Add invariant to let Flow understand BatchItemNotFound is a subclass of Error

__tests__/runtimeHelpers.test.js

@@ -0,0 +1,29 @@
+import { unPartitionResults } from '../src/runtimeHelpers';
+
+describe('unPartitionResults', () => {
+    it('should perform inverse mapping for result without Error', () => {
+        expect(unPartitionResults([[0, 2], [1]], [[{ foo: 'foo' }, { bar: 'bar' }], [{ baz: 'baz' }]])).toEqual([
+            { foo: 'foo' },
+            { baz: 'baz' },
+            { bar: 'bar' },
+        ]);
+    });
+
+    it('should perform inverse mapping for result with some Error in resultGroups', () => {
+        const customError = new Error('bar error');
+        expect(unPartitionResults([[0, 2], [1]], [[{ foo: 'foo' }, customError], [{ baz: 'baz' }]])).toEqual([
+            { foo: 'foo' },
+            { baz: 'baz' },
+            customError,
+        ]);
+    });
+
+    it('should perform inverse mapping for result with all Error in one resultGroup', () => {
+        const customError = new Error('foo error');
+        expect(unPartitionResults([[0, 2], [1]], [[customError, customError], [{ baz: 'baz' }]])).toEqual([
+            customError,
+            { baz: 'baz' },
+            customError,
+        ]);
+    });
+});

src/implementation.ts

@@ -249,6 +249,11 @@ function getBatchLoader(resourceConfig: BatchResourceConfig, resourcePath: Reado
                                         )} Resource returned \${response.length} items, but we requested \${requests.length} items.\`,
                                         'Add reorderResultsByKey to the config for this resource to be able to handle a partial response.',
                                     ].join(' '));
+
+                                    // Tell flow that BatchItemNotFoundError extends Error.
+                                    // It's an issue with flowgen package, but not an issue with Flow.
+                                    // @see https://github.com/Yelp/dataloader-codegen/pull/35#discussion_r394777533
+                                    invariant(response instanceof Error, 'expected BatchItemNotFoundError to be an Error');
                                 }
                             }
                         `;