ajax.js

// http://www.html5rocks.com/en/tutorials/es6/promises/

export function get(url) {
    // Return a new promise.
    return new Promise(function(resolve, reject) {
        // Do the usual XHR stuff
        var req = new XMLHttpRequest();
        req.open('GET', url);

        req.onload = function() {
            // This is called even on 404 etc
            // so check the status
            if (req.status == 200) {
                // Resolve the promise with the response text
                resolve(req.response);
            }
            else {
                // Otherwise reject with the status text
                // which will hopefully be a meaningful error
                reject(Error(`Failed with status ${req.status}: ${req.statusText}`));
            }
        };

        // Handle network errors
        req.onerror = function() {
            reject(Error('Network Error'));
        };

        // Make the request
        req.send();
    });
}