Making changes to data prop incorrectly updates the cache

[akteigland]

Bug report

Description / Observed Behavior

I made changes to data returned from useSWR, to format the data. This updated the SWR cache with the formatted data, which later resulted in a different useSWR hook returning the formatted data, which is incorrect.

Expected Behavior

I would expect that any changes made to the data object be made locally only and not propagated to the SWR cache.

Repro Steps / Code Example

CodeSandbox

Additional Context

I don't think making changes directly to data is great practice anyway, but it can be fairly easy to do so accidentally. In my case I had mistakenly expected a lodash function to return a copy of the data and not the data object itself. While I understand why this would make changes to data locally, I was surprised to find this affected the cache as I would not expect changes outside of the fetcher function to make changes to the cache.