typeof 'data' is always optional even with 'fallbackData' provided.

[kjsik11]

Bug report

Description / Observed Behavior

type of data

Expected Behavior

type of data should be Data(not Data | undefined) if fallbackData has provided in type Data.

Repro Steps / Code Example

const { data } = useSWR<string>('my-key', {
  fallbackData: 'hello world',
});

SWR version.
1.0.0

[kayk1m]

Any plans to resolve this issue?

It's so confusing especially while I am using Shared Hook State with SWR which is updated feature at swr@1.0.0 (using undefined for fetcher)

for example use case,

export function useEmail() {
  const { data: email, mutate: setEmail } = useSWR('@email-key', {
    fallbackData: '',
    fetcher: undefined,
  });

  return [email!, setEmail] as const;
}

[tachibanayu24]

Is there any update on this issue?

[dnjsgur0629]

It is weird that data can still be undefined even though fallbackData is set.
Because of this issue, it is quite inconvenient to use useSWR and typeScript together.

[mrcaidev]

Still an issue in v2.1.5.

Anyway coercion is always a workaround, but it is still strange because #2301 seems to have fixed this with BlockingData, and my config parameter does extend { fallbackData: Data }.

[BoneTM]

Still an issue in v2.1.5.

Anyway coercion is always a workaround, but it is still strange because #2301 seems to have fixed this with BlockingData, and my config parameter does extend { fallbackData: Data }.

dont specify generic, it need fetcher and fallbackData to infer the type

[peterhirn]

dont specify generic, it need fetcher and fallbackData to infer the type

Thanks for the info! It works:

  // string | undefined
  const { data } = useSWR<string>("/api", { suspense: true })

  // string
  const { data } = useSWR("/api", fetcher<string>, { suspense: true })