currency-converter.js

// Use your CurrencyConverter API KEY
// Set the currency conversion API URL

const API_KEY = 'YOUR_API_KEY_HERE';
const API_URL = `https://free.currencyconverterapi.com/api/v6/convert?apiKey=${API_KEY}&compact=ultra`;

// Set the base currency (US Dollar in this case)
const BASE_CURRENCY = 'USD';

// Set the list of available currencies
const AVAILABLE_CURRENCIES = [
  'GBP', 'EUR', 'CNY', 'JPY', 'CAD',
  'AUD', 'NGN', 'ZAR', 'KES', 'INR'
];

/**
 * Fetches the exchange rate of a currency relative to the `BASE_CURRENCY`.
 * @param {string} currency The currency to convert to
 * @returns {Promise} A promise that is fulfilled with an array of the format [currency, rate]
 */
const exchangeRate = currency => {
  // The currency conversion symbol
  // For example: the symbol for NGN will be 'USD_NGN'
  
  const symbol = `${BASE_CURRENCY}_${currency}`;
  
  // Append the conversion symbol to the API URL and make a request with Fetch
  // The JSON response will look like this: `{ USD_NGN: 359.999687 }`
  // Return a promise that is fulfilled with an array of the format [currency, rate]
  // For example: ['NGN', 359.999687]
  
  return fetch(`${API_URL}&q=${symbol}`)
    .then(response => response.json())
    .then(data => [ currency, data[symbol] || null ]);
}

/**
 * Fetches the exchange rates of multiple currencies relative to the `BASE_CURRENCY`.
 * @param {Array} currencies The currencies to convert to from the `BASE_CURRENCY`
 * @returns {Promise} A promise that is fulfilled with an object map of each currency to exchange rate
 */
const fetchExchangeRates = currencies => {
  // Notice the use of `Array.map(exchangeRate)` to create Promises for each currency's exchange rate
  // `Promise.all()` is used to execute all the promises in parallel (batch)
  // When all the promises have been settled, `Array.reduce()` is used to construct the object map
  // Return a promise that is fulfilled with an object map of each currency to exchange rate
  
  return Promise.all(currencies.map(exchangeRate))
    .then(rates => {
      return rates.reduce((data, [ currency, rate ]) => {
        return { ...data, [currency]: rate };
      }, {});
    });
}

// Fetch the exchange rates for all the listed AVAILABLE_CURRENCIES
// `console.log()` the data on fulfillment and `console.error()` on rejection

fetchExchangeRates(AVAILABLE_CURRENCIES)
  .then(console.log, console.error);