# Developer Guide: About MVC in Angular: Understanding the Controller Component

In angular, a controller is a JavaScript function(type/class) that is used to augment instances of angular Scope, excluding the root scope. When you or angular create a new child scope object via the `scope.$new` API , there is an option to pass in a controller as a method argument. This will tell angular to associate the controller with the new scope and to augment its behavior.

Use controllers to:

- Set up the initial state of a scope object.
- Add behavior to the scope object.

## Setting up the initial state of a scope object

Typically, when you create an application you need to set up an initial state for an angular scope.

Angular applies (in the sense of JavaScript's `Function#apply`) the controller constructor function to a new angular scope object, which sets up an initial scope state. This means that angular never creates instances of the controller type (by invoking the `new` operator on the controller constructor). Constructors are always applied to an existing scope object.

You set up the initial state of a scope by creating model properties. For example:

function GreetingCtrl($scope) { $scope.greeting = 'Hola!'; }

The `GreetingCtrl` controller creates a `greeting` model which can be referred to in a template.

When a controller function is applied to an angular scope object, the `this` of the controller function becomes the scope of the angular scope object, so any assignment to `this` within the controller function happens on the angular scope object.

## Adding Behavior to a Scope Object

Behavior on an angular scope object is in the form of scope method properties available to the template/view. This behavior interacts with and modifies the application model.

As discussed in the Model section of this guide, any objects (or primitives) assigned to the scope become model properties. Any functions assigned to the scope, along with any prototype methods of the controller type, become functions available in the template/view, and can be invoked via angular expressions and `ng-` event handlers (e.g. `ng-click`). These controller methods are always evaluated within the context of the angular scope object that the controller function was applied to (which means that the `this` keyword of any controller method is always bound to the scope that the controller augments). This is how the second task of adding behavior to the scope is accomplished.

## Using Controllers Correctly

In general, a controller shouldn't try to do too much. It should contain only the business logic needed for a single view.

The most common way to keep controllers slim is by encapsulating work that doesn't belong to controllers into services and then using these services in controllers via dependency injection. This is discussed in the Dependency Injection Services sections of this guide.

Do not use controllers for:

- Any kind of DOM manipulation — Controllers should contain only business logic. DOM manipulation—the presentation logic of an application—is well known for being hard to test. Putting any presentation logic into controllers significantly affects testability of the business logic. Angular offers dev_guide.templates.databinding for automatic DOM manipulation. If you have to perform your own manual DOM manipulation, encapsulate the presentation logic in `directives`.
- Input formatting — Use angular form controls instead.
- Output filtering — Use angular filters instead.
- Run stateless or stateful code shared across controllers — Use angular services instead.
- Instantiate or manage the life-cycle of other components (for example, to create service instances).

## Associating Controllers with Angular Scope Objects

You can associate controllers with scope objects explicitly via the `scope.$new` api or implicitly via the `ng-controller directive` or `$route service`.

### Controller Constructor and Methods Example

To illustrate how the controller component works in angular, let's create a little app with the following components:

- A template with two buttons and a simple message
- A model consisting of a string named `spice`
- A controller with two functions that set the value of `spice`

The message in our template contains a binding to the `spice` model, which by default is set to the string "very". Depending on which button is clicked, the `spice` model is set to `chili` or `jalapeño`, and the message is automatically updated by data-binding.

### A Spicy Controller Example

    <body ng-controller="SpicyCtrl">
     <button ng-click="chiliSpicy()">Chili</button>
     <button ng-click="jalapenoSpicy()">Jalapeño</button>
     <p>The food is {{spice}} spicy!</p>
    </body>

    function SpicyCtrl($scope) {
     $scope.spice = 'very';
     $scope.chiliSpicy = function() {
       this.spice = 'chili';
     }
    }

    SpicyCtrl.prototype.jalapenoSpicy = function() {
     this.spice = 'jalapeño';
    }

Things to notice in the example above:

- The `ng-controller` directive is used to (implicitly) create a scope for our template, and the scope is augmented (managed) by the `SpicyCtrl` controller.
- `SpicyCtrl` is just a plain JavaScript function. As an (optional) naming convention the name starts with capital letter and ends with "Ctrl" or "Controller".
- The JavaScript keyword `this` in the `SpicyCtrl` function is bound to the scope that the controller augments.
- Assigning a property to `this` creates or updates the model.
- Controller methods can be created through direct assignment to scope (the `chiliSpicy` method) or as prototype methods of the controller constructor function(the `jalapenoSpicy` method)
- Both controller methods are available in the template (for the `body` element and and its children).

Controller methods can also take arguments, as demonstrated in the following variation of the previous example.

### Controller Method Arguments Example

    <body ng-controller="SpicyCtrl">
     <input ng-model="customSpice" value="wasabi">
     <button ng-click="spicy('chili')">Chili</button>
     <button ng-click="spicy(customSpice)">Custom spice</button>
     <p>The food is {{spice}} spicy!</p>
    </body>

    function SpicyCtrl($scope) {
     $scope.spice = 'very';
     $scope.spicy = function(spice) {
       this.spice = spice;
     }
    }

Notice that the `SpicyCtrl` controller now defines just one method called `spicy`, which takes one argument called `spice`. The template then refers to this controller method and passes in a string constant `'chili'` in the binding for the first button and a model property `spice` (bound to an input box) in the second button.

### Controller Inheritance Example

Controller inheritance in angular is based on `Scope` inheritance. Let's have a look at an example:

    <body ng-controller="MainCtrl">
     <p>Good {{timeOfDay}}, {{name}}!</p>
     <div ng-controller="ChildCtrl">
       <p>Good {{timeOfDay}}, {{name}}!</p>
       <p ng-controller="BabyCtrl">Good {{timeOfDay}}, {{name}}!</p>
    </body>

    function MainCtrl($scope) {
     $scope.timeOfDay = 'morning';
     $scope.name = 'Nikki';
    }

    function ChildCtrl($scope) {
     $scope.name = 'Mattie';
    }

    function BabyCtrl($scope) {
     $scope.timeOfDay = 'evening';
     $scope.name = 'Gingerbreak Baby';
    }

Notice how we nested three `ng-controller` directives in our template. This template construct will result in 4 scopes being created for our view:

- The root scope
- The `MainCtrl` scope, which contains `timeOfDay` and `name` models
- The `ChildCtrl` scope, which shadows the `name` model from the previous scope and inherits the `timeOfDay` model
- The `BabyCtrl` scope, which shadows both the `timeOfDay` model defined in `MainCtrl` and `name` model defined in the ChildCtrl

Inheritance works between controllers in the same way as it does with models. So in our previous examples, all of the models could be replaced with controller methods that return string values.

Note: Standard prototypical inheritance between two controllers doesn't work as one might expect, because as we mentioned earlier, controllers are not instantiated directly by angular, but rather are applied to the scope object.

### Testing Controllers

The way to test a controller depends upon how complicated the controller is.

- If your controller doesn't use DI or scope methods — create the controller with the `new` operator and test away. For example:

Controller Function:

    function myController() {
       this.spices = [{"name":"pasilla", "spiciness":"mild"},
                      {"name":"jalapeno", "spiceiness":"hot hot hot!"},
                      {"name":"habanero", "spiceness":"LAVA HOT!!"}];

       this.spice = "habanero";
    }

Controller Test:

    describe('myController function', function() {

      describe('myController', function() {
        var ctrl;

        beforeEach(function() {
          ctrl = new myController();
        });

        it('should create "spices" model with 3 spices', function() {
          expect(ctrl.spices.length).toBe(3);
        });

        it('should set the default value of spice', function() {
          expect(ctrl.spice).toBe('habanero');
        });
      });
    });

- If your controller does use DI or scope methods — create a root scope, then create the controller in the root scope with `scope.$new(MyController)`. Test the controller using `$eval`, if necessary.
- If you need to test a nested controller that depends on its parent's state — create a root scope, create a parent scope, create a child scope, and test the controller using $eval if necessary.

### Related Topics

- About MVC in Angular
- Understanding the Model Component
- Understanding the View Component