README.md

var HttpClient = require('./core/HttpClient');
var http = new HttpClient();

http.get('/api/values').then((res) => {
  // Handle the response
}, (err) => {
  // Handle the error
});

HttpClient.js

var { Promise } = require('es6-promise');

export class HttpClient {

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

      request.onload = () => {
        // This is called even on 404 etc.
        // so check the status
        if (request.status >= 200 && request.status < 400) {
          // Resolve the promise with the response JSON
          resolve(JSON.parse(request.responseText));
        } else {
          // Otherwise reject with the status text
          // which will hopefully be a meaningful error
          reject(Error(request.statusText));
        }
      };

      // Handle network errors
      request.onerror = () => {
        reject(Error('Network error'));
      };

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

}