Issue 5735 - Use JS Map instead of Object for global keyByFilter in elemHide.js

lib/elemHide.js

Index: lib/elemHide.js
diff --git a/lib/elemHide.js b/lib/elemHide.js
index e99bb8a7f269e4df2370fb95bd950031503540b4..81d584767bb94bb3423e43f4a02e85a159d9f488 100644
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -32,9 +32,9 @@ let filterByKey = [];
 
 /**
  * Lookup table, keys of the filters by filter text
- * @type {Object}
+ * @type {Map.<string,string>}

[sergei, 2017/09/21 12:02:10]

Pay attention that after switching from Object to Map one can use Objects as
keys, so here it could be Map.<Filter,string>, I will measure it and if it's
better then either update this codereview or create a  new one.

[kzar, 2017/09/22 10:50:15]

Yea, it looks like we only used the Filter's text as the key since we had to. I
agree using the Filter itself as the key would be nicer, please could you change
that in this review?

[Wladimir Palant, 2017/09/25 10:59:31]

Agreed, we should use Filter objects as keys here.

[sergei, 2017/09/25 13:41:23]

Done.

I have checked when the type of key here is string and object and there is no
visible change in the memory usage (RSS).

[Wladimir Palant, 2017/09/25 13:57:16]

A change in memory usage would be unexpected, both approaches reuse existing
objects as keys instead of copying data around. Performance might be better with
objects however because comparing them is simpler than comparing strings.

  */
-let keyByFilter = Object.create(null);
+let keyByFilter = new Map();
 
 /**
  * Nested lookup table, filter (or false if inactive) by filter key by domain.
@@ -91,7 +91,7 @@ let ElemHide = exports.ElemHide = {
   clear()
   {
     filterByKey = [];
-    keyByFilter = Object.create(null);
+    keyByFilter = new Map();
     filtersByDomain = new Map();
     filterKeyBySelector = Object.create(null);
     unconditionalSelectors = unconditionalFilterKeys = null;
@@ -143,11 +143,11 @@ let ElemHide = exports.ElemHide = {
     }
     else
     {
-      if (filter.text in keyByFilter)
+      if (keyByFilter.has(filter.text))
         return;
 
       let key = filterByKey.push(filter) - 1;
-      keyByFilter[filter.text] = key;
+      keyByFilter.set(filter.text, key);
 
       if (!(filter.domains || filter.selector in exceptions))
       {
@@ -204,12 +204,12 @@ let ElemHide = exports.ElemHide = {
     }
     else
     {
-      if (!(filter.text in keyByFilter))
+      let key = keyByFilter.get(filter.text);
+      if (typeof key == "undefined")
         return;
 
-      let key = keyByFilter[filter.text];
       delete filterByKey[key];
-      delete keyByFilter[filter.text];
+      keyByFilter.delete(filter.text);
       this._removeFilterKey(key, filter);
     }