Redux - Selector

initial-state.js

import { Record } from 'immutable'

const CartInitialState = new Record({
  items: [],
})

const initialState = new CartInitialState()

export default initialState

reducer.js

import initialState from './initial-state'
import { ADD_ITEM, REMOVE_ITEM } from './action-types'

export default function cart(state = initialState, action) {
  switch (action.type) {
    case ADD_ITEM: {
      const { item } = action
      const nextItems = state.update('items', items => items.push(item))
      
      return state.set('items', nextItems)
    }
      
    case REMOVE_ITEM: {
      const { itemId } = action
      const nextItems = state.get('items').filter(item => item.get('id') !== itemId)
                                                                   
      return state.set('items', nextItems)
    }
      
    default:
      return state
  }
}