xhr.js

module.exports = function (options) {

  return new Promise(function (resolve, reject) {

    var req = new XMLHttpRequest();

    req.open(options.method || 'GET', options.url, true);

    // Set request headers if provided.
    Object.keys(options.headers || {}).forEach(function (key) {
      req.setRequestHeader(key, options.headers[key]);
    });

    req.onreadystatechange = function (e) {

      if (req.readyState !== 4) {
        return;
      }

      if ([200, 304].indexOf(req.status) === -1) {
        reject(new Error('Server responded with a status of ' + req.status));
      } else {
        options.expect = options.expect || 'anything';
        switch (options.expect.toLowerCase()) {
          case 'json':
            resolve(JSON.parse(e.target.responseText));
            break;
          case 'xml':
            resolve(e.target.responseXML);
            break;
          default:
            resolve(e.target.responseText);
            break;
        }
      }

    };

    req.send(options.data || void 0);

  });

}