[Infinite] Return current data if failed to resolve pages from cache …

…when setting size (#1379)

infinite/index.ts

@@ -162,7 +162,7 @@ export const infinite = ((<Data, Error>(useSWRNext: SWRHook) => (
 
   const mutate = useCallback(
     (
-      data: Data[] | Promise<Data[]> | MutatorCallback<Data[]>,
+      data: Data[] | undefined | Promise<Data[]> | MutatorCallback<Data[]>,
       shouldRevalidate = true
     ) => {
       // It is possible that the key is still falsy.
@@ -185,23 +185,25 @@ export const infinite = ((<Data, Error>(useSWRNext: SWRHook) => (
   )
 
   // Function to load pages data from the cache based on the page size.
-  const resolvePagesFromCache = (pageSize: number): Data[] => {
+  const resolvePagesFromCache = (pageSize: number): Data[] | undefined => {
     // return an array of page data
     const data: Data[] = []
 
     let previousPageData = null
     for (let i = 0; i < pageSize; ++i) {
       const [pageKey] = serialize(getKey ? getKey(i, previousPageData) : null)
 
-      // Get the cached page data. Skip if we can't get it from the cache.
+      // Get the cached page data.
       const pageData = pageKey ? cache.get(pageKey) : UNDEFINED
-      if (isUndefined(pageData)) break
+
+      // Return the current data if we can't get it from the cache.
+      if (isUndefined(pageData)) return dataRef.current
 
       data.push(pageData)
       previousPageData = pageData
     }
 
-    // return the data
+    // Return the data
     return data
   }
 

test/use-swr-infinite.test.tsx

@@ -864,4 +864,27 @@ describe('useSWRInfinite', () => {
     await screen.findByText('data:response value,response value')
     expect(getData).toHaveBeenCalledTimes(3)
   })
+
+  it('should return fallbackData if cache is empty', async () => {
+    const key = createKey()
+
+    function Page() {
+      const { data, setSize } = useSWRInfinite<string, string>(
+        index => key + '-' + index,
+        () => sleep(30).then(() => 'response value'),
+        { fallbackData: ['fallback-1', 'fallback-2'] }
+      )
+      return (
+        <div onClick={() => setSize(2)}>data:{data ? data.join(',') : ''}</div>
+      )
+    }
+    render(<Page />)
+
+    screen.getByText('data:fallback-1,fallback-2')
+
+    // Update size, it should still render the fallback
+    fireEvent.click(screen.getByText('data:fallback-1,fallback-2'))
+    await nextTick()
+    screen.getByText('data:fallback-1,fallback-2')
+  })
 })