Mocking Dependencies using [rewire]

Despite all my efforts to try to encapsulate some of my code into my modules, I was always founding myself exposing too much just for unit testing purpose. And even thought, it was often very complex to mock out some of the external libraries I was using. That was before I found the rewire library. As they say, it only: "adds a special setter and getter to modules so you can modify their behaviour for better unit testing".

This is all you need!

Let's see an example

I have a very simple controller like this:

'use strict';

var userService = require('../service/user-service');

exports.allUsers = function (req, res, next) {

  userService.getUsers(function (err, users) {
    if (err) {
      return next(err);
    }
    res.json(users);
  });
};

In my test driven development all I want to test is:

• this method will call getUsers on userService
• if the callback receive an error then it will call next passing the error
• else json is called on the response with the users as arguments

Setting up a mock is easy (I use sinon but any mocking will do it):

// given
var userController = require('./user-controller'),
    userServiceMock = {};

// and
userServiceMock.getUser = sinon.stub().callsArgWith(1, null, [{id: 'user1'}, {id: 'user2'}]);

// when
userController.allUsers();

// then
userServiceMock.getUser.calledOnce.should.equal(true);

But how can I inject the mock instead of the real user service ?

=>Rewire!

Instead of require the user-controller I'm using rewire:

var userController = rewire('./user-controller')

Then I can get and set any properties on the module. So I can do:

before(function(){
    userController.__set__({
        'userService': userServiceMock
    });
});

Et voila!

Here is the full test for the previous controller:

'use strict';

var sinon = require('sinon'),
    rewire = require('rewire'),
    userController = rewire('./user-controller');

describe('allUsers', function() {

  it ('should return all users',function() {

    // given
    var userServiceMock = {},
        res             = { json: sinon.spy()},
        users           = [{id: 'user1'}, {id: 'user2'}],
        nextStub        = sinon.stub();

    // and
    userServiceMock.getUser = sinon.stub().callsArgWith(1, null, users);

    // when
    userController.allUsers({}, res, nextStub);

    // then
    res.json.calledWith(users).should.equal(true);
    nextStub.called.should.equal(false);
  });
});