Testing initialization logic in closure in angular service - unit-testing

Question

I have an angular service that is used as a factory function to instatiate many object instances of the type Engine like this:
angular.module('parts.engine', []).factory('Engine', function() {
var Engine = function( settings ) {
this.hp = settings.engine.hp;
this.miles = 0;
};
Engine.prototype.setMiles = function( miles ) { this.miles = miles; }
return Engine;
});
Say I have another angular service, that is also used to create instances of an object like this:
angular.module('car', ['parts.engine']).factory('carCreator', function( Engine ) {
var carCreator = function( settings ) {
var engine = new Engine( settings );
engine.setMiles( settings.engine.miles )
return {
brand: settings.brand;
engine: engine;
}
};
return carCreator;
});
So I now instatiate a new instance of a car object like this:
angular.module('carApp', ['car']).controller('AppCtrl', function( carCreator ) {
var settings = {
brand: 'Ford',
engine: {
hp: 125,
miles: 12000
}
};
var newCar = carCreator(settings);
});
Does anyone have an idea how to test the initialization logic:
var engine = new Engine( settings );
engine.setMiles( settings.engine.miles )
in the carCreator factory? I know I can instantiate an object with the carCreator class and check, if the returned objects engine.miles property is set to the correct value. But I have cases, where checking for this will not be as easy, because the initalization logic and the values returned are much more complex. What I would like to do is test the businesslogic of setMiles in the Engine class, and just setup a SpyOn on Engine and engine.setMiles when testing the carCreator class, but how do I do that, when engine is created in the closure?

Answer 1

You can use $provide to provide a mock version of the Engine service to carCreator. You'll then be able to spy on it and assert that the correct steps have been taken.
One of the trickier bits of this is that you're testing a constructor function, which jasmine mocks doesn't play so nicely with, you need to return a mockInstance and spy on that instance.
var mockEngine, mockEngineInstance, carCreatorService;
beforeEach(function () {
module('car', function ($provide) {
// Define a mock instance
mockEngineInstance = {
setMiles: function () {}
};
// Mock the engine service to give a mocked instance
mockEngine = function () {
return mockEngineInstance;
};
$provide.value('Engine', mockEngine);
});
inject(function (carCreator) {
carCreatorService = carCreator;
});
});
it('should test that miles are set when a car is created',
function () {
// Arrange.
spyOn(mockEngineInstance, 'setMiles');
// Act.
carCreatorService({
engine: {
miles: 100
}
});
// Assert.
expect(mockEngineInstance.setMiles).toHaveBeenCalledWith(100);
}
);