main-es6.js

(function(window){
  'use strict';

  let bitcoinField = document.querySelector('.bitcoinfield__input');
  let inrField = document.querySelector('.inrfield__input');
  let urls = ['https://blockchain.info/ticker',
              'http://data.fixer.io/api/latest?access_key=YOUR_API_KEY&symbols=USD,INR'];
  function formSubmitted(event) {
    let bitcoinToUSD, eurToUSD, eurToINR;
    fetch(urls[0])
    .then(response => {
      // A fetch() promise will reject with a TypeError when a network error is encountered or CORS is misconfigured on the server-side, although this usually means permission issues or similar — a 404 does not constitute a network error, for example. An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the Response.ok property has a value of true.
      if (!response.ok) {
        throw new Error('Network response was not ok');
      }
      return response.json();
    })
    .then(text => {
      if (!isNaN(text.USD.last)) {
        bitcoinToUSD = text.USD.last;
      }
      else {
        throw new Error('Malformed JSON returned from blockchain.info !');
      }
    })
    .then(() => fetch(urls[1]))
    .then(response => {
      if (!response.ok) {
        throw new Error('Network response was not ok');
      }
      return response.json();
    })
    .then(text => {
      if (!isNaN(text.rates.INR) && !isNaN(text.rates.USD)) {
        eurToINR = text.rates.INR;
        eurToUSD = text.rates.USD;
        inrField.value = bitcoinField.value * bitcoinToUSD * eurToINR / eurToUSD;
      }
      else {
        throw new Error('Malformed JSON returned from fixer.io !');
      }
    })
    .catch(e => {
      console.error(e);
      console.error('Error in fetching from APIs!');
    })
  }
  window.formSubmitted = formSubmitted;
})(window);