angular-docs-cn/public/docs/ts/latest/guide/ngmodule.jade

block includes
  include ../_util-fns

// TODO
  Images
  Confirm plunkers

:marked
  **Angular Modules** help organize an application into cohesive blocks of functionality.

  An Angular Module _class_ is adorned with the **NgModule** decorator that defines metadata about the module. 

  This chapter explains how to **create** `NgModule` classes and how to load them, 
  either immediately when the application launches or later, as needed, via the Router.  

  ## Contents
  * [Angular modularity](#angular-modularity "Add structure to the app with NgModule")
  * [The application root module](#root-module "The startup module that every app requires")
  * [Bootstrap](#bootstrap "Launch the app in a browser with the root module as the entry point") the root module
  * [Declarations](#declarations "Declare the components, directives, and pipes that belong to a module")
  * [Providers](#providers "Extend the app with additional services")
  * [Imports](#imports "Import components, directives, and pipes for use in component templates")
  * [Resolve conflicts](#resolve-conflicts "When two directives have the same selector ...")
  * [Feature modules](#feature-modules "Partition the app into feature modules")
  * [Lazy loaded modules](#lazy-load "Load modules asynchronously") with the Router
  * [Shared modules](#shared-module "Create modules for commonly used components, directives, and pipes") 
  * [The Core module](#core-module "Create a core module with app-wide singleton services and single-use components") 
  * [Configure core services with _forRoot_](#core-for-root "Configure providers during module import")
  * [Prevent reimport of the _CoreModule_](#prevent-reimport "because bad things happen if a lazy loaded module imports Core")
  * [NgModule metadata properties](#ngmodule-properties "A technical summary of the @NgModule metadata properties")
  * [FAQ](#faq "Frequently asked questions")

  ### Live examples
  This chapter explains Angular Modules through a progression of improvements to a sample with a "Tour of Heroes" theme.
  Here's an index to live examples at key moments in the evolution of that sample:

  * <live-example plnkr="minimal.0">A minimal NgModule app</live-example>
  * <live-example plnkr="contact.1b">The first contact module</live-example>
  * <live-example plnkr="contact.2">The revised contact module</live-example>
  * <live-example plnkr="pre-shared.3">Just before adding _SharedModule_</live-example> 
  * <live-example>The final version</live-example>

.l-hr

a#angular-modularity  
.l-main-section
:marked
  ## Angular Modularity
  
  Modules are a great way to organize the application and extend it with capabilities from external libraries.
  
  Many Angular libraries are modules (e.g, `FormsModule`, `HttpModule`, `RouterModule`). 
  Many third party libraries are available as Angular modules (e.g., 
  <a href="https://material.angular.io/" target="_blank">Material Design</a>,
  <a href="http://ionicframework.com/" target="_blank">Ionic</a>,
  <a href="https://github.com/angular/angularfire2" target="_blank">AngularFire2</a>).

  Angular modules consolidate components, directives and pipes into
  cohesive blocks of functionality, each focused on a 
  feature area, application business domain, workflow, or common collection of utilities.
  
  Modules can also add services to the application.
  Such services might be internally-developed such as the application logger.
  They can come from outside sources such as the Angular router and Http client.

  Modules can be loaded eagerly when the application starts.
  They can also be _lazy loaded_ asynchronously by the router.
  
  An Angular module is a class decorated with `@NgModule` metadata. The metadata:

  * declare which components, directives and pipes  _belong together_.
  * make some of those classes public so that other component templates can use them.
  * hide other classes as implementation details.
  * import other modules with the components, directives and pipes it needs.
  * provide services at the application level that any application component can use.

  Every Angular app has at least one module class, the _root module_. 
  We bootstrap that module to launch the application.

  The _root module_ is all we need in a simple application with a few components.
  As the app grows, we refactor the _root module_ into **feature modules** 
  that represent collections of related functionality. 
  We then import these modules into the _root module_.

  We'll see how later in the chapter. Let's start with the _root module_.

a#root_module 
.l-main-section
:marked
  ## _AppModule_ - the application root module
  
  Every Angular app has a **root module** class. 
  By convention it's a class called `AppModule` in a file named `app.module.ts`.

  This `AppModule` is about as minimal as it gets:
+makeExample('ngmodule/ts/app/app.module.0.ts', '', 'app/app.module.ts (minimal)')(format=".")
:marked
  The `@NgModule` decorator defines the metadata for the module. 
  We'll take an intuitive approach to understanding the metadata and fill in details as we go.

  This metadata imports a single helper module, `BrowserModule`, the module every browser app must import.
  
  `BrowserModule` registers critical application service providers.
  It also includes common directives like `NgIf` and `NgFor` which become immediately visible and usable
  in any of this modules component templates. 
  
  The `declarations` list identifies the application's only component, 
  the _root component_, the top of this app's rather bare component tree.

  The example `AppComponent` simply displays a data-bound title:
+makeExample('ngmodule/ts/app/app.component.0.ts', '', 'app/app.component.ts (minimal)')(format=".")
:marked
  Lastly, the `@NgModule.bootstrap` property identifies this `AppComponent` as the _bootstrap component_. 
  When Angular launches the app, it places the HTML rendering of `AppComponent` in the DOM,
  inside the `<my-app>` element tags of the `index.html`

a#bootstrap
.l-main-section
:marked
  ## Bootstrapping in _main.ts_
  We launch the application by bootstrapping the `AppModule` in the `main.ts` file.
  
  Angular offers a variety of bootstrapping options, targeting multiple platforms. 
  In this chapter we consider two options, both targeting the browser.

  ### Dynamic bootstrapping with the Just-In-Time (JIT) compiler
  In the first, _dynamic_ option, the [Angular compiler](#q-angular-compiler "About the Angular Compiler") 
  compiles the application in the browser and then launches the app.

+makeExample('ngmodule/ts/app/main.ts', '', 'app/main.ts (dynamic)')(format=".")
:marked
  The samples in this chapter demonstrate the dynamic bootstrapping approach. 

  <live-example embedded plnkr="minimal.0">Try the live example.</live-example>
  

  ### Static bootstrapping with the Ahead-Of-Time (AOT) compiler

  Consider the static alternative which can produce a much smaller application that
  launches faster, especially on mobile devices and high latency networks.

  In the _static_ option, the Angular compiler runs ahead of time as part of the build process,
  producing a collection of class factories in their own files. 
  Among them is the `AppModuleNgFactory`.

  The syntax for bootstrapping the pre-compiled `AppModuleNgFactory` is similar to 
  the dynamic version that bootstraps the `AppModule` class.

+makeExample('ngmodule/ts/app/main-static.ts', '', 'app/main.ts (static)')(format=".")
:marked
  Because the entire application was pre-compiled, 
  we don't ship the _Angular Compiler_ to the browser and we don't compile in the browser.
  
  The application code downloaded to the browser is much smaller than the dynamic equivalent
  and it is ready to execute immediately. The performance boost can be significant.

