...
import {
  CART_ADD_ITEM,
} from '../constants/cartConstants'
  
export const cartReducer = (
  state = { cartItems: []},
  action
) => {
  switch (action.type) {
    case CART_ADD_ITEM:
      const item = action.payload
  
      const existItem = state.cartItems.find((x) => x.product === item.product)
  
      if (existItem) {
        return {
          ...state,
          cartItems: state.cartItems.map((x) =>
            x.product === existItem.product ? item : x
          ),
        }
      } else {
        return {
          ...state,
          cartItems: [...state.cartItems, item],
        }
      }
    default:
      return state
  }
}

误差

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'find')
    at cartReducer