Alternative to now deprecated existential type (*)

extractReturn.js

// To get the return type of a function you can use this helper

export type ExtractReturn<Fn> = $Call<<T>((...Iterable<any>) => T) => T, Fn>;
// Example of usage:

const mapDispatchToProps = (dispatch: Dispatch) => ({
  actions: {
    setCompany: ({ companyId, domainName }) =>
      dispatch(
        setCompany({
          companyId,
          domainName,
        }),
      ),
  },
});

type ReduxActions = ExtractReturn<typeof mapDispatchToProps>;