  Both the JIT and AOT compilers generate an `AppModuleNgFactory` class from the same `AppModule` source code.
  The JIT compiler creates that factory class on the fly, in memory, in the browser.
  The AOT compiler outputs the factory to a physical file
  that we're importing here in the static version of `main.ts`.

  In general, the `AppModule` should neither know nor care how it is bootstrapped.

  Although the `AppModule` evolves as the app grows, the bootstrap code in `main.ts` doesn't change.
  This is the last time we'll look at `main.ts`.

.l-hr

a#declarations
.l-main-section
:marked
  ## Declare directives and components
  The app evolves. 
  The first addition is a `HighlightDirective`, an [attribute directive](attribute-directives.html)
  that sets the background color of the attached element.
+makeExample('ngmodule/ts/app/highlight.directive.ts', '', 'app/highlight.directive.ts')(format=".")
:marked
  We update the `AppComponent` template to attach the directive to the title:
+makeExample('ngmodule/ts/app/app.component.1.ts', 'template')(format=".")
:marked
  If we ran the app now, Angular would report an error in the console because
  it doesn't recognize the `highlight` binding. 
  
  We must declare the directive in `AppModule`.
  Import the `HighlightDirective` class and add it to the module's `declarations` like this:
+makeExample('ngmodule/ts/app/app.module.1.ts', 'directive')(format=".")

:marked
  ### Add a component

  We decide to refactor the title into its own `TitleComponent`. 
  The component's template binds to the component's `title` and `subtitle` properties like this:
+makeExample('ngmodule/ts/app/title.component.html', 'v1', 'app/title.component.html')(format=".")

+makeExample('ngmodule/ts/app/title.component.ts', 'v1', 'app/title.component.ts')(format=".")

:marked
  We rewrite the `AppComponent` to display the new `TitleComponent` in the `<app-title>` element,
  using an input binding to set the `subtitle`.
+makeExample('ngmodule/ts/app/app.component.1.ts', '', 'app/app.component.ts (v1)')(format=".")
:marked
  Angular won't recognize the `<app-title>` tag until we declare it in `AppModule`. 
  Import the `TitleComponent` class and add it to the module's `declarations`:
+makeExample('ngmodule/ts/app/app.module.1.ts', 'component')(format=".")

a#providers
.l-main-section
:marked
  ## Service Providers

  Modules are a great way to provide services for all of the module's components.

  The [Dependency Injection](dependency-injection.html) chapter describes
  the Angular hierarchical dependency injection system and how to configure that system
  with [providers](dependency-injection.html#providers) at different levels of the
  application's component tree.

  A module can add providers to the application's root dependency injector, making those services
  available everywhere in the application.

  Many applications capture information about the currently logged-in user and make that information 
  accessible through a user service. 
  This sample application has a dummy implementation of such a `UserService`.

+makeExample('ngmodule/ts/app/user.service.ts', '', 'app/user.service.ts')(format=".")
:marked
  The sample application should display a welcome message to the logged in user just below the application title.
  Update the `TitleComponent` template to show the welcome message below the application title.
+makeExample('ngmodule/ts/app/title.component.html', '', 'app/title.component.html')(format=".")
:marked
  Update the `TitleComponent` class with a constructor that injects the `UserService`
  and sets the component's `user` property from the service.
+makeExample('ngmodule/ts/app/title.component.ts', '', 'app/title.component.ts')(format=".")
:marked
  We've _defined_ and _used_ the service. Now we _provide_ it for all components to use by
  adding it to a `providers` property in the `AppModule` metadata:
+makeExample('ngmodule/ts/app/app.module.1.ts', 'providers', 'app/app.module.ts (providers)')(format=".")

a#imports
.l-main-section
:marked
  ## Import supporting modules

  The app shouldn't welcome a user if there is no user.
  
  Notice in the revised `TitleComponent` that an `*ngIf` directive guards the message.
  There is no message if there is no user.
+makeExample('ngmodule/ts/app/title.component.html', 'ngIf', 'app/title.component.html (ngIf)')(format=".")
:marked
  Although `AppModule` doesn't declare `NgIf`, the application still compiles and runs.
  How can that be? The Angular compiler should either ignore or complain about unrecognized HTML.
  
  Angular _does_ recognize `NgIf` because we imported it earlier. 
  The initial version of `AppModule` imports `BrowserModule`.
+makeExample('ngmodule/ts/app/app.module.0.ts', 'imports', 'app/app.module.ts (imports)')(format=".")
:marked
  Importing `BrowserModule` made all of its public components, directives and pipes visible 
  to the component templates in `AppModule`. They are ready to use without further ado.
.l-sub-section
  :marked
    More accurately, `NgIf` is declared in `CommonModule` from `@angular/common`.

    `CommonModule` contributes many of the common directives that applications need including `ngIf` and `ngFor`.
    
    `BrowserModule` imports `CommonModule` and _re-exports_ it.
    We'll cover re-exporting a module [later](#q-re-export) in the chapter.
    The net effect is that an importer of `BrowserModule` gets `CommonModule` directives automatically.
:marked
  Many familiar Angular directives do not belong to`CommonModule`. 
  For example,  `NgModel` and `RouterLink` belong to Angular's `FormsModule` and `RouterModule` respectively.
  We must _import_ those modules before we can use their directives.

  To illustrate this point, we extend the sample app with `ContactComponent`, 
  a form component that imports form support from the Angular `FormsModule`.

  ### Add the _ContactComponent_

  [Angular Forms](forms.html) are a great way to manage user data entry.

  The `ContactComponent` presents a "contact editor", 
  implemented with _Angular Forms_ in the [_template-driven form_](forms.html) style.

.l-sub-section
  :marked
    #### Angular Form Styles
    
    We write Angular form components in either the
    [_template-driven form_](forms.html) style or 
    the [_reactive form_](../cookbook/dynamic-form.html) style.

    This sample is about to import the `FormsModule` from `@angular/forms` because
    the `ContactComponent` is written in the _template-driven_ style.
    Modules with components written in the _reactive_ style,
    should import the `ReactiveFormsModule` instead.

:marked
  The `ContactComponent` selector matches an element named `<app-contact>`. 
  Add an element with that name to the `AppComponent` template just below the `<app-title>`:
+makeExample('ngmodule/ts/app/app.component.1b.ts', 'template', 'app/app.component.ts (template)')(format=".")

:marked
  The `ContactComponent` has a lot going on. 
  Form components are often complex anyway and this one has its own `ContactService`, 
  its own [custom pipe](#pipes.html#custom-pipes) called `Awesome`,
  and an alternative version of the `HighlightDirective`.

