With `useSWR` the return type is export interface SWRResponse<Data = any, Error = any> { /** * The returned data of the fetcher function.

With `useSWR` the return type is export interface SWRResponse<Data = any, Error = any> { /** * The returned data of the fetcher function. */ data: Data | undefined /** * The error object thrown by the fetcher function. */ error: Error | undefined mutate: KeyedMutator<Data> isValidating: boolean isLoading: boolean } https://github.com/vercel/swr/blob/14956a840ac9b75fe321bf84691511e423a2f498/_internal/types.ts#L340-L352 https://github.com/vercel/swr/blob/14956a840ac9b75fe321bf846... so if I check `isLoading` in my component then typescript isn't able to narrow/refine the type to something more specific since it isn't a sum type. While with react-query, it's a proper sum type that can be differentiated by checking `isLoading`. The type is actually quite long so here's just a bit export type QueryObserverResult<TData = unknown, TError = unknown> = | QueryObserverIdleResult<TData, TError> | QueryObserverLoadingErrorResult<TData, TError> | QueryObserverLoadingResult<TData, TError> | QueryObserverRefetchErrorResult<TData, TError> | QueryObserverSuccessResult<TData, TError> export interface QueryObserverLoadingResult<TData = unknown, TError = unknown> extends QueryObserverBaseResult<TData, TError> { data: undefined error: null isError: false isIdle: false isLoading: true isLoadingError: false isRefetchError: false isSuccess: false status: 'loading' } https://github.com/TanStack/query/blob/9b21609350d9ad41faf5c2fb0ac6c67e4324a637/packages/query-core/src/types.ts#L368-L457 https://github.com/TanStack/query/blob/9b21609350d9ad41faf5c... The key point in the type above is `isLoading: true` and `data: undefined`, which means if I check `isLoading === true`, then typescript knows the data is not available. SWR doesn't provide these guarantees.