@ngdoc overview
@name  Modules
@sortOrder 320
@description

# What is a Module?

You can think of a module as a container for the different parts of your app – controllers,
services, filters, directives, etc.

# Why?

Most applications have a main method that instantiates and wires together the different parts of
the application.

Angular apps don't have a main method. Instead modules declaratively specify how an application
should be bootstrapped. There are several advantages to this approach:

  * The declarative process is easier to understand.
  * You can package code as reusable modules.
  * The modules can be loaded in any order (or even in parallel) because modules delay execution.
  * Unit tests only have to load relevant modules, which keeps them fast.
  * End-to-end tests can use modules to override configuration.


# The Basics

I'm in a hurry. How do I get a Hello World module working?

<example ng-app-included="true">
  <file name="index.html">
    <div ng-app="myApp">
      <div>
        {{ 'World' | greet }}
      </div>
    </div>
  </file>

  <file name="script.js">
    // declare a module
    var myAppModule = angular.module('myApp', []);

    // configure the module.
    // in this example we will create a greeting filter
    myAppModule.filter('greet', function() {
     return function(name) {
        return 'Hello, ' + name + '!';
      };
    });
  </file>

  <file name="protractor.js" type="protractor">
    it('should add Hello to the name', function() {
      expect(element(by.binding("'World' | greet")).getText()).toEqual('Hello, World!');
    });
  </file>
</example>

Important things to notice:

  * The {@link angular.Module Module} API
  * The reference to `myApp` module in `<div ng-app="myApp">`.
    This is what bootstraps the app using your module.
  * The empty array in `angular.module('myApp', [])`.
    This array is the list of modules `myApp` depends on.


# Recommended Setup

While the example above is simple, it will not scale to large applications. Instead we recommend
that you break your application to multiple modules like this:

  * A module for each feature
  * A module for each reusable component (especially directives and filters)
  * And an application level module which depends on the above modules and contains any
    initialization code.

We've also
[written a document](http://blog.angularjs.org/2014/02/an-angularjs-style-guide-and-best.html)
on how we organize large apps at Google.

The above is a suggestion. Tailor it to your needs.

<example module='xmpl'>
  <file name="index.html">
    <div ng-controller="XmplController">
      {{ greeting }}
    </div>
  </file>

  <file name="script.js">
    angular.module('xmpl.service', [])

      .value('greeter', {
        salutation: 'Hello',
        localize: function(localization) {
          this.salutation = localization.salutation;
        },
        greet: function(name) {
          return this.salutation + ' ' + name + '!';
        }
      })

      .value('user', {
        load: function(name) {
          this.name = name;
        }
      });

    angular.module('xmpl.directive', []);

    angular.module('xmpl.filter', []);

    angular.module('xmpl', ['xmpl.service', 'xmpl.directive', 'xmpl.filter'])

      .run(function(greeter, user) {
        // This is effectively part of the main method initialization code
        greeter.localize({
          salutation: 'Bonjour'
        });
        user.load('World');
      })

      .controller('XmplController', function($scope, greeter, user){
        $scope.greeting = greeter.greet(user.name);
      });

  </file>

  <file name="protractor.js" type="protractor">
    it('should add Hello to the name', function() {
      expect(element(by.binding("greeting")).getText()).toEqual('Bonjour World!');
    });
  </file>

</example>



# Module Loading & Dependencies

A module is a collection of configuration and run blocks which get applied to the application
during the bootstrap process. In its simplest form the module consist of a collection of two kinds
of blocks:

  1. **Configuration blocks** - get executed during the provider registrations and configuration
    phase. Only providers and constants can be injected into configuration blocks. This is to
    prevent accidental instantiation of services before they have been fully configured.
  2. **Run blocks** - get executed after the injector is created and are used to kickstart the
    application. Only instances and constants can be injected into run blocks. This is to prevent
    further system configuration during application run time.

```js
angular.module('myModule', []).
  config(function(injectables) { // provider-injector
    // This is an example of config block.
    // You can have as many of these as you want.
    // You can only inject Providers (not instances)
    // into config blocks.
  }).
  run(function(injectables) { // instance-injector
    // This is an example of a run block.
    // You can have as many of these as you want.
    // You can only inject instances (not Providers)
    // into run blocks
  });
```

