Methods missing from Javascript Class inside Vuex state

I'm using Nuxt framework alongside Vuex to store data in my web site but I'm facing trouble when I want to use a class directly in the state.

With a model cart.js defined like this:

export class Cart {

    constructor(ownedID) {
        this._created = new Date();
        this._lastUpdated = new Date();
        this._ownerID = ownedID || 'visitor'
        this._items = []
    }

     getItem (articleNumber) {
          console.log(this._items)
    }

    ...
}

And my store's module cart.js

import { Cart } from "~/models/cart";

const state = () => ({
    cart: new Cart()
})

const mutations = {
    ADD_ITEM(state, newItem) {
        console.log(state.cart)
    }
}

...

When the ADD_ITEM(state, newItem) mutation is called the getItem(articleNumber) function is missing and thus I receive the TypeError: state.cart.getItem is not a function error.

This is the result of the console.log:

__ob__: Object { value: {…}, dep: {…}, vmCount: 0 }
​
   _created: 
   _item:
   _lastUpdated:
   _ownerID:

This is a sandbox link of my setup.

Nuxt vuex sandbox error

Does anyone have a clue about my issue.

Thank you.