4/26/15 - Mocking with Sinon

When I wrote the Testing with Sinon post a couple days ago I meant to talk about how I was able to use mocking.

Given a test like this: (there's a lot of code here, but you can definitely get through it—read the comments)

	describe('BookPlayerPageGenerator()', function () {
	var bppg, dlManager, dlManagerMock;
	before(function () {
	// ... (omitted) set up audio and button elements and append them to body ...
	// Set up a dummy object with a stubbed BookDownloadManager interface
	dlManager = { downloadBook: function (book_id, chapter_obj) {},
	downloadChapter: function (book_obj) {} };
	dlManagerMock = sinon.mock(dlManager); // setup a mock of the dlManager object
	// dlManager, NOT dlManagerMock, passed in args
	bookPlayerArgs = {/* other args omitted, */ 'bookDownloadManager': dlManager };
	bppg = new BookPlayerPageGenerator(bookPlayerArgs);
	});
	describe('#generatePage()', function () {
	it('adds onclick to the book download DOM element', function () {
	bppg.generatePage(/* ... args */); // run method
	dlManagerMock.expects("downloadBook").once().withExactArgs(BOOK_OBJECT); // set up expectations for mock
	$('#downloadBook').trigger('click'); // should call dlManager's #downloadBook() method matching expectations
	dlManagerMock.verify(); // verify that #downloadBook() has been called or fail if it hasn't
	});
	});
	});

view raw bookpagegenerator.js hosted with ❤ by GitHub

Sinon's mocking is interesting, because it keeps the behavior of the object being mocked intact while still allowing you to test how the methods have been called. In the case of this test, I don't care about what the BookDownloadManager#downloadBook method is actually doing, so I created a dummy object with stubbed methods (line 8), and used sinon to mock that (line 9).

At first, I was confused about whether to use the original dlManager object or the dlManagerMock I had created with sinon, but I found that the mock you get from sinon.mock() is an object for you as the tester to verify that methods are being called, while the original object passed into .mock() continues to be what you actually use to interact with the objects being tested. Sam pointed out that this was done to preserve the namespace of the original object. For example, what if dlManager had a function called #verify()?