orderToPayload.js

const orderJSON = require('./order.json');
let simulation = {}
let arrayOfItems = []
const items = Object.values(orderJSON.content.items).map(value => {
    let obj = {
        "id": value.id,
        "quantity": value.quantity,
        "seller": "1"
    }

    return obj
})
const getPostalCode = Object.values(orderJSON.content.shippingData)[0].postalCode
const getCountry = Object.values(orderJSON.content.shippingData)[0].country

simulation = {
    "items": items,
    "postalCode": getPostalCode,
    "country": getCountry
}

console.log(JSON.stringify(simulation))