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JavaScript is a dynamically typed language which comes with great power of expression, but it also comes with almost no help from the compiler. For this reason we feel very strongly that any code written in JavaScript needs to come with a strong set of tests. We have built many features into Angular which makes testing your Angular applications easy. So there is no excuse for not testing.

## Separation of Concerns

Unit testing, as the name implies, is about testing individual units of code. Unit tests try to answer questions such as "Did I think about the logic correctly?" or "Does the sort function order the list in the right order?"

In order to answer such a question it is very important that we can isolate the unit of code under test. That is because when we are testing the sort function we don't want to be forced into creating related pieces such as the DOM elements, or making any XHR calls to fetch the data to sort.

While this may seem obvious it can be very difficult to call an individual function on a typical project. The reason is that the developers often mix concerns resulting in a piece of code which does everything. It makes an XHR request, it sorts the response data and then it manipulates the DOM.

With Angular we try to make it easy for you to do the right thing, and so we provide dependency injection for your XHR requests, which can be mocked, and we provide abstractions which allow you to test your model without having to resort to manipulating the DOM. The test can then assert that the data has been sorted without having to create or look at the state of the DOM or wait for any XHR requests to return data. The individual sort function can be tested in isolation.

## With great power comes great responsibility

Angular is written with testability in mind, but it still requires that you do the right thing. We tried to make the right thing easy, but if you ignore these guidelines you may end up with an untestable application.

## Dependency Injection

Angular comes with dependency injection built-in, which makes testing components much easier, because you can pass in a component's dependencies and stub or mock them as you wish.

Components that have their dependencies injected allow them to be easily mocked on a test by test basis, without having to mess with any global variables that could inadvertently affect another test.

## Additional tools for testing Angular applications

For testing Angular applications there are certain tools that you should use that will make testing much easier to set up and run.

### Karma

Karma is a JavaScript command line tool that can be used to spawn a web server which loads your application's source code and executes your tests. You can configure Karma to run against a number of browsers, which is useful for being confident that your application works on all browsers you need to support. Karma is executed on the command line and will display the results of your tests on the command line once they have run in the browser.

Karma is a NodeJS application, and should be installed through npm. Full installation instructions are available on the Karma website.

### Jasmine

Jasmine is a test driven development framework for JavaScript that has become the most popular choice for testing Angular applications. Jasmine provides functions to help with structuring your tests and also making assertions. As your tests grow, keeping them well structured and documented is vital, and Jasmine helps achieve this.

In Jasmine we use the `describe` function to group our tests together:

    describe("sorting the list of users", function() {
      // individual tests go here
    });

And then each individual test is defined within a call to the `it` function:

    describe('sorting the list of users', function() {
      it('sorts in descending order by default', function() {
        // your test assertion goes here
      });
    });

Grouping related tests within `describe` blocks and describing each individual test within an `it` call keeps your tests self documenting.

Finally, Jasmine provides matchers which let you make assertions:

    describe('sorting the list of users', function() {
      it('sorts in descending order by default', function() {
        var users = ['jack', 'igor', 'jeff'];
        var sorted = sortUsers(users);
        expect(sorted).toEqual(['jeff', 'jack', 'igor']);
      });
    });

Jasmine comes with a number of matchers that help you make a variety of assertions. You should read the Jasmine documentation to see what they are. To use Jasmine with Karma, we use the karma-jasmine test runner.

### angular-mocks

Angular also provides the `ngMock` module, which provides mocking for your tests. This is used to inject and mock Angular services within unit tests. In addition, it is able to extend other modules so they are synchronous. Having tests synchronous keeps them much cleaner and easier to work with. One of the most useful parts of ngMock is `$httpBackend`, which lets us mock XHR requests in tests, and return sample data instead.

