Add notNull filter with type guard

Summary: You need to use a type guard when narrowing types in a filter.

Reviewed By: danielbuechele

Differential Revision: D17163782

fbshipit-source-id: aa78bdd392653ebf1080a04e62e131b607e5181b

src/utils/typeUtils.tsx

@@ -0,0 +1,12 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+
+// Typescript doesn't know Array.filter(Boolean) won't contain nulls.
+// So use Array.filter(notNull) instead.
+export function notNull<T>(x: T | null | undefined): x is T {
+  return x !== null && x !== undefined;
+}