Painless typed Redux reducer with Flow and Immer

counter.js

// @flow
import { createAction, handleActions, type ActionType } from 'redux-actions';
import produce from 'immer';

/* ACTION TYPE */
const INCREASE = 'counter/INCREASE';


/* ACTION CREATOR */
const increase = createAction(INCREASE, (value: number) => value);

/* ACTION FLOW TYPE */
type increaseAction = ActionType<typeof increase>;

/* ACTION CREATORS INTERFACE */
export interface CounterActionCreators {
  increase(value: number): increaseAction
}

/* EXPORT ACTION CREATORS */
export const actionCreators: CounterActionCreators = {
  increase
};

/* STATE TYPES */
export type Counter = {
  value: number
};

/* INITIAL STATE */
const initialState: Counter = {
  value: 0
};

/* REDUCER */
export default handleActions({
  [INCREASE]: (state, action: increaseAction) => {
    return produce(state, draft => {
      draft.value += 1;
    });
  }
}, initialState);