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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 in getting 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 (which you can mock out) and we created abstractions which allow you to sort your model without having to resort to manipulating the DOM. So that in the end, it is easy to write a sort function which sorts some data, so that your test can create a data set, apply the function, and assert that the resulting model is in the correct order. The test does not have to wait for the XHR response to arrive, create the right kind of test DOM, nor assert that your function has mutated the DOM in the right way.

## 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

There are several ways in which you can get a hold of a dependency. You can:

1. Create it using the `new` operator.
2. Look for it in a well-known place, also known as a global singleton.
3. Ask a registry (also known as service registry) for it. (But how do you get a hold of the registry? Most likely by looking it up in a well known place. See #2.)
4. Expect it to be handed to you.

Out of the four options in the list above, only the last one is testable. Let's look at why:

### Using the `new` operator

While there is nothing wrong with the `new` operator fundamentally, a problem arises when calling `new` on a constructor. This permanently binds the call site to the type. For example, lets say that we try to instantiate an `XHR` that will retrieve data from the server.

    function MyClass() {
      this.doWork = function() {
        var xhr = new XHR();
        xhr.open(method, url, true);
        xhr.onreadystatechange = function() {...}
        xhr.send();
      }
    }

A problem surfaces in tests when we would like to instantiate a `MockXHR` that would allow us to return fake data and simulate network failures. By calling `new XHR()` we are permanently bound to the actual XHR and there is no way to replace it. Yes, we could monkey patch, but that is a bad idea for many reasons which are outside the scope of this document.

Here's an example of how the class above becomes hard to test when resorting to monkey patching:

    var oldXHR = XHR;
    XHR = function MockXHR() {};
    var myClass = new MyClass();
    myClass.doWork();
    // assert that MockXHR got called with the right arguments
    XHR = oldXHR; // if you forget this bad things will happen

### Global look-up:

Another way to approach the problem is to look for the service in a well-known location.

    function MyClass() {
      this.doWork = function() {
        global.xhr({
          method:'...',
          url:'...',
          complete:function(response){ ... }
        })
      }
    }

While no new dependency instance is created, it is fundamentally the same as `new` in that no way exists to intercept the call to `global.xhr` for testing purposes, other than through monkey patching. The basic issue for testing is that a global variable needs to be mutated in order to replace it with call to a mock method. For further explanation of why this is bad see: Brittle Global State & Singletons

The class above is hard to test since we have to change the global state:

    var oldXHR = global.xhr;
    global.xhr = function mockXHR() {};
    var myClass = new MyClass();
    myClass.doWork();
    // assert that mockXHR got called with the right arguments
    global.xhr = oldXHR; // if you forget this bad things will happen

### Service Registry:

It may seem that this can be solved by having a registry of all the services and then having the tests replace the services as needed.

    function MyClass() {
      var serviceRegistry = ????;
      this.doWork = function() {
        var xhr = serviceRegistry.get('xhr');
        xhr({
          method:'...',
          url:'...',
          complete:function(response){ ... }
        })
    }

However, where does the serviceRegistry come from? If it is:

- `new`-ed up, the test has no chance to reset the services for testing.
- a global look-up then the service returned is global as well (but resetting is easier, since only one global variable exists to be reset).

The class above is hard to test since we have to change the global state:

    var oldServiceLocator = global.serviceLocator;
    global.serviceLocator.set('xhr', function mockXHR() {});
    var myClass = new MyClass();
    myClass.doWork();
    // assert that mockXHR got called with the right arguments
    global.serviceLocator = oldServiceLocator; // if you forget this bad things will happen

### Passing in Dependencies:

Last, the dependency can be passed in.

    function MyClass(xhr) {
      this.doWork = function() {
        xhr({
          method:'...',
          url:'...',
          complete:function(response){ ... }
        })
    }

This is the preferred method since the code makes no assumptions about the origin of `xhr` and cares instead about whoever created the class responsible for passing it in. Since the creator of the class should be different code than the user of the class, it separates the responsibility of creation from the logic. This is dependency-injection in a nutshell.

The class above is testable, since in the test we can write:

    function xhrMock(args) {...}
    var myClass = new MyClass(xhrMock);
    myClass.doWork();
    // assert that xhrMock got called with the right arguments

Notice that no global variables were harmed in the writing of this test.

Angular comes with dependency injection built-in, making the right thing easy to do, but you still need to do it if you wish to take advantage of the testability story.

## Controllers

What makes each application unique is its logic, and the logic is what we would like to test. If the logic for your application contains DOM manipulation, it will be hard to test. See the example below:

    function PasswordCtrl() {
      // get references to DOM elements
      var msg = $('.ex1 span');
      var input = $('.ex1 input');
      var strength;

      this.grade = function() {
        msg.removeClass(strength);
        var pwd = input.val();
        password.text(pwd);
        if (pwd.length > 8) {
          strength = 'strong';
        } else if (pwd.length > 3) {
          strength = 'medium';
        } else {
          strength = 'weak';
        }
        msg
         .addClass(strength)
         .text(strength);
      }
    }

The code above is problematic from a testability point of view since it requires your test to have the right kind of DOM present when the code executes. The test would look like this:

    var input = $('<input type="text"/>');
    var span = $('<span>');
    $('body').html('<div class="ex1">')
      .find('div')
        .append(input)
        .append(span);
    var pc = new PasswordCtrl();
    input.val('abc');
    pc.grade();
    expect(span.text()).toEqual('weak');
    $('body').empty();

In angular the controllers are strictly separated from the DOM manipulation logic and this results in a much easier testability story as the following example shows:

    function PasswordCtrl($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';
        }
      };
    }

and the test is straight forward:

    var $scope = {};
    var pc = $controller('PasswordCtrl', { $scope: $scope });
    $scope.password = 'abc';
    $scope.grade();
    expect($scope.strength).toEqual('weak');

Notice that the test is not only much shorter, it is also easier to follow what is happening. We say that such a test tells a story, rather than asserting random bits which don't seem to be related.

## 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;
      }
    });

    var length = $filter('length');
    expect(length(null)).toEqual(0);
    expect(length('abc')).toEqual(3);

## 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;
        var $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.

### 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.