🐛 Prevent closure over fetch

packages/toolkit/src/query/fetchBaseQuery.ts

@@ -27,6 +27,13 @@ export interface FetchArgs extends CustomRequestInit {
   validateStatus?: (response: Response, body: any) => boolean
 }
 
+/**
+ * A mini-wrapper that passes arguments straight through to
+ * {@link [fetch](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API)}.
+ * Avoids storing `fetch` in a closure, in order to permit mocking/monkey-patching.
+ */
+const defaultFetchFn: typeof fetch = (...args) => fetch(...args)
+
 const defaultValidateStatus = (response: Response) =>
   response.status >= 200 && response.status <= 299
 
@@ -118,7 +125,7 @@ export type FetchBaseQueryMeta = { request: Request; response: Response }
 export function fetchBaseQuery({
   baseUrl,
   prepareHeaders = (x) => x,
-  fetchFn = fetch,
+  fetchFn = defaultFetchFn,
   ...baseFetchOptions
 }: FetchBaseQueryArgs = {}): BaseQueryFn<
   string | FetchArgs,
@@ -127,6 +134,11 @@ export function fetchBaseQuery({
   {},
   FetchBaseQueryMeta
 > {
+  if (typeof fetch === 'undefined' && fetchFn === defaultFetchFn) {
+    console.warn(
+      'Warning: `fetch` is not available. Please supply a custom `fetchFn` property to use `fetchBaseQuery` on SSR environments.'
+    )
+  }
   return async (arg, { signal, getState }) => {
     let {
       url,

packages/toolkit/src/query/tests/fetchBaseQuery.test.tsx

@@ -531,6 +531,34 @@ describe('fetchFn', () => {
 
     expect(request.url).toEqual(`${baseUrl}/echo?apple=fruit`)
   })
+
+  test('respects mocking window.fetch after a fetch base query is created', async () => {
+    const baseUrl = 'http://example.com'
+    const baseQuery = fetchBaseQuery({ baseUrl })
+
+    const fakeResponse = {
+      ok: true,
+      status: 200,
+      text: async () => `{ "url": "mock-return-url" }`,
+      clone: () => fakeResponse
+    }
+
+    const spiedFetch = jest.spyOn(window, 'fetch');
+    spiedFetch.mockResolvedValueOnce(fakeResponse as any);
+
+    const { data } = await baseQuery(
+      { url: '/echo' },
+      {
+        signal: new AbortController().signal,
+        dispatch: storeRef.store.dispatch,
+        getState: storeRef.store.getState,
+      },
+      {}
+    )
+    expect(data).toEqual({url: 'mock-return-url'})
+
+    spiedFetch.mockClear();
+  })
 })
 
 describe('FormData', () => {