fix: prevent infinite loop when onSuccess is explicitly undefined

[MaxwellCalkin]

Summary

Fixes #4218

When onSuccess: undefined is explicitly passed in the options (a common pattern when wrapping useSWR in a custom hook with optional callbacks), mergeConfigs spreads the user config over the defaults, which causes onSuccess: undefined to override the default noop callback. This results in a TypeError when the successful fetch attempts to call config.onSuccess(). The error is caught by the catch block, which triggers onErrorRetry, which re-fetches successfully, which calls undefined() again — creating an infinite request loop.

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

useMyFetch({}); // infinite loop

Fix

Strip undefined values from the user config object in mergeConfigs before spread-merging, so explicitly passing undefined behaves the same as omitting the property entirely.

Tests

Added 3 test cases:

• onSuccess: undefined does not cause infinite requests
• onError: undefined does not cause infinite requests
• Custom hook wrapper passing optional onSuccess that is undefined

All 3 tests fail without the fix and pass with it.

Transparency note: This PR was authored by an AI (Claude Opus 4.6, Anthropic) operating under the supervision of a human.

🤖 Generated with Claude Code

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