Infinite loop when passing `onSuccess: undefined` explicitly

[mori-hisayuki]

Bug report

Description / Observed Behavior

When onSuccess: undefined is explicitly passed as a property in the useSWR options, it causes infinite requests. This issue does not occur when:

• The onSuccess property is omitted entirely
• onError: undefined is passed instead
• An empty function () => {} is passed as onSuccess

Expected Behavior

onSuccess: undefined should behave the same as omitting the onSuccess property entirely.

Repro Steps / Code Example

// ❌ Causes infinite loop
const { data } = useSWR(key, fetcher, {
  onSuccess: undefined,
});

// ✅ Works fine when onSuccess is omitted
const { data } = useSWR(key, fetcher);

// ✅ onError: undefined does not cause the issue
const { data } = useSWR(key, fetcher, {
  onError: undefined,
});

// ✅ Works fine with an empty function
const { data } = useSWR(key, fetcher, {
  onSuccess: () => {},
});

A common scenario where this occurs is when a custom hook accepts an optional onSuccess callback and passes it directly to useSWR:

const useMyFetch = ({ onSuccess }: { onSuccess?: (data: Data) => void }) => {
  return useSWR(key, fetcher, {
    onSuccess, // becomes undefined when the caller doesn't provide it
  });
};

useMyFetch({});                                          // → infinite loop
useMyFetch({ onSuccess: (data) => console.log(data) });  // → works fine

Workaround: Strip undefined values from the options object before passing it to useSWR.

const sanitizedConfig = Object.fromEntries(
  Object.entries(config).filter(([, v]) => v !== undefined)
);

Additional Context

SWR version: ^2.3.8