Type-narrowing doesn't work

[lcswillems]

Bug report

Description / Observed Behavior

const { data, error, isLoading } = useSWR<number>('/api/user/123', fetcher);

if (!isLoading && !error) {
  data // number | undefined
}

Expected Behavior

It should be of type number.

[Ram4GB]

I don't think useSWR returns isSuccess. Are you mistyping something @lcswillems ?

[lcswillems]

@Ram4GB My bad. Fixed the example!

[Ram4GB]

I assume you request the user who is not found in your database or has been deleted by other actions. In this case, the API must throw the error with the status 400, so you won't access your data, and it has to return undefined. @lcswillems

[lcswillems]

Not sure to understand. I'm just saying if "isLoading" is false, then "data" should become of type "number".

It is a basic functionality of TanStack query: https://tanstack.com/query/latest/docs/framework/react/typescript#type-narrowing

[icyJoseph]

But if fetching fails/errors, there won't be data, and it won't be loading either.

[lcswillems]

@icyJoseph Updated my example. Makes sense? Basically I'm asking for the same thing than TanStack Query.

[Ram4GB]

Yes @lcswillems, your example is not clear to me. In your case, I believe we should get the type number instead of number | undefined.

[lcswillems]

Yes we should get the type number, it is what I say in the "expected behavior". Currently, the type is number | undefined (Observed behavior).