DefaultConfig of useSWR should not be overwritten with undefined

[kazukinagata]

We have a custom hook called useUsers that wraps useSWR as follows. This hook accepts the revalidateIfStale SWR option, with a default value of undefined.

function useUsers({
  revalidateIfStale
}) {
  const key = `path_to_the_users`;
  return useSWR(key, fetcher, {revalidateIfStale});
}

I expected that defaultConfig.revalidateIfStale /** this is true **/ would be applied when I passed an empty object to useUsers. However, the config.revalidateIfStale was merged with mergeObjects as undefined, which was contrary to my expectations.

You can find the code here:

swr/_internal/utils/helper.ts

Line 22 in 342b82c

export const mergeObjects = (a: any, b?: any) => ({ ...a, ...b })

Is this the intended behavior?

I would like to suggest the following implementation to avoid overwriting defaultConfig when the user passes undefined.

const mergeObjects = (a, b) => {
  const merged = { ...a };
  for (const key in b) {
    if (typeof b[key] !== "undefined") {
      merged[key] = b[key];
    }
  }
  return merged;
}

[devjiwonchoi]

@kazukinagata What is the expected output for revalidateIfStale param passed to useUsers?