Asynchronous request using promises

xhr.promise.js

function xhr(options) {
  var deferred = Q.defer(),
      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) {
      deferred.reject(new Error('Server responded with a status of ' + req.status));
    } else {
      deferred.resolve(e.target.result);
    }
  };

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

  return deferred.promise;
}