A promise that also accepts a callback.

promise.js

exports.get = function(fn) {
    var deferred = Q.defer();

    request('http://example.com', function(err, response) {
      if (err) {
        return deferred.reject(err);
      }

      if (response.statusCode !== 200) {
        return deferred.reject(new Error(response.body || 'Response failed with code ' + response.statusCode));
      }

      deferred.resolve(response.body);

    });

    return deferred.promise.nodeify(fn);
  };