Issue 6559 - Use maps and sets where appropriate

lib/downloader.js

Index: lib/downloader.js
===================================================================
--- a/lib/downloader.js
+++ b/lib/downloader.js
@@ -43,28 +43,29 @@
 {
   this.dataSource = dataSource;
   this._timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
   this._timer.initWithCallback(() =>
   {
     this._timer.delay = checkInterval;
     this._doCheck();
   }, initialDelay, Ci.nsITimer.TYPE_REPEATING_SLACK);
-  this._downloading = Object.create(null);
+  this._downloading = new Set();
 };
 Downloader.prototype =
 {
   /**
    * Timer triggering the downloads.
    * @type {nsITimer}
    */
   _timer: null,
 
   /**
-   * Map containing the URLs of objects currently being downloaded as its keys.
+   * Set containing the URLs of objects currently being downloaded.
+   * @type {Set.<string>}
    */
   _downloading: null,
 
   /**
    * Function that will yield downloadable objects on each check.
    * @type {Function}
    */
   dataSource: null,
@@ -177,17 +178,17 @@
 
   /**
    * Checks whether an address is currently being downloaded.
    * @param {string} url
    * @return {boolean}
    */
   isDownloading(url)
   {
-    return url in this._downloading;
+    return this._downloading.has(url);
   },
 
   /**
    * Starts downloading for an object.
    * @param {Downloadable} downloadable
    */
   download(downloadable)
   {
@@ -296,26 +297,26 @@
       Cu.reportError(e);
     }
 
     request.addEventListener("error", event =>
     {
       if (onShutdown.done)
         return;
 
-      delete this._downloading[downloadable.url];
+      this._downloading.delete(downloadable.url);
       errorCallback("synchronize_connection_error");
     }, false);
 
     request.addEventListener("load", event =>
     {
       if (onShutdown.done)
         return;
 
-      delete this._downloading[downloadable.url];
+      this._downloading.delete(downloadable.url);
 
       // Status will be 0 for non-HTTP requests
       if (request.status && request.status != 200)
       {
         errorCallback("synchronize_connection_error");
         return;
       }
 
@@ -333,17 +334,17 @@
             this._download(downloadable, redirects + 1);
           }
         }
       );
     });
 
     request.send(null);
 
-    this._downloading[downloadable.url] = true;
+    this._downloading.add(downloadable.url);
     if (this.onDownloadStarted)
       this.onDownloadStarted(downloadable);
   },
 
   /**
    * Produces a soft and a hard expiration interval for a given supplied
    * expiration interval.
    * @param {number} interval