  To make it manageable, we place all contact-related material in an `app/contact` folder
  and break the component into three constituent HTML, TypeScript, and css files:
+makeTabs(
  `ngmodule/ts/app/contact/contact.component.html,
  ngmodule/ts/app/contact/contact.component.ts,
  ngmodule/ts/app/contact/contact.component.css,
  ngmodule/ts/app/contact/contact.service.ts,
  ngmodule/ts/app/contact/awesome.pipe.ts,
  ngmodule/ts/app/contact/highlight.directive.ts
  `, 
  null,
  `app/contact/contact.component.html,
  app/contact/contact.component.ts,
  app/contact/contact.component.css,
  app/contact/contact.service.ts,
  app/contact/awesome.pipe.ts,
  app/contact/highlight.directive.ts
  `)
:marked
  Focus on the component template.
  Notice the two-way data binding `[(ngModel)]` in the middle of the template.
  `ngModel` is the selector for the `NgModel` directive.

  Although `NgModel` is an Angular directive, the _Angular Compiler_ won't recognize it
  because (a) `AppModule` doesn't declare it and (b) it wasn't imported via `BrowserModule`.

  Less obviously, even if Angular somehow recognized `ngModel`,
  this `ContactComponent` would not behave like an Angular form because
  form features such as validation are not yet available.

  ### Import the FormsModule

  Add the `FormsModule` to the `AppModule` metadata's `imports` list.
+makeExample('ngmodule/ts/app/app.module.1.ts', 'imports')(format=".")
:marked
  Now `[(ngModel)]` binding works and the user input is validated by Angular Forms.
.alert.is-critical
  :marked
    **Do not** add `NgModel` &mdash; or the `FORMS_DIRECTIVES` &mdash; 
    to the `AppModule` metadata's declarations!

    These directives belong to the `FormsModule`.
    Components, directives and pipes belong to one module &mdash; and _one module only_.

    **Never re-declare classes that belong to another module.**

a#declare-pipe
:marked
  ### Declare the contact component, directive and pipe

  The application fails to compile until we declare the contact component, directive and pipe.
  Update the `declarations` in the  `AppModule` accordingly:
+makeExample('ngmodule/ts/app/app.module.1.ts', 'declarations', 'app/app.module.ts (declarations)')(format=".")

a#import-name-conflict
.l-sub-section
  :marked
    There are two directives with the same name, both called `HighlightDirective`.

    We work around it by creating an alias for the second, contact version using the `as` JavaScript import keyword:
  +makeExample('ngmodule/ts/app/app.module.1b.ts', 'import-alias')(format=".")
  :marked
    This solves the immediate problem of referencing both directive _types_ in the same file but 
    leaves another problem unresoved as we discuss [below](#resolve-conflicts).

:marked
  ### Provide the _ContactService_
  The `ContactComponent` displays contacts retrieved by the `ContactService`
  which Angular injects into its constructor. 
  
  We have to provide that service somewhere.
  The `ContactComponent` _could_ provide it. 
  But then it would be scoped to this component _only_. 
  We want to share this service with other contact-related components that we will surely add later.

  In this app we chose to add `ContactService` to the `AppModule` metadata's `providers` list:
+makeExample('ngmodule/ts/app/app.module.1b.ts', 'providers', 'app/app.module.ts (providers)')(format=".")
:marked
  Now `ContactService` (like `UserService`) can be injected into any component in the application.

a#application-scoped-providers
.l-sub-section
  :marked
    #### Application-scoped Providers
      The `ContactService` provider is _application_-scoped because Angular 
      registers a module's `providers` with the application's **root injector**.

      Architecturally, the `ContactService` belongs to the Contact business domain. 
      Classes in _other_ domains don't need the `ContactService` and shouldn't inject it.
      
      We might expect Angular to offer a _module_-scoping mechanism to enforce this design.
      It doesn't. Angular module instances, unlike components, do not have their own injectors
      so they can't have their own provider scopes.

      This omission is intentional. 
      Angular modules are designed primarily to extend an application, 
      to enrich the entire app with the module's capabilities.
      
      Service scoping is rarely a problem in practice.
      Non-contact components can't inject the `ContactService` by accident.
      To inject `ContactService`, you must first import its _type_.
      Only Contact components should import the `ContactService` _type_.

      [An FAQ below](#q-component-scoped-providers) pursues this issue and its mitigations in greater detail.

:marked
  ### Run the app
  Everything is now in place to run the application with its contact editor.

  The app file structure looks like this:
.filetree
  .file app
  .children
    .file app.component.ts
    .file app.module.ts
    .file highlight.directive.ts
    .file main.ts
    .file title.component.(html|ts)
    .file user.service.ts
    .file contact
    .children
      .file awesome.pipe.ts
      .file contact.component.(css|html|ts)
      .file contact.service.ts
      .file highlight.directive.ts

:marked
  <live-example plnkr="contact.1b">Try the live example.</live-example>
  
a#resolve-conflicts
.l-main-section
:marked
  ## Resolve directive conflicts

  We ran into trouble [above](#import-name-conflict) when we declared the contact's `HighlightDirective` because
  we already had a `HighlightDirective` class at the application level.

  That both directives have the same name smells of trouble.

  A look at their selectors reveals that they both highlight the attached element with a different color.
+makeTabs(
  `ngmodule/ts/app/highlight.directive.ts,
   ngmodule/ts/app/contact/highlight.directive.ts`,
   '',
  `app/highlight.directive.ts,
   app/contact/highlight.directive.ts`)

:marked
  Will Angular use only one of them? No. 
  Both directives are declared in this module so _both directives are active_.
  
  When the two directives compete to color the same element, 
  the directive declared later wins because its DOM changes overwrite the first.
  In this case, the contact's `HighlightDirective` colors the application title text blue
  when it should stay gold.

.l-sub-section
  :marked
    The real problem is that there are _two different classes_ trying to do the same thing.

    It's OK to import the _same_ directive class multiple times. 
    Angular removes duplicate classes and only registers one of them.

    But these are actually two different classes, defined in different files, that happen to have the same name.

    They're not duplicates from Angular's perspective. Angular keeps both directives and
    they take turns modifying the same HTML element.

:marked
  At least the app still compiles. 
  If we define two different component classes with the same selector specifying the same element tag, 
  the compiler reports an error. It can't insert two components in the same DOM location.

  What a mess! 
  
  We can eliminate component and directive conflicts by creating feature modules
  that insulate the declarations in one module from the declarations in another.

a#feature-modules
.l-main-section
:marked
  ## Feature Modules

  This application isn't big yet. But it's already suffering structural problems.

  * The root `AppModule` grows larger with each new application class and shows no signs of stopping.

  * We have conflicting directives. 
  The `HighlightDirective` in contact is re-coloring the work done by the `HighlightDirective` declared in `AppModule`.
  And it's coloring the application title text when it should only color the `ContactComponent`.

  * A change to a contact class could break an application part in some unrelated section of the app.
  The app is brittle and hard to test.  

  * The app lacks clear boundaries between contact functionality and other application features.
  That lack of clarity makes it harder to assign development responsibilities to different teams.

  We mitigate these problems with _feature modules_.

  ### _Feature Module_

  A _feature module_ is a class adorned by the `@NgModule` decorator and its metadata,
  just like a root module. 
  Feature module metadata have the same properties as the metadata for a root module.

  The root module and the feature module share the same execution context.
  They share the same dependency injector which means the services in one module
  are available to all.

