Fix finish method

Author: timursevimli

Diff for: JavaScript/6-promise.js

@@ -42,8 +42,9 @@ class Queue {
 
   finish(err, res) {
     const { onFailure, onSuccess, onDone, onDrain } = this;
-    if (err && onFailure) onFailure(err, res);
-    else if (onSuccess) onSuccess(res);
+    if (err) {
+      if (onFailure) onFailure(err, res);
+    } else if (onSuccess) onSuccess(res);
     if (onDone) onDone(err, res);
     if (this.count === 0 && this.waiting.length === 0 && onDrain) onDrain();
   }