Bradley Braithwaite
1 import {
2 Learning,
3 JavaScript,
4 AngularJS
5 } from 'bradoncode.com'
6 |

ngMock Fundamentals for AngularJS - Understanding Inject

Final part of a two part series taking a deeper dive into the two key concepts of ngMock for Unit Testing - Module and Inject.

on
on javascript, angularjs, testing, ngmock

This is the final post of a two part series that takes a deeper dive into two main concepts from ngMock:

In the previous post, we looked at the angular.mock.module. In this post we will take a look into the angular.mock.inject funtionality and detail how this works in conjunction with angular.mock.module, and the Angular framework itself.

In the last post we established how we could load modules into unit tests via the mgMock functionality. Next we need to fill in the blanks as to what we can assert in the tests and how we can go about obtaining instances of the things we wish to test. As a reminder, here’s an example test from the last post:

describe('string alias arg', function () {
  it('should load module with string alias', function () {
    angular.mock.module('products');

    // TBC - we will build upon this!
    expect('what?').toBe('what?');
  });
});

To fill in the blanks, let’s use the following module and simple service as an example. How would we go about unit testing this?

// create a new module
angular.module('products', []);

// register the service
angular.module('products').service('CategoryService', function CategoryService() {
  return {
    getCategories: function() {
      return { 1: 'Beverages', 2: 'Condiments' };
    }
  }; 
});

In the last post we covered three ways of registering modules via mock.module. It’s important to keep these in mind, since this post will explain how these concepts link together in testing elements of an application in isolation. They are:

  1. string
  2. function()
  3. Object

Using the String alias form to Register Modules

Starting with the string form of registering a module, let’s update our test as follows:

describe('products category service', function () {
  it('should return the expected categories', function () {
    // registers module by its string alias
    angular.mock.module('products');

    // Where does this instance come from?
    var categories = CategoryService.getCategories();

    expect(categories).toEqual({ 1: 'Beverages', 2: 'Condiments' });
  });
});

The assertion is straight forward, we call the getCategories function from the service and check that the object returned is what we expect. The unanswered question is, how can we get hold of an instance of the CategoryService? This is where the angular.mock.inject functionality helps us.

Here’s the same test with the angular.mock.inject functionality being used:

describe('products category service', function () {
  it('should return the expected categories', function () {
    angular.mock.module('products');

    var service;
    
    // Get the service from the injector
    angular.mock.inject(function GetDependencies(CategoryService) {
      service = CategoryService;
    });

    // call the function on our service instance
    var categories = service.getCategories();

    expect(categories).toEqual({ 1: 'Beverages', 2: 'Condiments' });
  });
});

What does inject do? At a high level, the mock.inject function acts a wrapper for the angular injector. The angular injector acts as a service locator for our application.

The mock.module and mock.injector we just saw work together as follows:

  1. We registered the products module via mock.module.
  2. We called the inject function, passing a function with an argument called CategoryService.
  3. The injector looks for an object within the loaded module(s) (products, in our example) for something called CategoryService.
  4. It finds the CategoryService within the products module, and passes an instance to our GetDependencies function.
  5. We can use this instance for our tests.

Taking our example further, let’s introduce another dependency to the products module:

// create a new module
angular.module('products', []);

// register the service
angular.module('products').service('CategoryService', function CategoryService() {
  return {
    getCategories: function() {
      return { 1: 'Beverages', 2: 'Condiments' };
    }
  }; 
});

// register a service. **This has a dependency on the CategoryService**
angular.module('products').service('ProductsService', function ProductService (CategoryService) {
  return {
    getProducts: function () {
      var product1 = { name: 'Chai', categoryId: 1 };
      var product2 = { name: 'Aniseed Syrup', categoryId: 2 };
      var products = [product1, product2];

      var categories = CategoryService.getCategories();

      products.forEach(function (p) {
        // append the category name for the category service to each product.
        p.categoryName = categories[p.categoryId];
      });

      return products;
    }
  };
});

This new ProductsService relies on the new CategoryService to get the category names.

