Try with including

app/scripts/main.tsx

@@ -19,8 +19,8 @@ import { PageLoading } from '$components/common/loading-skeleton';
 // Views
 import UhOh from '$components/uhoh';
 import ErrorBoundary from '$components/uhoh/fatal-error';
-// eslint-disable-next-line
-import { Array } from '$types/array';
+
+import '$types/array';
 
 const Home = lazy(() => import('$components/home'));
 const About = lazy(() => import('$components/about'));

app/scripts/types/array.d.ts

@@ -1,5 +1,5 @@
 
 // Extend Array interface to include the last property
-export interface Array<T> {
-  readonly last: T | undefined;
+interface Array<T> {
+  last: T | undefined;
 }