diff --git a/src/typings/lib.array-ext.d.ts b/src/typings/lib.array-ext.d.ts
new file mode 100644
index 00000000000..59e1c0fc956
--- /dev/null
+++ b/src/typings/lib.array-ext.d.ts
@@ -0,0 +1,8 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+interface ArrayConstructor {
+    isArray<T>(arg: any): arg is Array<T>;
+}
\ No newline at end of file