Redux - Selector

cart-reducer.js

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

const getTotalPrice = items => items.reduce((acc, item) => {
  return acc + item.get('price')
}, 0)

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)
        .set('totalPrice', getTotalPrice(nextItems))
    }
      
    case REMOVE_ITEM: {
      const { itemId } = action
      const nextItems = state.get('items').filter(item => item.get('id') !== itemId)
                                                                   
      return state
        .set('items', nextItems)
        .set('totalPrice', getTotalPrice(nextItems))
    }
      
    default:
      return state
  }
}