app.js

var responseFromGoogle = axios.get(googleBooksUrl);
var responseFromCrossRef = = axios.get(crossRefUrl);
Promise.all([responseFromGoogle, responseFromCrossRef]).then(function(responses){
  var googleData = cleanGoogleBooks(responses[2]);
  var crossRefData = cleanCrossRef(responses[1]);
  response.send({google: googleData, crossRef: crossRefData});
});