Redux - Immutability

initial-state.js

import { Record, List } from 'immutable'
        
const CartInitialState = new Record({
 items: List()
})

const initialState = CartInitialState({
  items: List()
})

export default initialState

reducer.js

import initialState from './initial-state'

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