Issue 6559 - Use Map object for known subscriptions

lib/subscriptionClasses.js

Index: lib/subscriptionClasses.js
===================================================================
--- a/lib/subscriptionClasses.js
+++ b/lib/subscriptionClasses.js
@@ -34,17 +34,17 @@
  * @constructor
  */
 function Subscription(url, title)
 {
   this.url = url;
   this.filters = [];
   if (title)
     this._title = title;
-  Subscription.knownSubscriptions[url] = this;
+  Subscription.knownSubscriptions.set(url, this);
 }
 exports.Subscription = Subscription;
 
 Subscription.prototype =
 {
   /**
    * Download location of the subscription
    * @type {string}
@@ -149,31 +149,32 @@
     let buffer = [];
     this.serialize(buffer);
     return buffer.join("\n");
   }
 };
 
 /**
  * Cache for known filter subscriptions, maps URL to subscription objects.
- * @type {Object}
+ * @type {Map.<string,Subscription>}
  */
-Subscription.knownSubscriptions = Object.create(null);
+Subscription.knownSubscriptions = new Map();
 
 /**
  * Returns a subscription from its URL, creates a new one if necessary.
  * @param {string} url
  *   URL of the subscription
  * @return {Subscription}
  *   subscription or null if the subscription couldn't be created
  */
 Subscription.fromURL = function(url)
 {
-  if (url in Subscription.knownSubscriptions)
-    return Subscription.knownSubscriptions[url];
+  let subscription = Subscription.knownSubscriptions.get(url);
+  if (subscription)
+    return subscription;
 
   if (url[0] != "~")
     return new DownloadableSubscription(url, null);
   return new SpecialSubscription(url);
 };
 
 /**
  * Deserializes a subscription
@@ -301,17 +302,17 @@
  * @return {SpecialSubscription}
  */
 SpecialSubscription.create = function(title)
 {
   let url;
   do
   {
     url = "~user~" + Math.round(Math.random() * 1000000);
-  } while (url in Subscription.knownSubscriptions);
+  } while (Subscription.knownSubscriptions.has(url));
   return new SpecialSubscription(url, title);
 };
 
 /**
  * Creates a new user-defined filter group and adds the given filter to it.
  * This group will act as the default group for this filter type.
  * @param {Filter} filter
  * @return {SpecialSubscription}