07.js

// We declare our mapper function, context aware
function mapProductId(transactionsArray) {
  return transactionsArray
    .map(transaction => ({
      ...transaction,
      productId: this.getProductId(transaction.id)
    }))
}

// And now, we can transform our data like so
const example3Context = {
  // This function gets the product somehow
  getProductId: function(transactionId) {}
}
responseMapper(
  example3,
  [
    mapProductName,
    mapProductId.bind(example3Context)
  ]
)