More Filter Testing

I recently wrote a post on how to test whether a filter was called from a controller.  Today I needed to test whether a filter was called from a factory, which is slightly different.  When we test controllers we instantiate the controller and inject what we want.  That means when we inject the $filter service we can just supply our own spy instead.  In a factory, though, we don't really instantiate the factory.  Instead the factory is just kinda there and we inject other factories and services into it.  (I'm sure there's a way to "instantiate" a factory for testing purposes and I know you don't actually "instantiate" things in JavaScript, but get over it.)  I needed a way to globally tell Angular that during testing I didn't want to use the normal $filter service when it came across it in my factory.  Fortunately, I also recently wrote a post on how to test the $mdSidenav service that's part of Angular Material. I put the two posts together and came up with a solution that works very well.

What I ultimately did was checked into the Angular source code for the $filter service (here) and found that it's pretty straightforward.  It just uses the $injector service to find the registered filter by name and return it.  So I mimicked that in my spy, except that I returned another spy when I came across the filter I wanted to test.  It sounds a bit confusing (even to me) writing it out so why don't you just check out the code below.  That should make more sense.

The code:

angular.module('app', []).factory('myFactory', function($filter) {
  var factory = {};

  factory.states = [
    { name: 'Alabama', id: 'AL' },
    { name: 'Alaskas', id: 'AK' },
    { name: 'Arizona', id: 'AZ' },
    { name: 'Arkansas', id: 'AR' }
  ];
  
  factory.sort = function() {
    return $filter('orderBy')(factory.states, 'id');
  };

  return factory;
});

The spec:

describe('test suite', function() {
  var myFactory, orderByFilterSpy, filterSpy;
  
  beforeEach(module('app'));
  
  beforeEach(module(function($provide, $injector){
    orderByFilterSpy = jasmine.createSpy('orderBy');
    filterSpy = jasmine.createSpy('$filter').and.callFake(function(name) {
      switch(name) {
        case 'orderBy':
          return orderByFilterSpy;
        default:
          return $injector.get(name + 'Filter');
      }
    });

    $provide.factory('$filter', function() {
      return filterSpy;
    });
  }));

  beforeEach(inject(function(_myFactory_) {
    myFactory = _myFactory_;
  });
  
  it('should call orderByFilter', function() {
    // arrange
    myFactory.states = [{id: 'AZ', name: 'Arizona'}, {id: 'AL', name: 'Alabama'}, {id: 'AK', name: 'Alaska'}, {id: 'AR', name: 'Arkansas'}];

    // act
    myFactory.sort();

    // assert
    expect(filterSpy).toHaveBeenCalledWith('orderBy');
    expect(orderByFilterSpy).toHaveBeenCalledWith([{id: 'AZ', name: 'Arizona'}, {id: 'AL', name: 'Alabama'}, {id: 'AK', name: 'Alaska'}, {id: 'AR', name: 'Arkansas'}], 'id');
  });
});

That's it!  As a little added bonus, any filters other than orderBy that happen to get called in my factory should get passed through (I haven't validated that part yet, but it looks like it would do that so I'm rolling with it).