this-is-js.class.js

class Cart {
  constructor() {
    this.cart = []
  }

  onItemAddedToCart() {
    alert(`The cart has ${this.cart.length} items`)
  }

  addItemToCart(item) {
    new Promise((resolve, reject) => // Simulação de requisição http
      setTimeout(() => {
        this.cart.push(item)
        resolve()
      }, 500)
    )
      .then(this.onItemAddedToCart)
  }
}

const cart = new Cart()
cart.addItemToCart('Notebook') // this.cart is undefined