mutate(undefined) doesn't work

[hazae41]

Bug report

const [fallback, setFallback] = useState<Data>(...)
const res = useSWR<Data>(..., { fallbackData: fallback })

console.log("fallback", fallback)
console.log("data", res.data)

function unset(){
  setFallback(undefined)
  res.mutate(undefined, false)
}

With this code, I expect res.data to be undefined on next render when calling unset()
However, it is still the same value even though the fallback has been set to undefined

before unset:

fallback {...}
data {...}

after unset:

fallback undefined
data {...}

Additional Context

[email protected]

[shuding]

Currently in SWR, undefined isn't a valid data type. It's marked as an initial value which stands for "data is missing", but you can't mutate a resource to be undefined. This is something SWR should fix (e.g. mutating a resource as deleted), while I'd suggest using null or an extra layer like { data: undefined } for now.

Note: this is unrelated to fallbackData.

[shuding]

Hey! I opened #1515 which should fix this issue.

I've noticed that mutate(undefined) works correctly with the latest version — gonna close this issue, but feel free to reopen if you can provide a reproduction!