isLoading ignoring fallback

[Dragate]

Bug report

Description / Observed Behavior

isLoading becomes true on the first request even though data is already available for the given key through SWRConfig's fallback, which makes the UI show a loading indicator for the first revalidation (as if isValidating was used).

Expected Behavior

Expected isLoading to remain false when initial data for a given url/key is provided from server (getServerSideProps/getStaticProps).

Repro Steps / Code Example

CodeSandbox
Trigger page reload in CSB to see the loading indicator.

Additional Context

[email protected]

Documentation says

Fallback data and previous data are not considered "loaded data"

although I'm not sure if this includes fallback or only fallbackData.

Previously was using https://swr.vercel.app/docs/middleware#keep-previous-result but wanted to reduce code with the new isLoading and keepPreviousData. Should I keep using the middleware?

[koba04]

As the documentation says, this is an expected behavior.
https://swr.vercel.app/docs/advanced/understanding#fallback

I'd say say you can check data === undefined for that case.

[Dragate]

data === undefined wouldn't work, as in my case data is always present. It's pre-populated from getServerSideProps (using fallback) and keepPreviousData makes sure to persist data until the next key provides another set of data.

In any case, thanks for confirming the intended behaviour. I'll stick to the middleware then.