Option to treat fallback data as initial data for comparison

[ADTC]

Currently even if we give fallback data using fallbackData option, the initial data for comparison (as proven by logging the a, b inside compare(a, b)) would still be undefined. Because of this, despite providing some sort of initial data, the first fetch would still set a new data object replacing it. Even if the new data and the initial data are exactly the same.

It would be great to have an option that treats fallbackData as the initial data (like the now-defunct initialData option). This way, even in the first fetch, the fetched data is deep-compared to the fallback data, not to undefined, and if they are exactly the same, a replacement of the data object should not happen. (This would be useful for preventing re-renders based on object pointer/address change without data change, which I believe is the point of using SWR with deep comparison in the first place.)

There are so many issues and discussions asking for this alternative behavior in one way or another:

• Caching and fallbackData are not working #2561
• Assign default values to returned data #537
• Is there a way to prevent fallbackData fetching on client side initially? #1550
• Do preloaded resources not get cached in v2.0.0? #2304
• Force `useSWR` return data fetching not to be undefined with `config.fallbackData` #1765
• How to pre-fill (not preload) data for a key in SWR cache before useSWR hook has mounted for that key #2829
• useSWRInfinite with first page as initial data #2425
• uswSWRImmutable always load fallbackData in suspense mode #2436

But no resolution yet. 🙁

To developers using SWR: If you think this will be useful, please vote Yes 🙂