Mapping abstraction?

There will be a huge object mapping (transforming) with a lot of calculations and logic for some fields.

I would like to refactor this to classes or some kind of separation so it will be easily testable (jest) for each field. How would you abstract these to a few classes or which design pattern that is best suited for this?

For example:

function mapData(data) {
  const orderItems = data.items.map(item => {
    return {
      Name: item.title,
      Qty: parseInt(item.qty),
      Price: parseFloat(item.price),
      PriceExcVat: getItemPriceExcVat(item),
      TotalIncVat: getItemTotalIncVat(item),
      TotalExcVat: getItemTotalExcVat(item),
      // and many more fields
    }
  });

  return {
     Id:  data.data,
     Total: data_grand_total,
     TotalIncVat: getOrderIncVat(data),
     Items: orderItems,
     DeliveryAddress: {
       Name: data.address.name,
       Address1: data.address.line_1,
       Address2: data.address.line_2,
       Address3: data.address.line_3,
     },
      // and many more fields
  }
}

function getOrderIncVat(data) { return /*do some math calculation logic here*/ }
function getItemPriceExcVat(data) {  return /*do some math calculation logic here*/ }
function getItemTotalIncVat(data) {  return /*do some math calculation logic here*/ }
function getItemTotalExcVat(data) {  return /*do some math calculation logic here*/ }