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?

Edit in Plunker

<div ng-app="myApp">
  <div>
    {{ 'World' | greet }}
  </div>
</div>

// 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 + '!';
  };
});

it('should add Hello to the name', function() {
  expect(element(by.binding("'World' | greet")).getText()).toEqual('Hello, World!');
});

Important things to notice:

The 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.

You can find a community
style guide to help yourself when application grows.

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

Edit in Plunker

<div ng-controller="XmplController">
  {{ greeting }}
</div>

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);
  });

it('should add Hello to the name', function() {
  expect(element(by.binding("greeting")).getText()).toEqual('Bonjour World!');
});

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 consists of a collection of two kinds of blocks:

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.
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.

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:

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', ...);
  });

When bootstrapping, first Angular applies all constant definitions. Then Angular applies configuration blocks in the same order they were registered.

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 services 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 the 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.

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.

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.

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

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.

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!');
    });
  });
});