Cannot call ... because an indexer property is missing in ...

[steida]

Just spend the whole day trying to figure out why simple Flow generic code does not work.

Leaving it here, in order to help the future me or anyone else.

Flow cannot say in humanly comprehensible language that generic type needs to be bounded. https://flow.org/en/docs/types/generics/#toc-generic-types-act-as-bounds

Still, don't understand why Flow needs it and what actually fixes it.

[steida]

: Object!

export default function useMutation<Mutation: Object, Name>(
  config: Config,
  validate?: (Input<Mutation>) => Errors<Mutation, Name>,
): [Commit<Mutation>, Errors<Mutation, Name>] {
  const [errors, setErrors] = useState<Errors<Mutation, Name>>(null);
  const environment = useRelayEnvironment();
  function commit(input) {
    if (validate) {
      const errors = validate(input);
      setErrors(errors);
    }
  }
  return [commit, errors];
}