Reducer returned undefined during initialization

I am stuck on this from a long time, and none of the online support till now helped much. I am trying to use immer for immutability in my React-Native app. But the reducer part gives an error saying reducer "count" returned undefined during initialization. My count reducer looks like this -

import produce from "immer";
import {
    INCREMENT,
    DECREMENT,
} from '../action/index.js';

const INITIAL_STATE = {
    count: 0,
};

const countReducer = (state, action) =>
    produce(state, draft => {
        switch (action.type) {
            case INCREMENT: {
                draft = draft + 1;
                break;
            }
            case DECREMENT: {
                draft = draft - 1;
                break;
            }
        }
});

export default countReducer;

My rootReducer is -

import {combineReducers} from 'redux';
import countReducer from './countReducer.js';
const rootReducer = combineReducers({
  count: countReducer,
});
export default rootReducer;

How can I fix this?

From the looks of it you are not using INITIAL_STATE anywhere in your code.

I haven't used immer, but typically you would initialise your state like this

const reducer = (state = initialState, action) => {...}

So I imagine you'd want to do something like:

const INITIAL_STATE = {
  count: 0
};

const countReducer = (
  state = INITIAL_STATE, // initialises state
  action
) =>
  produce(state, draft => {
    switch (action.type) {
      case INCREMENT: {
        draft = draft + 1;
        break;
      }
      case DECREMENT: {
        draft = draft - 1;
        break;
      }
    }
  });