  There are two significant technical differences:

  1. We _boot_ the root module to _launch_ the app;
  we _import_ a feature module to _extend_ the app.

  2. A feature module can expose or hide its implementation from other modules.

  Otherwise, a feature module is distinguished primarily by its intent.

  A feature module delivers a cohesive set of functionality. 
  focused on an application business domain, a user workflow, a facility (forms, http, routing), 
  or a collection of related utilities.
  
  While we can do everything within the root module,
  feature modules help us partition the app into areas of specific interest and purpose.

  A feature module collaborates with the root module and with other modules 
  through the services it provides and 
  the components, directives, and pipes that it chooses to share.
 
  In the next section, we carve the contact functionality out of the root module
  and into a dedicated feature module.

  <a id="contact-module-v1"></a>
  ### Make _Contact_ a feature module

  It's easy to refactor the contact material into a contact feature module.

  1. Create the `ContactModule` in the `app/contact` folder.
  1. Move the contact material from `AppModule` to `ContactModule`.
  1. Replace the imported  `BrowserModule` with `CommonModule`.
  1. Import the `ContactModule` into the `AppModule`.

  `AppModule` is the only _existing_ class that changes. But we do add one new file.
  
  ### Add the _ContactModule_
  
  Here's the new `ContactModule`
+makeExample('ngmodule/ts/app/contact/contact.module.2.ts', '', 'app/contact/contact.module.ts')
:marked
  We copy from `AppModule` the contact-related import statements and the `@NgModule` properties
  that concern the contact and paste them in `ContactModule`.
  
  We _import_ the `FormsModule` because the contact component needs it. 
.alert.is-important
  :marked
    Modules do not inherit access to the components, directives or pipes that are declared in other modules.
    The fact that `AppModule` imports `FormsModule` is irrelevant.
    The `ContactModule` must import `FormsModule` explicitly so that
    `ContactComponent` can data bind with `ngModel`.
:marked
  We also replaced `BrowserModule` by `CommonModule` for reasons explained [below](#q-browser-vs-common-module).

  We _declare_ the contact component, directive, and pipe in the module `declarations`.

  We _export_ the `ContactComponent` so
  other modules that import the `ContactModule` can include it in their component templates.

  All other declared contact classes are private by default.
  The `AwesomePipe` and `HighlightDirective` are hidden from the rest of the application. 
  The `HighlightDirective` can no longer color the `AppComponent` title text.

:marked
 ### Refactor the _AppModule_
 Return to the `AppModule` and remove everything specific to the contact feature set.

 Delete the contact import statements.
 Delete the contact declarations and contact providers.
 Remove the `FormsModule` from the `imports` list (`AppComponent` doesn't need it).
 Leave only the classes required at the application root level.

 Then import the `ContactModule` so the app can continue to display the exported `ContactComponent`.

 Here's the refactored version of the `AppModule` side-by-side with the previous version.
+makeTabs(
  `ngmodule/ts/app/app.module.2.ts,
   ngmodule/ts/app/app.module.1b.ts`,
   '', 
  `app/app.module.ts (v2),
   app/app.module.ts (v1)`)
:marked
 ### Improvements
:marked
  There's a lot to like in the revised `AppModule`
  * It does not change as the _Contact_ domain grows.
  * It only changes when we add new modules.
  * It's simpler:
    * Fewer import statements
    * No `FormsModule` import
    * No contact-specific declarations
    * No `ContactService` provider
    * No `HighlightDirective` conflict

  <live-example embedded plnkr="contact.2">Try the live example.</live-example>

a#lazy-load
.l-main-section
:marked
  ## Lazy loading modules with the Router

  The Heroic Staffing Agency sample app has evolved. 
  It has two more modules, one for managing the heroes-on-staff and another for matching crises to the heroes.
  Both modules are in the early stages of development. 
  Their specifics aren't important to the story and we won't discuss every line of code.
.l-sub-section
  :marked
    Examine and download the complete source for this version from the <live-example plnkr="pre-shared.3">live example.</live-example>
:marked
  Some facets of the current application merit discussion.

  * The app has three feature modules: Contact, Hero, and Crisis.
  * The Angular router helps users navigate among these modules.
  * The `ContactComponent` is the default destination when the app starts.
  * The `ContactModule` continues to be "eagerly" loaded when the application starts.
  * `HeroModule` and the `CrisisModule` are lazy loaded.

  <a id="app-component-template"></a>
  Let's start at the top with the new `AppComponent` template:
  a title, three links, and a `<router-outlet>`.
+makeExample('ngmodule/ts/app/app.component.3.ts', 'template', 'app/app.component.ts (v3 - Template)')(format='.')
:marked
  The `<app-contact>` element is gone; we're routing to the _Contact_ page now.

  The `AppModule` has changed modestly:
+makeExample('ngmodule/ts/app/app.module.3.ts', '', 'app/app.module.ts (v3)')
.l-sub-section
  :marked
    Some file names bear a `.3` extension indicating
    a difference with prior or future versions.
    We'll explain differences that matter in due course.
    
:marked
  The module still imports `ContactModule` so that its routes and components are mounted when the app starts. 

  The module does _not_ import `HeroModule` or `CrisisModule`. 
  They'll be fetched and mounted asynchronously when the user navigates to one of their routes.
  
  The significant change from version 2 is the addition of a ***routing*** object to the `imports`.
  The routing object, which provides a configured `Router` service, is defined in the `app.routing.ts` file.

  ### App routing
+makeExample('ngmodule/ts/app/app.routing.ts', '', 'app/app.routing.ts')(format='.')
:marked
  The router is the subject of [its own chapter](router.html) so we'll skip lightly over the details and 
  concentrate on the intersection of Angular modules and routing.

  This file defines three routes. 
  
  The first redirects the empty URL (e.g., `http://host.com/`) 
  to another route whose path is `contact` (e.g., `http://host.com/contact`).
  
  The `contact` route isn't defined here.
  It's defined in the _Contact_ feature's _own_ routing file, `contact.routing.ts`.
  It's standard practice for feature modules with routing components to define their own routes.
  We'll get to that file in a moment.

  The remaining two routes use lazy loading syntax to tell the router where to find the modules:
+makeExample('ngmodule/ts/app/app.routing.ts', 'lazy-routes')(format='.')
.l-sub-section
  :marked
    Note that the module location is a _string_, not a _type_. 

    To reference the _type_ we'd have to import the module, which loads the module immediately,
    defeating our intent to load the module later. 
    A string, on the other hand, is just a string. It has no side-effects.
:marked
  The module location strings in this app identify module _files_, not module _classes_.
  That works because each module class is marked as the default export in its file.
+makeExample('ngmodule/ts/app/crisis/crisis.module.ts', 'export-default', '/app/crisis/crisis.module.ts (export default)')(format='.')
:marked
    _Remember to use_ `export default`_, not just_ `export`.

