Mockhard: Contract Testing for JavaScript

Mockhard makes mocking more robust by ensuring mocked methods are tested.

The Problem

Let's assume I have a Student which depends on a Library:

var Student = {
  learn: function(library) {
    return 'I learned ' + library.checkout();
  }
};

We can test Student like this:

describe('Student', function() {
  it('does something', function() {
    var library = {
      checkout: function() { return 'Moby Dick'; }
    };

    expect(Student.learn(library)).toEqual('I learned Moby Dick');
  });
});

However, we have a problem: we have no idea whether the real Library has a function called checkout that takes no arguments and returns Moby Dick. Our mocks are not synchronised with their real counterparts.

The Solution

Mockhard ensures that the things you do with your mock object can be done with a real object. If you change the real object without changing the mock, Mockhard will let you know (and vice versa).

If you want more information on why this is cool, watch Integration Tests are a Scam. (You could say that Mockhard is a contract testing helper.)

In Mockhard you would write:

describe('Student', function() {
  it('does something', function() {
    var library = fake('Library');
    library.mock('checkout').map([], 'Moby Dick');

    expect(Student.learn(library)).toEqual('I learned Moby Dick');
  });
});

If you leave it at this, Mockhard will moan when your tests run because you haven't linked your mock with a real object.

Let's do what it says and test the real object:

describe('Library', function() {
  var library;

  beforeEach(function() {
    library = real('Library', Library('Moby Dick'));
  });

  describe('.checkout', function() {
    it('returns the first book', function() {
      expect(library.checkout()).toEqual('Moby Dick');
    });
  });
});

Congratulations, you have appeased Mockhard and it will no longer whinge at you.

Limitations

• Mockhard doesn't link into any test frameworks for auto-verifying.
• Mockhard mocks methods only, not properties.
• Mockhard is not async-safe at the moment. Well, I'm not confident it is anyway.

For Developers

Get set up by cloning the repository and running npm install.

Run tests once using npm test. Use karma start to watch your tests while developing.