Spying on a constructor using Jasmine

I am using Jasmine to test if certain objects are created and methods are called on them.

I have a jQuery widget that creates flipcounter objects and calls the setValue method on them. The code for flipcounter is here: https://bitbucket.org/cnanney/apple-style-flip-counter/src/13fd00129a41/js/flipcounter.js

The flipcounters are created using:

var myFlipCounter = new flipCounter("counter", {inc: 23, pace: 500});

I want to test that the flipcounters are created and the setValue method is called on them. My problem is that how do I spy on these objects even before they are created? Do I spy on the constructor and return fake objects? Sample code would really help. Thanks for your help! :)


I've tried spying on the flipCounter like this:

myStub = jasmine.createSpy('myStub');
spyOn(window, 'flipCounter').andReturn(myStub);


Then testing for the setValue call by flipCounter:

spyOn(myStub, 'setValue');


the first test for initializing flipCounter is fine, but for testing the setValue call, all I'm getting is a 'setValue() method does not exist' error. Am I doing this the right way? Thanks!



flipCounter is just another function, even if it also happens to construct an object. Hence you can do:

var cSpy = spyOn(window, 'flipCounter');

to obtain a spy on it, and do all sorts of inspections on it or say:

var cSpy = spyOn(window, 'flipCounter').andCallThrough();
var counter = flipCounter('foo', options);

However, this seems overkill. It would be enough to do:

var myFlipCounter = new flipCounter("counter", options);

I would suggest using jasmine.createSpyObj() when you want to mock objects with properties that need to be spied on.

myStub = jasmine.createSpyObj('myStub', ['setValue']);
spyOn(window, 'flipCounter').andReturn(myStub);

This tests interactions with the expected flipCounter interface, without depending on the flipCounter implementation.


The following does not rely on 'window'. Lets say this is the code you want to test -

function startCountingFlips(flipCounter) {
    var myFlipCounter = new flipCounter("counter", {inc: 23, pace: 500});

Your test could be -

var initSpy = jasmine.createSpy('initFlipCounter');
var flipCounter = function(id, options) {
    initSpy(id, options);
expect(initSpy).toHaveBeenCalledWith("counter", {inc:23, pace:500});

You have to implement a fake constructor for flipCounter that sets the setValue property to a spy function. Let's say the function you want to test is this:

function flipIt() {
  var myFlipCounter = new flipCounter("counter", {inc: 23, pace: 500});

Your spec should look like this:

describe('flipIt', function () {
  var setValue;
  beforeEach(function () {
    setValue = jasmine.createSpy('setValue');
    spyOn(window, 'flipCounter').and.callFake(function () {
      this.setValue = setValue;
  it('should call flipCounter constructor', function () {
      .toHaveBeenCalledWith("counter", {inc: 23, pace: 500});
  it('should call flipCounter.setValue', function () {

My version to test a constructor is to spy on the prototype:

spyOn(flipCounter.prototype, 'setValue').and.callThrough();
var myFlipCounter = new flipCounter("counter", {inc: 23, pace: 500});

Don't know how to do this using jasmine mocks, but if you want powerful mocking/spy/stubs I recommend sinon.js, wich works very well with jasmine.

From docs:

A test spy is a function that records arguments, return value, the value of this and exception thrown (if any) for all its calls. A test spy can be an anonymous function or it can wrap an existing function.

Mocks (and mock expectations) are fake methods (like spies) with pre-programmed behavior (like stubs) as well as pre-programmed expectations. A mock will fail your test if it is not used as expected.

With sinon.js you could create a mock of the flipCounter constructor that returns another spy.

Then assert that the constructor was called using constructorMock.calledWithNew(), and assert that the returned spy was called with returnedSpy.calledWith(arg1, arg2...).


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