:marked
  ### RouterModule.forRoot

  The last line calls the `forRoot` static class method of the `RouterModule`, passing in the configuration.
+makeExample('ngmodule/ts/app/app.routing.ts', 'forRoot')(format='.')
:marked
  The returned `routing` object is a `ModuleWithProviders` containing both the `RouterModule` directives
  and the Dependency Injection providers that produce a configured `Router`. 

  This `routing` object is intended for the app _root_ module _only_.

.alert.is-critical
  :marked
    Never call `RouterModule.forRoot` in a feature module.
:marked
  Back in the root `AppModule`, we add this `routing` object to its `imports` list, 
  and the app is ready to navigate.
+makeExample('ngmodule/ts/app/app.module.3.ts', 'imports', 'app/app.module.ts (imports)')(format='.')

:marked
  ### Routing to a feature module
  The `app/contact` folder holds a new file, `contact.routing.ts`.
  It defines the `contact` route we mentioned a bit earlier and also creates a `routing` object like so:
+makeExample('ngmodule/ts/app/contact/contact.routing.ts', 'routing', 'app/contact/contact.routing.ts (routing)')(format='.')
:marked
  This time we pass the route list to the `forChild` method of the `RouterModule`.
  It produces a different kind of object intended for feature modules.

.alert.is-important
  :marked
    Always call `RouterModule.forChild` in a feature module.

.alert.is-helpful
  :marked
    **_forRoot_** and **_forChild_** are conventional names for methods that
    deliver different `import` values to root and feature modules.
    Angular doesn't recognize them but Angular developers do.

    [Follow this convention](#q-for-root) if you write a similar module
    that has both shared [_declarables_](#q-declarable) and services.

:marked
  `ContactModule` has changed in two small but important details
+makeTabs(
  `ngmodule/ts/app/contact/contact.module.3.ts,
   ngmodule/ts/app/contact/contact.module.2.ts`,
   'class, class', 
  `app/contact/contact.module.3.ts,
   app/contact/contact.module.2.ts`)
:marked
  1. It imports the `routing` object from `contact.routing.ts`
  1. It no longer exports `ContactComponent`

  Now that we navigate to `ContactComponent` with the router there's no reason to make it public.
  Nor does it need a selector. 
  No template will ever again reference this `ContactComponent`.
  It's gone from the [_AppComponent_ template](#app-component-template).

a#hero-module
:marked
  ### Lazy loaded routing to a module

  The lazy loaded `HeroModule` and `CrisisModule` follow the same principles as any feature module.
  They don't look different from the eagerly loaded `ContactModule`.

  The `HeroModule` is a bit more complex than the `CrisisModule` which makes it 
  a more interesting and useful example. Here's its file structure:

.filetree
  .file hero
  .children
    .file hero-detail.component.ts
    .file hero-list.component.ts
    .file hero.component.ts
    .file hero.module.ts
    .file hero.routing.ts
    .file hero.service.ts
    .file highlight.directive.ts
:marked
  This is the child routing scenario familiar to readers of [Router](router.html#child-routing-component) chapter.
  The `HeroComponent` is the feature's top component and routing host. 
  Its template has a `<router-outlet>` that displays either a list of heroes (`HeroList`) 
  or an editor of a selected hero (`HeroDetail`).
  Both components delegate to the `HeroService` to fetch and save data.

  There's yet _another_ `HighlightDirective` that colors elements in yet a different shade.
  We should [do something](#shared-module "Shared modules") about the repetition and inconsistencies.
  We endure for now.

  The `HeroModule` is a feature module like any other.
+makeExample('ngmodule/ts/app/hero/hero.module.3.ts', 'class', 'app/hero/hero.module.ts (class)')(format='.')
:marked
  It imports the `FormsModule` because the `HeroDetailComponent` template binds with `[(ngModel)]`. 
  It imports a `routing` object from `hero.routing.ts` just as `ContactModule` and `CrisisModule` do.

  The `CrisisModule` is much the same. There's nothing more to say that's new. 

  <live-example embedded plnkr="pre-shared.3">Try the live example.</live-example>

a#shared-module
.l-main-section
:marked
  ## Shared modules
  
  The app is shaping up. 
  One thing we don't like is carrying three different versions of the `HighlightDirective`.
  And there's a bunch of other stuff cluttering the app folder level that could be tucked away.

  Let's add a `SharedModule` to hold the common components, directives, and pipes 
  and share them with the modules that need them.

  * create an `app/shared` folder
  * move the `AwesomePipe` and `HighlightDirective` from `app/contact` to `app/shared`.
  * delete the `HighlightDirective` classes from `app/` and `app/hero`
  * create a `SharedModule` class to own the shared material
  * update other feature modules to import `SharedModule`

  Most of this is familiar blocking and tackling. Here is the `SharedModule`
+makeExample('ngmodule/ts/app/shared/shared.module.ts', '', 'app/app/shared/shared.module.ts')
:marked
  Some highlights
  * It imports the `CommonModule` because its component needs common directives.
  * It declares and exports the utility pipe, directive, and component classes as expected.
  * It re-exports the `CommonModule` and `FormsModule`

  #### Re-exporting other modules

  While reviewing our application, we noticed that many components requiring `SharedModule` directives
  also use `NgIf` and `NgFor` from `CommonModule`
  and bind to component properties with `[(ngModel)]`, a directive in the `FormsModule`.
  Modules that declare these components would have to import `CommonModule`, `FormsModule` and `SharedModule`.

  We can reduce the repetition by having `SharedModule` re-export `CommonModule` and `FormsModule`
  so that importers of `SharedModule` get `CommonModule` and `FormsModule` _for free_.

  As it happens, the components declared by `SharedModule` itself don't bind with `[(ngModel)]`. 
  Technically,  there is no need for `SharedModule` to import `FormsModule`.
  
  `SharedModule` can still export `FormsModule` without importing it.

  ### Why _TitleComponent_ isn't shared

  `SharedModule` exists to make commonly used components, directives and pipes available
  for use in the templates of components in _many_ other modules.

  The `TitleComponent` is used _only once_ by the `AppComponent`. 
  There's no point in sharing it.

  <a id="no-shared-module-providers"></a>
  ### Why _UserService_ isn't shared  

  While many components share the same service _instances_,
  they rely on Angular dependency injection to do this kind of sharing, not the module system.
 
  Several components of our sample inject the `UserService`.
  There should be _only one_ instance of the `UserService` in the entire application 
  and _only one_ provider of it.
  
  `UserService` is an application-wide singleton.
  We don't want each module to have its own separate instance. 
  Yet there is [a real danger](#q-why-it-is-bad) of that happening 
  if the `SharedModule` provides the `UserService`.

.alert.is-critical
  :marked
    Do **not** specify singleton `providers` in shared modules.

a#core-module
.l-main-section
:marked
  ## The Core module
  At the moment, our root folder is cluttered with the `UserService`
  and the `TitleComponent` that only appears in the root `AppComponent`.
  We did not include them in the `SharedModule` for reasons just explained.
  
  Instead, we'll gather them in a single `CoreModule` that we **import _once_ when the app starts**
  and _never import anywhere else_.