How would we write a test for this new service given the extra dependency? Here’s the updated test code:

describe('products service tests', function () {
  
  // Note that we can move the call to module in the beforeEach block,
  // thus making it available for each test, keeping our tests DRY.
  beforeEach(angular.mock.module('products'));

  it('should append category names to products', function () {
    var service;
    
    // Get the service from the injector
    angular.mock.inject(function GetDependencies(ProductsService) {
      service = ProductsService;
    });

    var products = service.getProducts();
    expect(products[0].categoryName).toBe('Beverages');
    expect(products[1].categoryName).toBe('Condiments');
  });
});

The key point with this second test, is that the call to inject for the ProductsService also resolved the reference to the CategoryService that is used by the ProductsService itself. This is down to the magic of the angular injector.

Using the Object form to Register Modules

Unit testing means testing in isolation, so given that the test is for the ProductsService, we should be mocking the CategoryService that it uses. How can we do this? In this test we attach our own instance of the CategoryService service to the injector. Note the usage of the Object argument when registering the module via mock.module:

it('should append category names to products', function () {

  // Note that this version of the CategoryService overrides the version we added to the products module.
  angular.mock.module({
    'CategoryService': { 
      getCategories: function() { 
        return { 1: 'Electronics', 2: 'DVDs' }; 
      } 
    }
  });

  var service;
  
  angular.mock.inject(function GetDependencies(ProductsService) {
    service = ProductsService;
  });

  var products = service.getProducts();
  expect(products[0].categoryName).toBe('Electronics');
  expect(products[1].categoryName).toBe('DVDs');
});

The result of this test demonstrates that we are using this new version of CategoryService and not the actual implementation as we saw with previous tests. This is a trivial example, but it demonstrates the concept simply.

The key point of this test, is that we can have multiple modules within an app but there is only one injector. This means that names of services can clash, even if they are in different modules! But of course it also allows us to override previously registered components with our own versions for testing.

Another point is that order matters. The products module had already been registered in the beforeEach block that we introduced in the test before last. Our mock service took precedence as it was registered after the products module.

Using the Anonymous Function form to Register Modules

Now let’s introduce a controller that has a dependency on the ProductsService:

angular.module('products').controller('ProductsController', function ProductsController($scope, ProductsService) {
  $scope.products = [];
  $scope.load = function() {
    $scope.products = ProductsService.getProducts();
  };
});

How can we test this? We need to mock the $scope and ProductsService arguments, and then obtain an instance of the ProductsController. Here’s the code:

describe('products module', function () {

  beforeEach(module('products'));

  beforeEach(angular.mock.module({
    'ProductsService': { 
      getProducts: function() { 
        return [{ name: 'Aniseed Syrup', categoryId: 2, categoryName: 'Condiments' }]; 
      }
    }
  }));

  var $controller;

  beforeEach(inject(function(_$controller_){
    $controller = _$controller_;
  }));

  describe('load', function () {
    it('products should be loaded to scope', function () {
      var $scope = {};
      var controller = $controller('ProductsController', { $scope: $scope });
      $scope.load();
      expect($scope.products).toEqual([{ name: 'Aniseed Syrup', categoryId: 2, categoryName: 'Condiments' }]);
    }); 

    it('products should default to empty array', function () {
      var $scope = {};
      var controller = $controller('ProductsController', { $scope: $scope });
      expect($scope.products).toEqual([]);
    });
  });

});

This first thing to note, is that the mechanism for injecting controllers is different from the method we’ve seen thus far. This is due to the fact that angular works differently under the covers when registering controllers.

We make use of the $controller decorator from ngMock. We can get an instance of the controller by passing its name to the $controller function instance. As a side note, if you noticed the use of underscores for the $controller function argument, it’s a convention in angular that allows the use of underscores for the function arguments to inject. The injector unwraps the underscores thus allowing us to use the literal $controller for the variable assignment inside the function.

Secondly, note that we can easily provide a mock $scope argument, since it’s a simple JavaScript object.

Lastly, as we registered a test version of the ProductsService before calling inject, the injector uses this version for the ProductController dependency.

