reducer.js

counterReducer = (state, action) => {
  switch (action.type) {
    case 'INCREASE':
      return { ...state, count: state.count + 1 };
    case 'DECREASE':
      return { ...state, count: state.count - 1 };
    default:
      return state;
  }
};
/*
This kind of reducer above doesn't make use of any payload
in the action so supplying a payload in the action is not needed.
That is why i said payload is optional.
*/