## Configuration Blocks

There are some convenience methods on the module which are equivalent to the `config` block. For
example:

```js
angular.module('myModule', []).
  value('a', 123).
  factory('a', function() { return 123; }).
  directive('directiveName', ...).
  filter('filterName', ...);

// is same as

angular.module('myModule', []).
  config(function($provide, $compileProvider, $filterProvider) {
    $provide.value('a', 123);
    $provide.factory('a', function() { return 123; });
    $compileProvider.directive('directiveName', ...);
    $filterProvider.register('filterName', ...);
  });
```

<div class="alert alert-info">
When bootstrapping, first Angular applies all constant definitions.
Then Angular applies configuration blocks in the same order they were registered.
</div>

## Run Blocks

Run blocks are the closest thing in Angular to the main method. A run block is the code which
needs to run to kickstart the application. It is executed after all of the service have been
configured and the injector has been created. Run blocks typically contain code which is hard
to unit-test, and for this reason should be declared in isolated modules, so that they can be
ignored in the unit-tests.

## Dependencies

Modules can list other modules as their dependencies. Depending on a module implies that required
module needs to be loaded before the requiring module is loaded. In other words the configuration
blocks of the required modules execute before the configuration blocks of the requiring module.
The same is true for the run blocks. Each module can only be loaded once, even if multiple other
modules require it.

## Asynchronous Loading

Modules are a way of managing $injector configuration, and have nothing to do with loading of
scripts into a VM. There are existing projects which deal with script loading, which may be used
with Angular. Because modules do nothing at load time they can be loaded into the VM in any order
and thus script loaders can take advantage of this property and parallelize the loading process.

## Creation versus Retrieval

Beware that using `angular.module('myModule', [])` will create the module `myModule` and overwrite any
existing module named `myModule`. Use `angular.module('myModule')` to retrieve an existing module.

```js
var myModule = angular.module('myModule', []);

// add some directives and services
myModule.service('myService', ...);
myModule.directive('myDirective', ...);

// overwrites both myService and myDirective by creating a new module
var myModule = angular.module('myModule', []);

// throws an error because myOtherModule has yet to be defined
var myModule = angular.module('myOtherModule');
```

# Unit Testing

A unit test is a way of instantiating a subset of an application to apply stimulus to it.
Small, structured modules help keep unit tests concise and focused.

<div class="did you know...">
Each module can only be loaded once per injector.
Usually an Angular app has only one injector and modules are only loaded once.
Each test has its own injector and modules are loaded multiple times.
</div>

In all of these examples we are going to assume this module definition:

```js
angular.module('greetMod', []).

  factory('alert', function($window) {
    return function(text) {
      $window.alert(text);
    }
  }).

  value('salutation', 'Hello').

  factory('greet', function(alert, salutation) {
    return function(name) {
      alert(salutation + ' ' + name + '!');
    }
  });
```

Let's write some tests to show how to override configuration in tests.

```js
describe('myApp', function() {
  // load application module (`greetMod`) then load a special
  // test module which overrides `$window` with a mock version,
  // so that calling `window.alert()` will not block the test
  // runner with a real alert box.
  beforeEach(module('greetMod', function($provide) {
    $provide.value('$window', {
      alert: jasmine.createSpy('alert')
    });
  }));

  // inject() will create the injector and inject the `greet` and
  // `$window` into the tests.
  it('should alert on $window', inject(function(greet, $window) {
    greet('World');
    expect($window.alert).toHaveBeenCalledWith('Hello World!');
  }));

  // this is another way of overriding configuration in the
  // tests using inline `module` and `inject` methods.
  it('should alert using the alert service', function() {
    var alertSpy = jasmine.createSpy('alert');
    module(function($provide) {
      $provide.value('alert', alertSpy);
    });
    inject(function(greet) {
      greet('World');
      expect(alertSpy).toHaveBeenCalledWith('Hello World!');
    });
  });
});
```
