object-spread.js

const countryCodes = {
  US: 'United States',
  CA: 'Canada',
  NG: 'Nigeria',
  GB: 'United Kingdom',
};

const sales = [
  { code: 'US', count: 233 },
  { code: 'CA', count: 200 },
  { code: 'GB', count: 150 },
  { code: 'NG', count: 111 },
];

const salesMix = sales.map(sale => {
  return {
    ...sale, // object spread takes does a shallow copy of the sale object
    country: countryCodes[sale.code]
  };
});

// or the hotshot way with implicit return:
const salesMixHotShot = sales.map(sale => ({...sale, country: countryCodes[sale.code] }));

console.table(salesMix);