正确使用sinon的假XMLHttpRequest

我正在创建XMLHttpRequest javascript模块以从服务器获取JSON数据。以下是代码：

I am creating XMLHttpRequest javascript module to get JSON data from server. Here is the code:

(function() {
  var makeRequest = function(url,callback,opt) {
    var xhr;
    if (XMLHttpRequest) { // Mozilla, Safari, ...
      xhr = new XMLHttpRequest();
    } else if (ActiveXObject) { // IE
      try {
        xhr = new ActiveXObject("Msxml2.XMLHTTP");
      }
      catch (e) {
        try {
          xhr = new ActiveXObject("Microsoft.XMLHTTP");
        }
        catch (e) {}
      }
    }

    if (!xhr) {
      callback.call(this,
        'Giving up :( Cannot create an XMLHTTP instance',
        null);
      return false;
    }
    xhr.onreadystatechange = function() {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
          var data = xhr.responseText;
          if(opt && !opt.raw) {
            try {
              data = JSON.parse(data);
            } catch (e) {
              callback.call(this, e,null);
              return;
            }
          }
          callback.call(this,null,data);
        } else {
          callback.call(this,
            'There was a problem with the request.',
            null);
        }
      }
    };
    var params = '';
    if (opt && opt.params && typeof(opt.params) == 'object') {
      for( var key in opt.params) {
        params += encodeURIComponent(opt.params[key]);
      }
    }
    var method = opt && opt.method ? opt.method : 'GET';
    if (method == 'GET') {
      url = params.length > 0 ? url+'?'+params : url;
      xhr.open('GET', url);
      xhr.send();
    } else if (method == 'POST') {
      var data = opt && opt.data ? opt.data : params;
      xhr.open('POST', url);
      xhr.send(JSON.stringify(data));
    }
    return xhr;
  }

  if(typeof module !== 'undefined' && module.exports) {
    module.exports = makeRequest;
  }
  if(typeof window!== 'undefined') {
    window.getJSONData = makeRequest;
  }
})();

现在我正在使用Mocha和Sinon在nodejs上编写测试用例。使用Sinon的fakeXMLHttpRequest测试模块和测试代码在这里：

Now I am writing the test case for this on nodejs with Mocha and Sinon. Using Sinon's fakeXMLHttpRequest to test the module and test code is here:

var expect = require('chai').expect,
  getJSON = require('../'),
  sinon = require('sinon');

describe('get-json-data test the request', function() {
  beforeEach(function() {
    this.xhr = sinon.useFakeXMLHttpRequest();
    var requests = this.requests = [];

    this.xhr.onCreate = function (xhr) {
      requests.push(xhr);
    };
  });
  afterEach(function() {
    this.xhr.restore();
  });

  it('get json data', function() {
    var callback = sinon.spy();
    getJSON('/some/json', callback);
    expect(this.requests.length).to.equal(1);
    this.requests[0].respond(200,
      {"Content-Type": "application/json"},
      '{"id": 1, "name": "foo"}');
    sinon.assert.calledWith(callback, {"id": 1, "name": "foo"});
  });
});

在运行测试时出现错误：

On running the test I get error:

ReferenceError：未定义XMLHttpRequest

这似乎是正确的，因为nodejs中没有XMLHttpRequest类/函数。但是Sinon的假XMLHttpRequest不应该这样做。我想在Sinon的setUp（Mocha的beforeEach）中我们用fakeXMLHttpRequest替换原生的XMLHttpRequest。
请建议我做错了什么？或者在nodejs上测试我的模块的正确方法是什么？

And it seems correct as there is no XMLHttpRequest class/function in nodejs. But is Sinon's fakeXMLHttpRequest not supposed to do that. I thought in Sinon's setUp (Mocha's beforeEach) we are replacing the native XMLHttpRequest with fakeXMLHttpRequest. Please suggest what I am doing wrong? Or what would be the correct way to test my module at nodejs?