TVariables default Type

[hossein-jorfi]

Shouldn't TVariables default be "any" or "unknown" so you dont have to define it unless its important to you to be defined

that is just because if you write:

  const { mutate } = useMutation((values) =>

    axios.post('/test', values),

  );

it is not defined what values should have for a type. The generic TVariables defaults to void and that's the error you are getting. If you write a function in typescript:

const myFunction = (variables) => console.log(variables)

the compiler needs to know what variables will be, otherwise, it will be implicitly any.

if you take my example where values has a type of values: Variables, you will see that it works.

Originally posted by @TkDodo in #2134

[puneet26082006]

You don’t need to specify the useMutation generics manually if the mutation function already has a typed parameter. For example, using the v5 API:

type Variables = {
  title: string;
};

const { mutate } = useMutation({
  mutationFn: async (values: Variables) => {
    const response = await axios.post('/test', values);
    return response.data;
  },
});

mutate({ title: 'Hello' });

Here, TVariables is inferred as Variables.

The practical benefit of the void default is that mutations without an input can be called as mutate(). Defaulting to any would lose input checking when inference is missing. unknown would allow arbitrary values at the call site, but you would need to narrow them before accessing properties inside the mutation function.

For your example, typing values at the function boundary gives you inference without repeating the type in useMutation<...>.