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:

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: