fix(streamedQuery): fall back to initialValue when stream yields no values (#9876)

* fix(streamedQuery): fall back to initialValue when stream yields no values

Add failing test for empty streams and update implementation to avoid returning undefined.
Includes changeset for patch release.

* chore: address review nitpicks (JSDoc + test cleanup)

* docs: note that initialValue is returned when the stream yields no values

---------

Co-authored-by: Dominik Dorfmeister <office@dorfmeister.cc>

Author: ahmethos

.changeset/puny-walls-eat.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Fix streamedQuery to avoid returning undefined when the stream yields no values

docs/reference/streamedQuery.md

@@ -40,6 +40,6 @@ const query = queryOptions({
   - If `TData` is not an array, you must provide a custom `reducer`.
 - `initialValue?: TData = TQueryFnData`
   - Optional
-  - Defines the initial data to be used while the first chunk is being fetched.
+  - Defines the initial data to be used while the first chunk is being fetched, and it is also returned when the stream yields no values.
   - It is mandatory when custom `reducer` is provided.
   - Defaults to an empty array.

packages/query-core/src/__tests__/streamedQuery.test.tsx

@@ -128,6 +128,35 @@ describe('streamedQuery', () => {
     unsubscribe()
   })
 
+  test('should handle empty streams', async () => {
+    const key = queryKey()
+
+    const observer = new QueryObserver(queryClient, {
+      queryKey: key,
+      queryFn: streamedQuery({
+        streamFn: async function* () {},
+      }),
+    })
+
+    const unsubscribe = observer.subscribe(vi.fn())
+
+    expect(observer.getCurrentResult()).toMatchObject({
+      status: 'pending',
+      fetchStatus: 'fetching',
+      data: undefined,
+    })
+
+    await vi.advanceTimersByTimeAsync(50)
+
+    expect(observer.getCurrentResult()).toMatchObject({
+      status: 'success',
+      fetchStatus: 'idle',
+      data: [],
+    })
+
+    unsubscribe()
+  })
+
   test('should replace on refetch', async () => {
     const key = queryKey()
     const observer = new QueryObserver(queryClient, {

packages/query-core/src/streamedQuery.ts

@@ -41,7 +41,7 @@ type StreamedQueryParams<TQueryFnData, TData, TQueryKey extends QueryKey> =
  * Set to `'replace'` to write all data to the cache once the stream ends.
  * @param reducer - A function to reduce the streamed chunks into the final data.
  * Defaults to a function that appends chunks to the end of the array.
- * @param initialValue - Initial value to be used while the first chunk is being fetched.
+ * @param initialValue - Initial value to be used while the first chunk is being fetched, and returned if the stream yields no values.
  */
 export function streamedQuery<
   TQueryFnData = unknown,
@@ -94,6 +94,6 @@ export function streamedQuery<
       context.client.setQueryData<TData>(context.queryKey, result)
     }
 
-    return context.client.getQueryData(context.queryKey)!
+    return context.client.getQueryData(context.queryKey) ?? initialValue
   }
 }