In Angular, a controller is a JavaScript function(type/class) that is used to augment instances of angular Scope, excluding the root scope.

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.

NOTE: Many of the examples in the documentation show the creation of functions in the global scope. This is only for demonstration purposes - in a real application you should use the .controller method of your Angular module for your application as follows:

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

    myApp.controller('GreetingCtrl', ['$scope', function($scope) {
        $scope.greeting = 'Hola!';
    }]);

Note also that we use the array notation to explicitly specify the dependency of the controller on the $scope service provided by Angular.

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 are available in the template/view, and can be invoked via angular expressions and ng event handler directives (e.g. ngClick).

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 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.
Sharing stateless or stateful code across controllers — Use angular services instead.
Managing 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 implicitly via the ngController 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() {
   $scope.spice = 'chili';
 }
 $scope.jalapenoSpicy = function() {
  $scope.spice = 'jalapeño';
 }
}

Things to notice in the example above:

The ngController 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".
Assigning a property to $scope creates or updates the model.
Controller methods can be created through direct assignment to scope (the chiliSpicy method)
Both controller methods are available in the template (for the body element and and its children).
NB: Previous versions of Angular (pre 1.0 RC) allowed you to use this interchangeably with the $scope method, but this is no longer the case. Inside of methods defined on the scope this and $scope are interchangeable (angular sets this to $scope), but not otherwise inside your controller constructor.
NB: Previous versions of Angular (pre 1.0 RC) added prototype methods into the scope automatically, but this is no longer the case; all methods need to be added manually to the scope.

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) {
   $scope.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>
 </div>
</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 ngController 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

Although there are many ways to test a controller, one of the best conventions, shown below, involves injecting the $rootScope and $controller

Controller Function:

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

   $scope.spice = "habanero";
}

Controller Test:

describe('myController function', function() {

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

    beforeEach(inject(function($rootScope, $controller) {
      scope = $rootScope.$new();
      var ctrl = $controller(myController, {$scope: scope});
    }));

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

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

If you need to test a nested controller you need to create the same scope hierarchy in your test that exists in the DOM.

describe('state', function() {
    var mainScope, childScope, babyScope;

    beforeEach(inject(function($rootScope, $controller) {
        mainScope = $rootScope.$new();
        var mainCtrl = $controller(MainCtrl, {$scope: mainScope});
        childScope = mainScope.$new();
        var childCtrl = $controller(ChildCtrl, {$scope: childScope});
        babyScope = childScope.$new();
        var babyCtrl = $controller(BabyCtrl, {$scope: babyScope});
    }));

    it('should have over and selected', function() {
        expect(mainScope.timeOfDay).toBe('morning');
        expect(mainScope.name).toBe('Nikki');
        expect(childScope.timeOfDay).toBe('morning');
        expect(childScope.name).toBe('Mattie');
        expect(babyScope.timeOfDay).toBe('evening');
        expect(babyScope.name).toBe('Gingerbreak Baby');
    });
});