this-is-js.factory-function.js

function createCart() {
    const data = {
        items: []
    }

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

    function addItem(item) {
        new Promise((resolve, reject) => {
            setTimeout(() => {
                data.items.push(item)
                resolve()
            }, 500)
        }).then(onItemAddedToCart)
    }

    return {
        addItem
    }
}

const myCart = createCart()
myCart.addItem('Notebook') // The cart has 1 item