promise-ajax.js

// Wrap the $.getJSON() call to return a real Promise
// properly handling rejection with the available `.fail` method
var fetchJSON = function(url) {  
  return new Promise((resolve, reject) => {
    $.getJSON(url)
      .done((json) => resolve(json))
      .fail((xhr, status, err) => reject(status + err.message));
  });
}