  **Steps:**

  * create an `app/core` folder
  * move the `UserService` and `TitleComponent` from `app/` to `app/core`
  * create a `CoreModule` class to own the core material
  * update the `AppRoot` module to  import `CoreModule`
  
  Again, most of this is familiar blocking and tackling. The interesting part is the `CoreModule`
+makeExample('ngmodule/ts/app/core/core.module.ts', 'v4', 'app/app/core/core.module.ts')
.l-sub-section
  :marked
    We're importing some extra symbols from the Angular core library that we're not using yet.
    They'll become relevant later in this chapter.
:marked
  The `@NgModule` metadata should be familiar. 
  We declare the `TitleComponent`  because this module _owns_ it and we export it
  because `AppComponent` (which is in `AppModule`) displays the title in its template.
  `TitleComponent` needs the Angular `NgIf` directive that we import from `CommonModule`.

  `CoreModule` _provides_ the `UserService`. Angular registers that provider with the app root injector,
  making a singleton instance of the `UserService` available to any component that needs it, 
  whether that component is eagerly or lazily loaded.

.l-sub-section
  :marked
    #### Why bother?
    This scenario is clearly contrived. 
    The app is too small to worry about a single service file and a tiny, one-time component.

    A `TitleComponent` sitting in the root folder isn't bothering anyone.
    The root `AppModule` can register the `UserService` itself,
    as it does currently, even if we decide to relocate the `UserService` file to the `app/core` folder.

    Real world apps have more to worry about. 
    They can have several single-use components (e.g., spinners, message toasts, and modal dialogs)
    that appear only in the `AppComponent` template. 
    We don't import them elsewhere so they're not _shared_ in that sense. 
    Yet they're too big and messy to leave loose in the root folder.

    Apps often have many singleton services like this sample's `UserService`.
    Each must be registered _exactly once_, in the app root injector, when the application starts.

    While many Components inject such services in their constructors &mdash;
    and therefore require JavaScript `import` statements to import their symbols &mdash;
    no other component or module should define or re-create the services themselves.
    Their _providers_ are not shared.

    We recommend collecting such single-use classes and hiding their gory details inside a `CoreModule`.
    A simplified root `AppModule` imports `CoreModule` in its capacity as orchestrator of the application as a whole.

.l-main-section
:marked
  ## Cleanup
  Having refactored to a `CoreModule` and a `SharedModule`, it's time to cleanup the other modules. 

  ### A trimmer _AppModule_
  
  Here is the updated `AppModule` paired with version 3 for comparison:
+makeTabs(
  `ngmodule/ts/app/app.module.ts,
   ngmodule/ts/app/app.module.3.ts`,
   'v4,',
  `app/app.module.ts (v4),
   app/app.module.ts (v3)`)

:marked
  Notice that `AppModule` is ...
  * a little smaller because many `app/root` classes have moved to other modules.
  * stable because we'll add future components and providers to other modules, not this one.
  * delegating to imported modules rather than doing work.
  * focused on its main task, orchestrating the app as a whole.

  ### A trimmer _ContactModule_
  Here is the new `ContactModule` paired with the prior version:
+makeTabs(
  `ngmodule/ts/app/contact/contact.module.ts,
   ngmodule/ts/app/contact/contact.module.3.ts`,
   '',
  `app/contact/contact.module.ts (v4),
   app/contact/contact.module.ts (v3)`)

:marked
  Notice that
  * The `AwesomePipe` and `HighlightDirective` are gone.
  * The imports include `SharedModule` instead of `CommonModule` and `FormsModule`
  * This new version is leaner and cleaner.

.l-hr

a#core-for-root
.l-main-section
:marked
  ## Configure core services with _CoreModule.forRoot_

  A module that adds providers to the application can offer a facility for configuring those providers as well.
  
  By convention, the **_forRoot_** static method both provides and configures services at the same time.
  It takes a service configuration object and returns a
  [ModuleWithProviders](../api/core/index/ModuleWithProviders-interface.html) which is
  a simple object with two properties:
  * `ngModule` - the `CoreModule` class
  * `providers` - the configured providers

  The root `AppModule` imports the `CoreModule` and adds the `providers` to the `AppModule` providers.
.l-sub-section
  :marked
    More precisely, Angular accumulates all imported providers _before_ appending the items listed in `@NgModule.providers`. 
    This sequence ensures that whatever we add explicitly to the `AppModule` providers takes precedence 
    over the providers of imported modules.
:marked
  Let's add a `CoreModule.forRoot` method that configures the core `UserService`.

  We've extended the core `UserService` with an optional, injected `UserServiceConfig`.
  If a `UserServiceConfig` exists, the `UserService` sets the user name from that config.
+makeExample('ngmodule/ts/app/core/user.service.ts', 'ctor', 'app/core/user.service.ts (constructor)')(format='.')
:marked
  Here's `CoreModule.forRoot` that takes a `UserServiceConfig` object:
+makeExample('ngmodule/ts/app/core/core.module.ts', 'for-root', 'app/core/core.module.ts (forRoot)')(format='.')
:marked
  Lastly, we call it _within the_ `imports` _list_ of the `AppModule`.
+makeExample('ngmodule/ts/app/app.module.ts', 'import-for-root', 'app//app.module.ts (imports)')(format='.')
:marked
  The app displays "Miss Marple" as the user instead of the default "Sherlock Holmes".

.alert.is-important
  :marked
    Call `forRoot` only in the root application module, `AppModule`.
    Calling it in any other module, particularly in a lazy loaded module,
    is contrary to the intent and is likely to produce a runtime error.

    Remember to _import_ the result; don't add it to any other `@NgModule` list.

.l-hr

a#prevent-reimport
.l-main-section
:marked
  ## Prevent reimport of the _CoreModule_

  Only the root `AppModule` should import the `CoreModule`. 
  [Bad things happen](#q-why-it-is-bad) if a lazy loaded module imports it.

  We could _hope_ that no developer makes that mistake. 
  Or we can guard against it and fail fast by adding the following `CoreModule` constructor.
+makeExample('ngmodule/ts/app/core/core.module.ts', 'ctor')(format='.')
:marked
  The constructor tells Angular to inject the `CoreModule` into itself.
  That seems dangerously circular.

  The injection _would be circular_ if Angular looked for `CoreModule` in the _current_ injector.
  The `@SkipSelf` decorator means "_look for_ `CoreModule` _in an ancestor injector, above me in the injector hierarchy._"
  
  If the constructor executes as intended in the `AppModule`, 
  there is no ancestor injector that could provide an instance of `CoreModule`.
  The injector should give up.

  By default the injector throws an error when it can't find a requested provider.
  The `@Optional` decorator means not finding the service is OK. 
  The injector returns `null`, the `parentModule` parameter is null,
  and the constructor concludes uneventfully.

  It's a different story if we improperly import `CoreModule` into a lazy loaded module such as `HeroModule` (try it).
  