If you recall from the previous post, we noted that the difference between the Object and function() method of creating an anonymous module, is that the Object cannot take any dependencies. This trivial example demonstrates this:

describe('products module', function () {

  // register the existing module from our app
  beforeEach(module('products'));

  describe('load', function () {

    it('products should be loaded to scope', function () {
    
      // Create an anonymous module with Object argument
      // NB this overrides the CategoryService from our products module
      angular.mock.module({
        'CategoryService': { 
          getCategories: function() { 
            return { 1: 'Electronics', 2: 'DVDs' }; 
          } 
        }
      });

      // Create an anonymous module with function() argument
      // NB this overrides the ProductsService from our products module and
      // will also use our previously mocked CategoryService.
      angular.mock.module(function($provide) { 
        $provide.service('ProductsService', function(CategoryService) { 
          return {
            getProducts: function() {
              var mockCategories = CategoryService.getCategories();
              return [{ name: 'Aniseed Syrup', categoryId: 2, categoryName: mockCategories[2] }];
            }
          }; 
        });
      });

      var $controller;

      angular.mock.inject(function(_$controller_){
        $controller = _$controller_;
      });

      var $scope = {};
      var controller = $controller('ProductsController', { $scope: $scope });
      $scope.load();
      expect($scope.products).toEqual([{ name: 'Aniseed Syrup', categoryId: 2, categoryName: 'DVDs' }]);
    }); 
  });

});

As noted, this is a trivial example, but it shows how our mock ProductService can make use of the mock CategoryService.

Taking a Dive into the Source Code

Now that we have connected the dots between registering modules in tests via angular.mock.module and calling angular.mock.inject, we can take a deeper look at the source code.

Below is a version of the angular.mock.inject function with some elements removed for brevity:

window.inject = angular.mock.inject = function() {
  var blockFns = Array.prototype.slice.call(arguments, 0);
  return isSpecRunning() ? workFn.call(currentSpec) : workFn;
  /////////////////////
  function workFn() {
    var modules = currentSpec.$modules || [];
    modules.unshift('ngMock');
    modules.unshift('ng');
    var injector = currentSpec.$injector = angular.injector(modules, strictDi);
    for (var i = 0, ii = blockFns.length; i < ii; i++) {
      injector.invoke(blockFns[i] || angular.noop, this);
    }
  }
};

You can see the full source code of the function here.

This process is like the ngApp and/or bootstrap functionality for starting an angular application from the browser.

What this code does, is:

ADD ng, ngMock to the front of any existing modules registered in our tests
SET injector = call to angular.injector(modules)
FOR EACH fn in arguments array:
  call injector.invoke(fn)

The angular injector is a complex concept, but to explain it briefly, it takes all services, controllers etc from all modules that make up an app and holds them in a single injector container. When we ask for an instance of a service, controller etc we must ask the injector. Since the injector knows about all the components that make up our app, if we ask for a service that has a dependency on a different service, the injector will resolve that dependency for us.

The injector acts as a wrapper for the provider service, which we cannot access directly, we have to use the injector’s API for this from the angular application code.

In our tests however, we can access the $provide service, that the injector calls. For example:

angular.mock.module(function($provide) { 
  $provide.service('FooService', function() { 
    return { ... } 
  });
});

The call to injector.invoke in the source code takes our function argument to mock.inject, passes it through the injector and along the way any arguments to that function which match the name of a service registered to the injector will be populated.

A call to mock inject:

angular.mock.inject(function(MyService){
  service = MyService;
});

Would make an equivalent call to the angular injector:

injector.invoke(function(MyService){
  service = MyService;
});

Conclusion

This post concludes a first look at the ngMock fundamentals giving us a solid foundation to build upon. Expect a lot more to come relating to testing with AngularJS.

SHARE
Don't miss out on the free technical content:

Subscribe to Updates

CONNECT WITH BRADLEY

Bradley Braithwaite Software Blog Bradley Braithwaite is a software engineer who works for the search engine duckduckgo.com. He is a published author at pluralsight.com. He writes about software development practices, JavaScript, AngularJS and Node.js via his website . Find out more about Brad. Find him via:
You might also like:
mean stack tutorial AngularJS Testing - Unit Testing Tutorials