mocha + async test isolation

email.js

module.exports.send = function (template, model, recipient, subject, callback) {

  async.waterfall([
    function readTemplate (wf_callback) {
      fs.readFile(path.join(__dirname, TEMPLATES_DIR, template), wf_callback);
    },

    function renderTemplate (data, wf_callback) {
      wf_callback(null, data.toString());
    },

    function sendEmail (html, wf_callback) {
      var params = {
        from:     'noreply@example.com',
        fromname: 'example.com',
        to: recipient,
        subject: subject,
        html: html
      };
      sendgrid.send(params, wf_callback);
    }
  ], function waterfallDone (err, json) {
    if (err) return callback(err);
    callback(null, true);
  });

};

email.test.js

describe('email', function () {

  var sandbox;
  beforeEach(function () {
      sandbox = sinon.sandbox.create();
  });

  afterEach(function () {
      sandbox.restore();
  });

  it('should read the given template', function (done) {
    var user = new User(fx.User.valid);
    var template = 'some_template.html';
    var stub = sandbox.stub(fs, 'readFile');
    stub.callsArgWith(1, null, new Buffer(''));
    email.send(template, user, user.email, 'Subject', function () {
      stub.calledOnce.should.be.ok;
      stub.firstCall.args[0].should.endWith(template);
      stub.firstCall.args[1].should.be.a.Function;
      done();
    });
  });

});