Fetch currencies rate and get ratio between two currencies

currencies_ratio.js

/**
* @function fetchRates
* @description fetch currencies rate
* @param {string} [url='https://bank.gov.ua/NBUStatService/v1/statdirectory/exchange?json'] - currencies rate api endpoint
* @returns {promise}
* @example
* let rates = [];
* fetchRates().then(newRates => {
*   rates = newRates;
* });
*/
const fetchRates = (url) => {
  return fetch(url || 'https://bank.gov.ua/NBUStatService/v1/statdirectory/exchange?json')
    .then(res => res.json())
    .catch(console.error);  
};


/**
* @function getRatio
* @description get currencies ratio
* @param {array} rates - list of rates data
* @param {string} currencySlugA
* @param {string} currencySlugB
* @returns {number}
* @example
* let ratio = getRatio(ratesList, 'EUR', 'USD'); // 1.2108000780045032
*/
const getRatio = (rates, currencySlugA, currencySlugB) => {
    let aData = rates.filter(i => i.cc === currencySlugA)[0];
    let bData = rates.filter(i => i.cc === currencySlugB)[0];
    
    if(!aData){
      throw(`Invalid value for $currencySlugA: ${currencySlugA}`);
    }
    if(!bData){
      throw(`Invalid value for currencySlugB: ${currencySlugB}`);
    }

    return Number(aData.rate) / Number(bData.rate);
};

Usage example:

fetchRates()
    .then((rates) => {
        const ratio = getRatio(rates, 'EUR', 'USD');
        console.log({ratio});
    });