  Angular creates a lazy loaded module with its own injector, a _child_ of the root injector.
  `@SkipSelf` causes Angular to look for a `CoreModule` in the parent injector which this time is the root injector.
  Of course it finds the instance imported by the root `AppModule`. 
  Now `parentModule` exists and the constructor throws the error.
:marked
  ### Conclusion
  We're done with the tutorial portion of the chapter. 
  You can examine and download the complete source for this final version from the <live-example>live example.</live-example>

  The next section summarizes the `NgModule` API. 
.l-hr

a#ngmodule-properties  
.l-main-section
:marked
  ## *NgModule* properties

  The following chart summarizes the `NgModule` metadata properties.
//
  export interface NgModuleMetadataType {
    providers?: any[];
    declarations?: Array<Type|any[]>;
    imports?: Array<Type|ModuleWithProviders|any[]>;
    exports?: Array<Type|any[]>;
    entryComponents?: Array<Type|any[]>;
    bootstrap?: Array<Type|any[]>;
    schemas?: Array<SchemaMetadata|any[]>;
  }

table
  tr
    th Property
    th Description
  tr
    td(style="vertical-align: top") <code>declarations</code>
    td
      :marked
        A list of [declarable](#q-declarables) classes, 
        the **component**, **directive** and **pipe** classes that _belong to this module_.

        These declared classes are visible within the module but invisible to
        components in a different module unless (a) they are _exported_ from this module and 
        (b) that other module _imports_ this one.

        Components, directives and pipes must belong to _exactly_ one module.
        The compiler emits an error if we try to declare the same class in more than one module.

        **Do not re-declare a class imported from another module.**

  tr
    td(style="vertical-align: top") <code>providers</code>
    td
      :marked
        A list of dependency injection providers.
        
        Angular registers these providers with the root injector of the module's execution context.
        That's the application's root injector for all modules loaded when the application starts.

        Angular can inject one of these provider services into any component in the application.
        If this module provides the `HeroService`, or any module loaded at launch provides the `HeroService`,
        Angular can inject the same `HeroService` intance into any app component.
        
        A lazy loaded module has its own sub-root injector which typically 
        is a direct child of the application root injector.

        Lazy loaded services are scoped to the lazy module's injector.
        If a lazy loaded module also provides the `HeroService`, 
        any component created within that module's context (e.g., by router navigation)
        gets the local instance of the service, not the instance in the root application injector.

        Components in external modules continue to receive the instance created for the application root. 

  tr
    td(style="vertical-align: top") <code>imports</code>
    td
      :marked
        A list of supporting modules.
        
        Specifically, the list of modules whose exported components, directives or pipes 
        are referenced by the component templates declared in this module.
        
        A component template can [reference](#q-template-reference) another component, directive or pipe
        on two conditions: either the referenced class is declared in this module
        or the class was imported from another module. 

        A component can use the `NgIf` and `NgFor` directives only because its parent module 
        imported the Angular `CommonModule` (perhaps indirectly by importing `BrowserModule`).
       
        We can import many standard directives with the `CommonModule`.
        But some familiar directives belong to other modules.
        A component template can bind with `[(ngModel)]` only after importing the Angular `FormsModule`.
  tr
    td(style="vertical-align: top") <code>exports</code>
    td
      :marked
        A list of declarations &mdash; **component**, **directive**, and **pipe** classes &mdash; that 
        an importing module can use.

        Exported declarations are the module's _public API_. 
        A component in another module can [reference](#q-template-reference) _this_ module's `HeroComponent` 
        if (a) it imports this module and (b) this module exports `HeroComponent`.
        
        Declarations are private by default.
        If this module does _not_ export `HeroComponent`, no other module can see it.

        Importing a module does _not_ automatically re-export the imported module's exports.
        Module 'B' can't use `ngIf` just because it imported module `A` which imported `CommonModule`.
        Module 'B' must import `CommonModule` itself.

        A module can list another module among its `exports` in which case
        all of that module's public components, directives, and pipes are exported.

        [Re-export](#q-re-export) makes module transitivity explicit.
        If Module 'A' re-exports `CommonModule` and Module 'B' imports Module 'A',
        Module 'B' components can use `ngIf` even though 'B' itself didn't import `CommonModule`.

  tr
    td(style="vertical-align: top") <code>bootstrap</code>
    td
      :marked
        A list of components that can be [bootstrapped](#bootstrap).

        Usually there is only one component in this list, the _root component_ of the application.
        
        Angular can launch with multiple bootstrap components, 
        each with its own location in the host web page.

        A bootstrap component is automatically an `entryComponent` 

  tr
    td(style="vertical-align: top") <code>entryComponents</code>
    td
      :marked
        A list of components that are _not_ [referenced](#q-template-reference) in a reachable component template.

        Most developers will never set this property. Here's why.

        The [_Angular Compiler_](#q-angular-compiler) must know about every component actually used in the application.
        The compiler can discover most components by walking the tree of references 
        from one component template to another.

        But there's always at least one component that is not referenced in any template:
        the root component, `AppComponent`, that we bootstrap to launch the app.
        That's why it's called an _entry component_.

        Routed components are also _entry components_ because they aren't referenced in a template either.
        The router creates them and drops them into the DOM near a `<router-outlet>`.

        While the bootstrapped and routed components are _entry components_, 
        we usally don't have to add them to a module's `entryComponents` list.

        Angular automatically adds components in the module's `bootstrap` list to the `entryComponents` list.
        The `RouterModule` adds routed components to that list.

        That leaves only two sources of undiscoverable components.
        1. Components bootstrapped using one of the imperative techniques.
        1. Components dynamically loaded into the DOM by some means other than the router.
        
        Both are advanced techniques that few developers will ever employ. 
        If you are one of those few, you'll have to add these components to the 
        `entryComponents` list yourself, either programmatically or by hand.

a#faq 
.l-main-section
:marked
  ## FAQ: Frequently Asked Questions

  Declarations
  * [What classes should I add to _declarations_?](#q-what-to-declare)
  * [What is a _declarable_?](#q-declarable)
  * [What classes should I *not* add to _declarations_?](#q-what-not-to-declare)
  * [Why list the same component in multiple _NgModule_ properties?](#q-why-multiple-mentions)
  * [What does "_Can't bind to 'x' since it isn't a known property of 'y'_" mean?](#q-why-cant-bind-to)

  Imports
  * [What should I import?](#q-what-to-import)
  * [Should I import _BrowserModule_ or _CommonModule_?](#q-browser-vs-common-module)
  * [What if I import the same module twice?](#q-reimport)

  Exports  
  * [What should I export?](#q-what-to-export)
  * [What should I *not* export?](#q-what-not-to-export)
  * [Can I re-export imported classes and modules?](#q-re-export)
  * [What is the _forRoot_ method?](#q-for-root)

