revalidateIfStale false and keepPreviousData true will break pages with fallback data

[ddhp]

Bug report

Description / Observed Behavior

If setting revalidateIfStale false and keepPreviousData true, changing between pages with fallback data wouldn't work.

Expected Behavior

returning fallback data from SSR

Repro Steps / Code Example

Reproduce codesandbox: navigating between /a and /b and swr hook always return old data.

I think it's from here

swr/core/use-swr.ts

Lines 206 to 216 in f7004ac

	const data = isUndefined(cachedData) ? fallback : cachedData
	const error = cached.error
	// Use a ref to store previously returned data. Use the initial data as its initial value.
	const laggyDataRef = useRef(data)
	const returnedData = keepPreviousData
	? isUndefined(cachedData)
	? laggyDataRef.current
	: cachedData
	: data

when revalidateIfStale is false, cachedData wouldn't be updated from fallback and when keepPreviousData is true, laggyRef data will be returned.

if modifies returnedData to

const returnedData = data ? data : keepPreviousData
    ? isUndefined(cachedData)
      ? laggyDataRef.current
      : cachedData

will solve the problem but I don't know if this is the expect behavior.

Please help to clarify, thanks!

Additional Context

tested on
swr v2.0.3
next v13.1.6

[koba04]

@ddhp
Thank you! I've confirmed the behavior. This is because we prioritize keepPreviousData over fallback. Why do you enable keepPreviousData even if you have fallback?

[ddhp]

Hi @koba04, thanks for your reply!

I think we want to keep the original data when the key changes and meanwhile the page/component supports SSR.
Is it like what this diagram explains?
https://swr.vercel.app/docs/advanced/understanding#key-change--previous-data--fallback

[koba04]

The dialog is a case that has the fallback data only for the initial fetch, so it's different from this.

[ddhp]

Hmm.. I can't get it here.

For our scenario, we have a fallback data for the initial fetch, then there is a key change caused by changing page but we can't see the hook getting revalidated.

From my understanding, keepPreviousData will keep the previous data of the previous key until the data for the new key is revalidated; And it should revalidate even when we set revalidateIfStale to false since the key has changed.