fix leaking internal state when setSize on null key

Author: shuding

src/use-swr-infinite.ts

@@ -159,6 +159,9 @@ function useSWRInfinite<Data = any, Error = any>(
 
   const mutate = useCallback(
     (data: MutatorCallback, shouldRevalidate = true) => {
+      // It is possible that the key is still falsy.
+      if (!contextCacheKey) return undefined
+
       if (shouldRevalidate && typeof data !== 'undefined') {
         // we only revalidate the pages that are changed
         const originalData = dataRef.current
@@ -178,6 +181,9 @@ function useSWRInfinite<Data = any, Error = any>(
   // extend the SWR API
   const setSize = useCallback(
     (arg: number | ((size: number) => number)) => {
+      // It is possible that the key is still falsy.
+      if (!pageSizeCacheKey) return undefined
+
       let size
       if (typeof arg === 'function') {
         size = arg(resolvePageSize())

test/use-swr-infinite.test.tsx

@@ -544,14 +544,31 @@ describe('useSWRInfinite', () => {
     await screen.findByText('B:page-2-2')
   })
 
-  it.only('should support null as getKey', async () => {
+  it('should support null as getKey', async () => {
     function Page() {
-      const { data } = useSWRInfinite<string, string>(null, () => 'data')
-      return <div>data:{data}</div>
+      const { data, setSize } = useSWRInfinite<string, string>(
+        null,
+        () => 'data'
+      )
+
+      return (
+        <div
+          onClick={() => {
+            // load next page
+            setSize(size => size + 1)
+          }}
+        >
+          data:{data || ''}
+        </div>
+      )
     }
 
     render(<Page />)
     screen.getByText('data:')
     await screen.findByText('data:')
+
+    // load next page
+    fireEvent.click(screen.getByText('data:'))
+    await screen.findByText('data:')
   })
 })