListener("error", function(event)
+  document.addEventListener("error", event =>
   {
     checkCollapse(event.target);
   }, true);
 
-  document.addEventListener("load", function(event)
-  {
-    var element = event.target;
+  document.addEventListener("load", event =>
+  {
+    let element = event.target;
     if (/^i?frame$/.test(element.localName))
       checkCollapse(element);
   }, true);
 }