Issue 6559 - Use maps and sets where appropriate

lib/elemHideEmulation.js

Index: lib/elemHideEmulation.js
===================================================================
--- a/lib/elemHideEmulation.js
+++ b/lib/elemHideEmulation.js
@@ -19,61 +19,60 @@
 
 /**
  * @fileOverview Element hiding emulation implementation.
  */
 
 const {ElemHide} = require("./elemHide");
 const {Filter} = require("./filterClasses");
 
-let filters = Object.create(null);
+let filters = new Set();
 
 /**
  * Container for element hiding emulation filters
  * @class
  */
 let ElemHideEmulation = {
   /**
    * Removes all known filters
    */
   clear()
   {
-    filters = Object.create(null);
+    filters.clear();
   },
 
   /**
    * Add a new element hiding emulation filter
    * @param {ElemHideEmulationFilter} filter
    */
   add(filter)
   {
-    filters[filter.text] = true;
+    filters.add(filter.text);
   },
 
   /**
    * Removes an element hiding emulation filter
    * @param {ElemHideEmulationFilter} filter
    */
   remove(filter)
   {
-    delete filters[filter.text];
+    filters.delete(filter.text);
   },
 
   /**
    * Returns a list of all rules active on a particular domain
    * @param {string} domain
    * @return {ElemHideEmulationFilter[]}
    */
   getRulesForDomain(domain)
   {
     let result = [];
-    let keys = Object.getOwnPropertyNames(filters);
-    for (let key of keys)
+    for (let text of filters.values())
     {
-      let filter = Filter.fromText(key);
+      let filter = Filter.fromText(text);
       if (filter.isActiveOnDomain(domain) &&
           !ElemHide.getException(filter, domain))
       {
         result.push(filter);
       }
     }
     return result;
   }