How to reflect mutation post in the actual (fetch) data?

 const {data: categories, isLoading: isCategoriesLoading, setData: setCategoriesData} = useQuery(
    ['categories'],
    fetchCategories
  );

 const postCategoriesMutation = useMutation({
    mutationFn: ({category: category}) => postCategory(category),
    onSuccess: (result, variables, context) => {
      queryClient.setQueryData(['categories'], (cats) =>{
        console.log(cats); // prints undefined
        return [...cats, result] // me trying to set the 'categories' data
      })
    },
  });