fix: prevent infinite loop when onSuccess is explicitly undefined

src/_internal/utils/merge-config.ts

@@ -1,12 +1,29 @@
 import { mergeObjects } from './shared'
 import type { FullConfiguration } from '../types'
 
+// Strip keys with `undefined` values from an object so they don't override
+// defaults when spread-merged. This prevents issues like passing
+// `{ onSuccess: undefined }` from overriding the default `noop` callback.
+const stripUndefinedValues = (obj: Partial<FullConfiguration>) => {
+  const result: Partial<FullConfiguration> = {}
+  for (const key in obj) {
+    if (obj[key as keyof FullConfiguration] !== undefined) {
+      ;(result as any)[key] = obj[key as keyof FullConfiguration]
+    }
+  }
+  return result
+}
+
 export const mergeConfigs = (
   a: Partial<FullConfiguration>,
   b?: Partial<FullConfiguration>
 ) => {
   // Need to create a new object to avoid mutating the original here.
-  const v: Partial<FullConfiguration> = mergeObjects(a, b)
+  // Strip undefined values from `b` so they don't override defaults from `a`.
+  const v: Partial<FullConfiguration> = mergeObjects(
+    a,
+    b ? stripUndefinedValues(b) : b
+  )
 
   // If two configs are provided, merge their `use` and `fallback` options.
   if (b) {

test/use-swr-config-callbacks.test.tsx

@@ -200,6 +200,67 @@ describe('useSWR - config callbacks', () => {
     expect(discardedEvents).toEqual([key])
   })
 
+  it('should not cause infinite requests when onSuccess is explicitly undefined', async () => {
+    let count = 0
+    const key = createKey()
+    function Page() {
+      const { data } = useSWR(key, () => createResponse(count++), {
+        onSuccess: undefined,
+        errorRetryInterval: 50,
+        dedupingInterval: 0
+      })
+      return <div>data: {data}</div>
+    }
+    renderWithConfig(<Page />)
+    await screen.findByText('data: 0')
+
+    // Wait longer than the retry interval. If onSuccess: undefined caused a
+    // TypeError that triggered error-retry cycles, count would exceed 1.
+    await act(() => sleep(200))
+    expect(count).toBe(1)
+  })
+
+  it('should not cause infinite requests when onError is explicitly undefined', async () => {
+    let count = 0
+    const key = createKey()
+    function Page() {
+      const { data } = useSWR(key, () => createResponse(count++), {
+        onError: undefined,
+        errorRetryInterval: 50,
+        dedupingInterval: 0
+      })
+      return <div>data: {data}</div>
+    }
+    renderWithConfig(<Page />)
+    await screen.findByText('data: 0')
+
+    await act(() => sleep(200))
+    expect(count).toBe(1)
+  })
+
+  it('should handle a custom hook passing optional onSuccess that is undefined', async () => {
+    let count = 0
+    const key = createKey()
+
+    function useMyFetch({ onSuccess }: { onSuccess?: (data: any) => void }) {
+      return useSWR(key, () => createResponse(count++), {
+        onSuccess,
+        errorRetryInterval: 50,
+        dedupingInterval: 0
+      })
+    }
+
+    function Page() {
+      const { data } = useMyFetch({})
+      return <div>data: {data}</div>
+    }
+    renderWithConfig(<Page />)
+    await screen.findByText('data: 0')
+
+    await act(() => sleep(200))
+    expect(count).toBe(1)
+  })
+
   it('should not trigger the onSuccess callback when discarded', async () => {
     const key = createKey()
     const discardedEvents = []