QueryClient DefaultOptions error type

[akinzalowevidation]

Hi,

I am wondering why it is not possible to define the error type of the DefaultOptions within a QueryClient?

I am using it in this way:

const handleError: (error: DefaultGenericError) => {
  ...
};

const queryClient = new QueryClient({
  defaultOptions: {
     queries: {
       onError: handleError,
     },
    mutations: {
      onError: handleError,
     },
   }}
});

Would new QueryClient<DefaultGenericError>(...) (which is passed to DefaultOptions) be reasonable here as a suggestion?

[TkDodo]

error defaults to unknown because everything can be thrown in javascript. If you're certain that error is always of a specific type, you can do an explicit type assertion:

const handleError = (error: unknown) => {
  const genericError = error as DefaultGenericError
};

the safer way would be to let typescript perform type narrowing by doing runtime checks:

const handleError = (error: unknown) => {
  if (error instanceOf Error) {
    // do something with error.message
  }
};

I've also written about that here: https://tkdodo.eu/blog/react-query-and-type-script#what-about-error