Type narrowing with useMutation not working? (or maybe as intended?)

[anddoutoi]

Edit: Type narrowing for useMutation was added in v3.23.2.

Context: using:

"react-query": "3.19.6",
"typescript": "4.3.5"

When using useQuery you get "type narrowing" as in:

const queryResult = useQuery<Gizmo, Error>(/* stuff */);

// queryResult.error is Error | null here

if (queryResult.isError) {
  // queryResult.error is Error here for sure
}

Super awesome!

But when using useMutation I get this:

const mutationResult = useMutation<Gizmo, Error>(/* stuff */);

if (mutationResult.isError) {
  // mutationResult.error is Error | null here
}

I followed the breadcrumbs in from react/types.ts:

export declare type UseQueryResult<TData = unknown, TError = unknown> = UseBaseQueryResult<TData, TError>;

to core/types.ts:

export interface QueryObserverLoadingErrorResult<TData = unknown, TError = unknown> extends QueryObserverBaseResult<TData, TError> {
    data: undefined;
    error: TError;
    isError: true;
    isIdle: false;
    isLoading: false;
    isLoadingError: true;
    isRefetchError: false;
    isSuccess: false;
    status: 'error';
}

and understand that's why it works for useQuery.

For useMutation it is:

export interface UseMutationResult<TData = unknown, TError = unknown, TVariables = unknown, TContext = unknown> {
    context: TContext | undefined;
    data: TData | undefined;
    error: TError | null;
    failureCount: number;
    isError: boolean;
    isIdle: boolean;
    isLoading: boolean;
    isPaused: boolean;
    isSuccess: boolean;
    mutate: UseMutateFunction<TData, TError, TVariables, TContext>;
    mutateAsync: UseMutateAsyncFunction<TData, TError, TVariables, TContext>;
    reset: () => void;
    status: MutationStatus;
    variables: TVariables | undefined;
}

Is this intended? Or maybe there is an upcoming fix for it?

Anyway, thanks for an awesome library

[TkDodo]

we have an open PR for that: #2620

[anddoutoi]

Awesome! Thanks for getting back to me. I did look in issues and discussion before posting but forgot to look in PRs ofc 😬