## Testing a Controller

Because Angular separates logic from the view layer, it keeps controllers easy to test. Let's take a look at how we might test the controller below, which provides `$scope.grade`, which sets a property on the scope based on the length of the password.

    angular.module('app', [])
    .controller('PasswordController', function PasswordController($scope) {
      $scope.password = '';
      $scope.grade = function() {
        var size = $scope.password.length;
        if (size > 8) {
          $scope.strength = 'strong';
        } else if (size > 3) {
          $scope.strength = 'medium';
        } else {
          $scope.strength = 'weak';
        }
      };
    });

Because controllers are not available on the global scope, we need to use `angular.mock.inject` to inject our controller first. The first step is to use the `module` function, which is provided by angular-mocks. This loads in the module it's given, so it is available in your tests. We pass this into `beforeEach`, which is a function Jasmine provides that lets us run code before each test. Then we can use `inject` to access `$controller`, the service that is responsible for instantiating controllers.

    describe('PasswordController', function() {
      beforeEach(module('app'));

      var $controller;

      beforeEach(inject(function(_$controller_){
        // The injector unwraps the underscores (_) from around the parameter names when matching
        $controller = _$controller_;
      }));

      describe('$scope.grade', function() {
        it('sets the strength to "strong" if the password length is >8 chars', function() {
          var $scope = {};
          var controller = $controller('PasswordController', { $scope: $scope });
          $scope.password = 'longerthaneightchars';
          $scope.grade();
          expect($scope.strength).toEqual('strong');
        });
      });
    });

Notice how by nesting the `describe` calls and being descriptive when calling them with strings, the test is very clear. It documents exactly what it is testing, and at a glance you can quickly see what is happening. Now let's add the test for when the password is less than three characters, which should see `$scope.strength` set to "weak":

    describe('PasswordController', function() {
      beforeEach(module('app'));

      var $controller;

      beforeEach(inject(function(_$controller_){
        // The injector unwraps the underscores (_) from around the parameter names when matching
        $controller = _$controller_;
      }));

      describe('$scope.grade', function() {
        it('sets the strength to "strong" if the password length is >8 chars', function() {
          var $scope = {};
          var controller = $controller('PasswordController', { $scope: $scope });
          $scope.password = 'longerthaneightchars';
          $scope.grade();
          expect($scope.strength).toEqual('strong');
        });

        it('sets the strength to "weak" if the password length <3 chars', function() {
          var $scope = {};
          var controller = $controller('PasswordController', { $scope: $scope });
          $scope.password = 'a';
          $scope.grade();
          expect($scope.strength).toEqual('weak');
        });
      });
    });

Now we have two tests, but notice the duplication between the tests. Both have to create the `$scope` variable and create the controller. As we add new tests, this duplication is only going to get worse. Thankfully, Jasmine provides `beforeEach`, which lets us run a function before each individual test. Let's see how that would tidy up our tests:

    describe('PasswordController', function() {
      beforeEach(module('app'));

      var $controller;

      beforeEach(inject(function(_$controller_){
        // The injector unwraps the underscores (_) from around the parameter names when matching
        $controller = _$controller_;
      }));

      describe('$scope.grade', function() {
        var $scope, controller;

        beforeEach(function() {
          $scope = {};
          controller = $controller('PasswordController', { $scope: $scope });
        });

        it('sets the strength to "strong" if the password length is >8 chars', function() {
          $scope.password = 'longerthaneightchars';
          $scope.grade();
          expect($scope.strength).toEqual('strong');
        });

        it('sets the strength to "weak" if the password length <3 chars', function() {
          $scope.password = 'a';
          $scope.grade();
          expect($scope.strength).toEqual('weak');
        });
      });
    });

We've moved the duplication out and into the `beforeEach` block. Each individual test now only contains the code specific to that test, and not code that is general across all tests. As you expand your tests, keep an eye out for locations where you can use `beforeEach` to tidy up tests. `beforeEach` isn't the only function of this sort that Jasmine provides, and the documentation lists the others.

## Testing Filters

Filters are functions which transform the data into a user readable format. They are important because they remove the formatting responsibility from the application logic, further simplifying the application logic.

    myModule.filter('length', function() {
      return function(text) {
        return ('' + (text || '')).length;
      }
    });

    describe('length filter', function() {
      it('returns 0 when given null', function() {
        var length = $filter('length');
        expect(length(null)).toEqual(0);
      });

      it('returns the correct value when given a string of chars', function() {
        var length = $filter('length');
        expect(length('abc')).toEqual(3);
      });
    });

## Testing Directives

Directives in angular are responsible for encapsulating complex functionality within custom HTML tags, attributes, classes or comments. Unit tests are very important for directives because the components you create with directives may be used throughout your application and in many different contexts.

