In this example we want to call the foo method, and validate the call to console.log. Note that log is a method defined in the console object. We need to create a 'spy' on this method. In the test-step method, define an argument of type SinonSpy. To make this argument become a spy object, we need to add decorator to the argument called spy and give to the decorator 2 arguments: the object the method to spy is on it, and the name of the method to spy