Typescript - useMutation variables can't handle unions

[bviebahn]

async function mutationFn(variables: { a: string } | { b: string }) {
    return true;
}

const useFoo = () => {
    const [mutate] = useMutation(mutationFn);
    mutate({ a: "" });
};

The code above produces this Typescript error:
Argument of type '{ a: string; }' is not assignable to parameter of type '{ a: string; } & { b: string; }'. Property 'b' is missing in type '{ a: string; }' but required in type '{ b: string; }'.

I have no idea why the expected type becomes the intersection { a: string; } & { b: string; } rather than the union I specified.

The type definition for MutateFunction is:

export type MutateFunction<
  TResult,
  TVariables,
  TError = Error
> = TVariables extends undefined
  ? (options?: MutateOptions<TResult, TVariables, TError>) => Promise<TResult>
  : (
      variables: TVariables,
      options?: MutateOptions<TResult, TVariables, TError>
    ) => Promise<TResult>

changing TVariables extends undefined to [TVariables] extends [undefined] seems to fix the problem. I don't understand why though.

I'm using version 2.4.13

[serhii-ohorodnyk]

I bumped into the same issue. And it is a feature of typescript which is called Distributive conditional types.

With current implementation it means that for mutation function with string | number we will get union type of function (string) => Promise<TResult> | (number) => Promise<TResult> which will fail compilation. Expected type should be (string | number) => Promise<TResult>.

And wrapping conditional type into 1-tuple (convert TVariables extends undefined to [TVariables] extends [undefined] as you mentioned @Bene8493) works because type is not naked anymore. And it is a common way to avoid type distribution.

I'd be happy to raise a PR with this change, but if I get contribution guide correctly I should be assigned to this issue first, or...?

[bviebahn]

Thanks for looking into it. There's an open pull request to completely move the project to typescript. Might be worth waiting for that. #767

[MichaelDeBoey]

Is this still a valid problem now #767 is merged? 🤔

[boschni]

Seems to be working correctly now when using the example in the React Query tests