Javascript: Mocking Constructor using Sinon

Question

I am pulling my hair out trying to figure out how to mock a constructor using sinon. I have a function that will create multiple widgets by calling a constructor that accepts a few arguments. I want to verify that the constructor is called the correct number of times with the correct parameters, but I don't want to actually construct the widgets. The following links seemingly explain a straightforward way of mocking the constructor, however it does not work for me:

Spying on a constructor using Jasmine

http://tinnedfruit.com/2011/03/25/testing-backbone-apps-with-jasmine-sinon-2.html

When I make the following call to stub the constructor:

sinon.stub(window, "MyWidget");

I get the following error:

Uncaught TypeError: Attempted to wrap undefined property MyWidget as function 

When debugging in Chrome I see MyWidget shows up in the Local section of the Scope Variables, however there is not MyWidget property off of window.

Any help would be greatly appreciated.

Answer 1

I needed a solution for this because my code was calling the new operator. I wanted to mock the object that the new call created.

var MockExample = sinon.stub();
MockExample.prototype.test = sinon.stub().returns("42");
var example = new MockExample();
console.log("example: " + example.test()); // outputs 42

Then I used rewire to inject it into the code that I was testing

rewiredModule = rewire('/path/to/module.js');
rewiredModule.__set__("Example", example);