fix: Ensure that using preload with useSWRInfinite returns back an array of data

_internal/src/utils/preload.ts

@@ -48,14 +48,15 @@ export const middleware: Middleware =
         const [key] = serialize(key_)
         const [, , , PRELOAD] = SWRGlobalState.get(cache) as GlobalState
 
-        let normalizedKey = key
         if (key.startsWith(INFINITE_PREFIX)) {
-          normalizedKey = key.slice(INFINITE_PREFIX.length)
+          // we want the infinite fetcher to be called.
+          // handling of the PRELOAD cache happens there.
+          return fetcher_(...args)
         }
 
-        const req = PRELOAD[normalizedKey]
+        const req = PRELOAD[key]
         if (isUndefined(req)) return fetcher_(...args)
-        delete PRELOAD[normalizedKey]
+        delete PRELOAD[key]
         return req
       })
     return useSWRNext(key_, fetcher, config)

infinite/src/index.ts

@@ -12,14 +12,17 @@ import {
   useIsomorphicLayoutEffect,
   serialize,
   withMiddleware,
-  INFINITE_PREFIX
+  INFINITE_PREFIX,
+  SWRGlobalState,
+  cache as defaultCache
 } from 'swr/_internal'
 import type {
   BareFetcher,
   SWRHook,
   MutatorCallback,
   Middleware,
-  MutatorOptions
+  MutatorOptions,
+  GlobalState
 } from 'swr/_internal'
 import type {
   SWRInfiniteConfiguration,
@@ -63,6 +66,7 @@ export const infinite = (<Data, Error>(useSWRNext: SWRHook) =>
       revalidateOnMount = false,
       parallel = false
     } = config
+    const [, , , PRELOAD] = SWRGlobalState.get(defaultCache) as GlobalState
 
     // The serialized key of the first page. This key will be used to store
     // metadata of this SWR infinite hook.
@@ -73,6 +77,7 @@ export const infinite = (<Data, Error>(useSWRNext: SWRHook) =>
     } catch (err) {
       // Not ready yet.
     }
+
     const [get, set, subscribeCache] = createCacheHelper<
       Data,
       SWRInfiniteCacheValue<Data, any>
@@ -164,9 +169,22 @@ export const infinite = (<Data, Error>(useSWRNext: SWRHook) =>
             SWRInfiniteCacheValue<Data, any>
           >(cache, pageKey)
 
+          const hasPreloadedRequest = pageKey in PRELOAD
           // Get the cached page data.
           let pageData = getSWRCache().data as Data
 
+          if (hasPreloadedRequest) {
+            const req = PRELOAD[pageKey]
+            // delete the preload cache key before resolving it
+            // in case there's an error
+            delete PRELOAD[pageKey]
+            // get the page data from the preload cache
+            pageData = await req
+            // set the SWR cache with the preloaded data
+            setSWRCache({ data: pageData, _k: pageArg })
+            // remove the preload cache key to prevent memory leak
+          }
+
           // should fetch (or revalidate) if:
           // - `revalidateAll` is enabled
           // - `mutate()` called
@@ -183,7 +201,6 @@ export const infinite = (<Data, Error>(useSWRNext: SWRHook) =>
             (cacheData &&
               !isUndefined(cacheData[i]) &&
               !config.compare(cacheData[i], pageData))
-
           if (fn && shouldFetchPage) {
             const revalidate = async () => {
               pageData = await fn(pageArg)

test/use-swr-infinite-preload.test.tsx

@@ -8,6 +8,21 @@ describe('useSWRInfinite - preload', () => {
   const getKeyFunction = (key: string) => (index: number) =>
     `page-${index}-${key}`
 
+  it('preloading useSWRInfinite should produce the same result', async () => {
+    const key = createKey()
+    const getKey = getKeyFunction(key)
+
+    const fetcher = jest.fn(() => createResponse('foo'))
+    function Page() {
+      const { data } = useSWRInfinite(getKey, fetcher)
+      return <div>data:{Array.isArray(data) ? 'true' : 'false'}</div>
+    }
+
+    preload(getKey(0), fetcher)
+    renderWithConfig(<Page />)
+    await screen.findByText('data:true')
+  })
+
   it('preload the fetcher function', async () => {
     const key = createKey()
     const getKey = getKeyFunction(key)

test/use-swr-infinite.test.tsx

@@ -24,6 +24,7 @@ describe('useSWRInfinite', () => {
       return (
         <div>
           <div>data:{data}</div>
+          <div>isArray:{Array.isArray(data) ? 'true' : 'false'}</div>
           <div>error:{error}</div>
           <div>isValidating:{isValidating.toString()}</div>
         </div>
@@ -34,6 +35,7 @@ describe('useSWRInfinite', () => {
     screen.getByText('data:')
 
     await screen.findByText(`data:page-0-${key}`)
+    await screen.findByText(`isArray:true`)
     await screen.findByText(`error:`)
     await screen.findByText(`isValidating:false`)
   })