Issue 5735 - Use JS Map instead of Object for property domains of Filter objects

lib/elemHide.js

Index: lib/elemHide.js
diff --git a/lib/elemHide.js b/lib/elemHide.js
index e9243853e3733c83632710b690e69224f1983bea..a6d2c5a7a27c05e91a0186320b560f3db405e721 100644
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -66,8 +66,7 @@ let unconditionalFilterKeys = null;
 /**
  * Object to be used instead when a filter has a blank domains property.
  */
-let defaultDomains = Object.create(null);
-defaultDomains[""] = true;
+let defaultDomains = new Map([["", true]]);
 
 /**
  * Lookup table, keys are known element hiding exceptions
@@ -104,13 +103,13 @@ let ElemHide = exports.ElemHide = {
   _addToFiltersByDomain(key, filter)
   {
     let domains = filter.domains || defaultDomains;
-    for (let domain in domains)
+    for (let [domain, isIncluded] of domains)
     {
       let filters = filtersByDomain[domain];
       if (!filters)
         filters = filtersByDomain[domain] = Object.create(null);
 
-      if (domains[domain])
+      if (isIncluded)
         filters[key] = filter;
       else
         filters[key] = false;
@@ -182,7 +181,7 @@ let ElemHide = exports.ElemHide = {
     // We haven't found this filter in unconditional filters, look in
     // filtersByDomain.
     let domains = filter.domains || defaultDomains;
-    for (let domain in domains)
+    for (let domain of domains.keys())
     {
       let filters = filtersByDomain[domain];
       if (filters)