refactor: use delete to remove the member

src/internal/Subscription.ts

@@ -40,16 +40,7 @@ export class Subscription implements SubscriptionLike {
    */
   constructor(unsubscribe?: () => void) {
     if (unsubscribe) {
-      // Wrap the unsubscribe teardown in a function so that the argument can
-      // be nulled. It's not possible to null the _unsubscribe member as there
-      // are many classes that are derived from Subscriber (which derives from
-      // Subscription) that implement an _unsubscribe method as a mechanism for
-      // obtaining unsubscription notifications and some of those subscribers
-      // are recycled.
-      (<any> this)._unsubscribe = () => {
-        unsubscribe!();
-        unsubscribe = undefined;
-      };
+      (this as any)._unsubscribe = unsubscribe;
     }
   }
 
@@ -66,13 +57,20 @@ export class Subscription implements SubscriptionLike {
       return;
     }
 
-    let { _parentOrParents, _unsubscribe, _subscriptions } = (<any> this);
+    let { _parentOrParents, _unsubscribe, _subscriptions } = (this as any);
 
     this.closed = true;
     this._parentOrParents = null;
     // null out _subscriptions first so any child subscriptions that attempt
     // to remove themselves from this subscription will noop
     this._subscriptions = null;
+    // It's not possible to null the _unsubscribe member as there are many
+    // classes that are derived from Subscriber (which derives from
+    // Subscription) that implement an _unsubscribe method as a mechanism for
+    // obtaining unsubscription notifications and some of those subscribers are
+    // recycled. Deleting the member will release the reference to any teardown
+    // functions passed in the constructor and will leave any methods intact.
+    delete (this as any)._unsubscribe;
 
     if (_parentOrParents instanceof Subscription) {
       _parentOrParents.remove(this);
@@ -140,7 +138,7 @@ export class Subscription implements SubscriptionLike {
   add(teardown: TeardownLogic): Subscription {
     let subscription = (<Subscription>teardown);
 
-    if (!(<any>teardown)) {
+    if (!teardown) {
       return Subscription.EMPTY;
     }