A test double is a pattern on unit testing. It replaces a test dependent component with an equivalent implementation that is specific to the test scenario. These implementations are called doubles because although their behavior might be specific to the test, they act like, and have the same API as, the object they impersonate.
Spies are Jasmine's solution to test doubles. At its core, a Jasmine spy is a special type of function that records all interactions that happen with it. Therefore, they are very useful when a returned value or change in an object's state can't be used to determine whether a test expectation was a success. In other words, Jasmine spies are perfect when a test success can only be determined by behavior checking.
To understand the concept of behavior checking, let's revisit an example presented in Chapter 3, Testing Frontend Code, and test the observable behavior of the NewInvestmentView test suite:
describe("NewInvestmentView", function() {
var view;
// setup and other specs ...
describe("with its inputs correctly filled", function() {
// setup and other specs ...
describe("and when an investment is created", function() {
var callbackSpy;
var investment;
beforeEach(function() {
callbackSpy = jasmine.createSpy('callback');
view.onCreate(callbackSpy);
investment = view.create();
});
it("should invoke the 'onCreate' callback with the created investment", function() {
expect(callbackSpy).toHaveBeenCalled();
expect(callbackSpy).toHaveBeenCalledWith(investment);
});
});
});
});During the spec setup, it creates a new Jasmine spy using the jasmine.createSpy function while passing a name for it (callback). A Jasmine spy is a special kind of function that tracks calls and arguments made to it.
Then, it sets this spy as an observer of the View's create event using the onCreate function, and finally it invokes the create function to create a new investment.
Later on, at the expectations, the spec uses the toHaveBeenCalled and toHaveBeenCalledWith matchers to check whether the callbackSpy was called and with the right parameters (investment), thereby making a behavior check.