From 16457e2adbfddaa1bc445df331a77f75b419b7a6 Mon Sep 17 00:00:00 2001
From: John Knox <jknox@fb.com>
Date: Wed, 4 Sep 2019 03:30:16 -0700
Subject: [PATCH] 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 | 12 ++++++++++++
 1 file changed, 12 insertions(+)
 create mode 100644 src/utils/typeUtils.tsx

diff --git a/src/utils/typeUtils.tsx b/src/utils/typeUtils.tsx
new file mode 100644
index 000000000..5f37ac42b
--- /dev/null
+++ b/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;
+}