diff --git a/src/renderers/shared/stack/event/ReactSyntheticEvent.js b/src/renderers/shared/stack/event/ReactSyntheticEvent.js
index 830344d98f..672e137da1 100644
--- a/src/renderers/shared/stack/event/ReactSyntheticEvent.js
+++ b/src/renderers/shared/stack/event/ReactSyntheticEvent.js
@@ -18,4 +18,4 @@ export type DispatchConfig = any;
 
 export class ReactSyntheticEvent extends SyntheticEvent {
   dispatchConfig: DispatchConfig;
-};
+}
diff --git a/src/renderers/shared/utils/CallbackQueue.js b/src/renderers/shared/utils/CallbackQueue.js
index 167403dc80..8f8e53a8c0 100644
--- a/src/renderers/shared/utils/CallbackQueue.js
+++ b/src/renderers/shared/utils/CallbackQueue.js
@@ -27,12 +27,14 @@ var invariant = require('invariant');
  * @implements PooledClass
  * @internal
  */
-function CallbackQueue() {
-  this._callbacks = null;
-  this._contexts = null;
-}
+class CallbackQueue<T> {
+  _callbacks: ?Array<() => void>;
+  _contexts: ?Array<T>;
 
-Object.assign(CallbackQueue.prototype, {
+  constructor() {
+    this._callbacks = null;
+    this._contexts = null;
+  }
 
   /**
    * Enqueues a callback to be invoked when `notifyAll` is invoked.
@@ -41,12 +43,12 @@ Object.assign(CallbackQueue.prototype, {
    * @param {?object} context Context to call `callback` with.
    * @internal
    */
-  enqueue: function(callback, context) {
+  enqueue(callback: () => void, context: T) {
     this._callbacks = this._callbacks || [];
-    this._contexts = this._contexts || [];
     this._callbacks.push(callback);
+    this._contexts = this._contexts || [];
     this._contexts.push(context);
-  },
+  }
 
   /**
    * Invokes all enqueued callbacks and clears the queue. This is invoked after
@@ -54,10 +56,10 @@ Object.assign(CallbackQueue.prototype, {
    *
    * @internal
    */
-  notifyAll: function() {
+  notifyAll() {
     var callbacks = this._callbacks;
     var contexts = this._contexts;
-    if (callbacks) {
+    if (callbacks && contexts) {
       invariant(
         callbacks.length === contexts.length,
         'Mismatched list of contexts in callback queue'
@@ -70,36 +72,35 @@ Object.assign(CallbackQueue.prototype, {
       callbacks.length = 0;
       contexts.length = 0;
     }
-  },
+  }
 
-  checkpoint: function() {
+  checkpoint() {
     return this._callbacks ? this._callbacks.length : 0;
-  },
+  }
 
-  rollback: function(len) {
-    if (this._callbacks) {
+  rollback(len: number) {
+    if (this._callbacks && this._contexts) {
       this._callbacks.length = len;
       this._contexts.length = len;
     }
-  },
+  }
 
   /**
    * Resets the internal queue.
    *
    * @internal
    */
-  reset: function() {
+  reset() {
     this._callbacks = null;
     this._contexts = null;
-  },
+  }
 
   /**
    * `PooledClass` looks for this.
    */
-  destructor: function() {
+  destructor() {
     this.reset();
-  },
-
-});
+  }
+}
 
 module.exports = PooledClass.addPoolingTo(CallbackQueue);