Fix useFormState and useField Flow types

[redbmk]

The exported flow typing in index.js.flow doesn't match what's actually used in useFormState. When using this externally, this causes Flow to think that useFormState might return null or undefined, when that appears not to be the case. It also thinks that the params are required when they're really optional.

src/index.js.flow

declare export function useFormState<FormValues>(
  params: UseFormStateParams<FormValues>
): ?FormState<FormValues>

src/useFormState.js

function useFormState<FormValues: FormValuesShape>({
  onChange,
  subscription = all
}: UseFormStateParams<FormValues> = {}): FormState<FormValues> {

It looks like a similar change was made in #486 to fix the TypeScript typings in the same way.

typescript/index.d.ts

export function useField<T extends HTMLElement>(
   config: UseFieldConfig
 ): FieldRenderProps<T>;
 export function useForm(): FormApi;
-export function useFormState(params: UseFormStateParams): FormState | void;
+export function useFormState(params?: UseFormStateParams): FormState;
 export const version: string;

I'm using this locally with npm link and it's working for me. I would add // @flow to useFormState.test.js and then import from ./index instead of ./useFormState, but it seems flow and jest aren't working together out of the box.

[codecov]

Codecov Report

Merging #630 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #630   +/-   ##
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  Coverage     100%   100%           
=====================================
  Files          17     17           
  Lines         248    248           
  Branches       57     57           
=====================================
  Hits          248    248

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[redbmk]

@erikras I also found that there were issues with the useField typings and figured out how to get flow working with jest, so now the tests have flow coverage as well. Since most people will be importing from index, I also changed the tests to import from index, which will pull in the exported flow typings.

[erikras]

Published in v6.3.1.

[lock]

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