### Simple HTML Element Directive

Let's start with an angular app with no dependencies.

    var app = angular.module('myApp', []);

Now we can add a directive to our app.

    app.directive('aGreatEye', function () {
        return {
            restrict: 'E',
            replace: true,
            template: '<h1>lidless, wreathed in flame, {{1 + 1}} times</h1>'
        };
    });

This directive is used as a tag `<a-great-eye></a-great-eye>`. It replaces the entire tag with the template `<h1>lidless, wreathed in flame, {{1 + 1}} times</h1>`. Now we are going to write a jasmine unit test to verify this functionality. Note that the expression `{{1 + 1}}` times will also be evaluated in the rendered content.

    describe('Unit testing great quotes', function() {
      var $compile,
          $rootScope;

      // Load the myApp module, which contains the directive
      beforeEach(module('myApp'));

      // Store references to $rootScope and $compile
      // so they are available to all tests in this describe block
      beforeEach(inject(function(_$compile_, _$rootScope_){
        // The injector unwraps the underscores (_) from around the parameter names when matching
        $compile = _$compile_;
        $rootScope = _$rootScope_;
      }));

      it('Replaces the element with the appropriate content', function() {
        // Compile a piece of HTML containing the directive
        var element = $compile("<a-great-eye></a-great-eye>")($rootScope);
        // fire all the watches, so the scope expression {{1 + 1}} will be evaluated
        $rootScope.$digest();
        // Check that the compiled element contains the templated content
        expect(element.html()).toContain("lidless, wreathed in flame, 2 times");
      });
    });

We inject the $compile service and $rootScope before each jasmine test. The $compile service is used to render the aGreatEye directive. After rendering the directive we ensure that the directive has replaced the content and "lidless, wreathed in flame, 2 times" is present.

Underscore notation: The use of the underscore notation (e.g.: `_$rootScope_`) is a convention wide spread in AngularJS community to keep the variable names clean in your tests. That's why the `$injector` strips out the leading and the trailing underscores when matching the parameters. The underscore rule applies only if the name starts and ends with exactly one underscore, otherwise no replacing happens.

### Testing Transclusion Directives

Directives that use transclusion are treated specially by the compiler. Before their compile function is called, the contents of the directive's element are removed from the element and provided via a transclusion function. The directive's template is then appended to the directive's element, to which it can then insert the transcluded content into its template.

Before compilation:

    <div translude-directive>
      Some transcluded content
    </div>

After transclusion extraction:

    <div transclude-directive></div>

After compilation:

    <div transclude-directive>
      Some Template
      <span ng-transclude>Some transcluded content</span>
    </div>

If the directive is using 'element' transclusion, the compiler will actually remove the directive's entire element from the DOM and replace it with a comment node. The compiler then inserts the directive's template "after" this comment node, as a sibling.

Before compilation

    <div element-transclude>
      Some Content
    </div>

After transclusion extraction

    <!-- elementTransclude -->

After compilation:

    <!-- elementTransclude -->
    <div element-transclude>
      Some Template
      <span ng-transclude>Some transcluded content</span>
    </div>

It is important to be aware of this when writing tests for directives that use 'element' transclusion. If you place the directive on the root element of the DOM fragment that you pass to `$compile`, then the DOM node returned from the linking function will be the comment node and you will lose the ability to access the template and transcluded content.

    var node = $compile('<div element-transclude></div>')($rootScope);
    expect(node[0].nodeType).toEqual(node.COMMENT_NODE);
    expect(node[1]).toBeUndefined();

To cope with this you simply ensure that your 'element' transclude directive is wrapped in an element, such as a `<div>`.

    var node = $compile('<div><div element-transclude></div></div>')($rootScope);
    var contents = node.contents();
    expect(contents[0].nodeType).toEqual(node.COMMENT_NODE);
    expect(contents[1].nodeType).toEqual(node.ELEMENT_NODE);

### Testing Directives With External Templates

If your directive uses `templateUrl`, consider using karma-ng-html2js-preprocessor to pre-compile HTML templates and thus avoid having to load them over HTTP during test execution. Otherwise you may run into issues if the test directory hierarchy differs from the application's.

## Sample project

See the angular-seed project for an example.