  Service Providers  
  * [Why is a service provided in a feature module visible everywhere?](#q-module-provider-visibility)
  * [Why is a service provided in a _lazy loaded_ module visible only to that module?](#q-lazy-loaded-module-provider-visibility) 
  * [What if two modules provide the _same_ service?](#q-module-provider-duplicates)
  * [How do I restrict service scope to a module?](#q-component-scoped-providers)
  * [Should I add providers to the root _AppModule_ or the root _AppComponent_?](#q-root-component-or-module)
  * [Why is it bad if _SharedModule_ provides a service to a lazy loaded module?](#q-why-it-is-bad)
  * [Why does lazy loading create a child injector?](#q-why-child-injector)  
  * [How can I tell if a module or service was previously loaded?](#q-is-it-loaded)

  Entry Components
  * [What is an _entry component_?](#q-entry-component-defined)
  * [What is the difference between a _bootstrap_ component and an _entry component_?](#q-bootstrap_vs_entry_component)
  * [When do I add components to _entryComponents_?](#q-when-entry-components)
  * [Why does Angular need _entryComponents_?](#q-why-entry-components)

  General
  * [What kinds of modules should I have and how should I use them?](#q-module-recommendations)
  * [What's the difference between Angular and JavaScript Modules?](#q-ng-vs-js-modules)
  * [What is a "template reference"?](#q-template-reference)
  * [How does Angular find components, directives, and pipes in a template?](#q-template-reference)  
  * [What is the Angular Compiler?](#q-angular-compiler)  

.l-hr

a#q-what-to-declare
.l-main-section
:marked
  ### What classes should I add to _declarations_?

  Add [declarable](#q-declarable) classes &mdash; components, directives, and pipes &mdash; to a `declarations` list.

  These classes must be declared in _exactly one_ module of the application.
  Declare them in _this_ module if they _belong_ to this module.

.l-hr

a#q-declarable
.l-main-section
:marked
  ### What is a _declarable_?
  
  _Declarables_ are the class types &mdash; components, directives, and pipes &mdash;
  that you can add to a module's `declarations` list. 
  They're the _only_ classes that you can add to `declarations`.

.l-hr

a#q-what-not-to-declare
.l-main-section
:marked
  ### What classes should I _not_ add to _declarations_?

  Only [declarable](#q-declarable) classes can be added to a module's `declarations` list.

  Do *not* declare 
  * a class that is already declared in another module, whether an app module, @angular module, or 3rd party module

  * an array of directives imported from another module. 
  For example, do not declare FORMS_DIRECTIVES from `@angular/forms`.

  * module classes
  
  * service classes

  * non-Angular classes and objects such as 
  strings, numbers, functions, entity models, configurations, business logic, and helper classes.

.l-hr

a#q-why-multiple-mentions
.l-main-section
:marked
  ### Why list the same component in multiple _NgModule_ properties?

  We often see `AppComponent` listed in both `declarations` and `bootstrap`.
  We might see `HeroComponent` listed in `declarations`, `exports`, and `entryComponents`.
  
  That _feels_ redundant but these properties have different functions 
  and we can't infer that membership in one list implies membership in another list.

  * `AppComponent` could be declared in this module but not bootstrapped.
  * `AppComponent` could be bootstrapped in this module but declared in a different feature module.
  * `HeroComponent` could be imported from another app module (so we can't declare it) and re-exported by this module.
  * `HeroComponent` could be exported for inclusion in an external component's template and also dynamically loaded in a pop-up dialog.

.l-hr

a#q-why-cant-bind-to
.l-main-section
:marked
  ### What does "_Can't bind to 'x' since it isn't a known property of 'y'_" mean?

  This error usually means either that you neglected to declare the directive "x" 
  or you haven't imported the module to which "x" belongs.

  For example, if "x" is `ngModel`, you probably haven't imported the `FormsModule` from `@angular/forms`.

  Perhaps you declared "x" in an application sub-module but forgot to export it? 
  The "x" class won't be visible to other modules until you add it to the `exports` list.

.l-hr

a#q-what-to-import
.l-main-section
:marked
  ### What should I import?

  Import modules whose public (exported) [declarable classes](#q-declarable) 
  you need to reference in this module's component templates.

  This invariably means importing `CommonModule` from `@angular/common` for access to
  the Angular directives such as `NgIf` and `NgFor`. 
  You can import it directly or from another module that [re-exports](#q-reexport) it.

  Import `FormsModule` from `@angular/forms` 
  if your components have `[(ngModel)]` two-way binding expressions.

  Import _shared_ and _feature_ modules when this module's components incorporate their
  components, directives, and pipes.

  Only [import _BrowserModule_](#q-browser-vs-common-module) in the root `AppModule`.

.l-hr

a#q-browser-vs-common-module
.l-main-section
:marked
  ### Should I import _BrowserModule_ or _CommonModule_?

  The **root application module** (`AppModule`) of almost every browser application
  should import `BrowserModule` from `@angular/core`.
  
  `BrowserModule` provides services that are essential to launch and run a browser app.

  `BrowserModule` also re-exports `CommonModule` from `@angular/common`
  which means that component in the `AppModule` module also have access to
  the Angular directives every app needs such as `NgIf` and `NgFor`.
  
  _Do not import_ `BrowserModule` in any other module.
  *Feature modules* and *lazy loaded modules* should import `CommonModule` instead.
  They need the common directives. They don't need to re-install the app-wide providers. 
.l-sub-section
  :marked
    `BrowserModule` throws an error if you try to lazy load  a module that imports it.
:marked
  Importing `CommonModule` also frees feature modules for use on _any_ target platform, not just browsers,
  a fact of some interest to authors of cross-platform libraries.

.l-hr 
a#q-reimport
.l-main-section
:marked
  ### What if I import the same module twice?
  
  That's not a problem. When three modules all import Module 'A', 
  Angular evaluates Module 'A' once, the first time it encounters it, and does not do so again. 

  That's true at whatever level `A` appears in a hierarchy of imported modules.
  When Module 'B' imports Module 'A', Module 'C' imports 'B', and Module 'D' imports `[C, B, A]`,
  then 'D' triggers the evaluation of 'C' which triggers the evaluation of 'B' which evaluates 'A'.
  When Angular gets to the 'B' and 'A' in 'D', they're already cached and ready to go.

  Angular does not like modules with circular references so don't let Module 'A' import Module 'B' which imports Module 'A'.

.l-hr 
a#q-what-to-export
.l-main-section
:marked
  ### What should I export?

  Export [declarable](#q-declarable) classes that components in _other_ modules
  should be able to reference in their templates. These are your _public_ classes.
  If you don't export a class, it stays _private_, visible only to other component
  declared in this module.

  You _can_ export any declarable class &mdash; components, directives, and pipes &mdash; 
  whether it is declared in this module or in an imported module.

  You _can_ re-export entire imported modules which effectively re-exports all of their exported classes.
  A module can even export a module that it doesn't import. 

.l-hr

a#q-what-not-to-export
.l-main-section
:marked
  ### What should I *not* export?

  Do *not* export

  * Private components, directives, and pipes that you need only within components declared in this module.
  If you don't want another module to see it, don't export it.

  * Non-declarable objects such as services, functions, configurations, entity models, etc.
  
  * Components that are only loaded dynamically by the router or by bootstrapping.
  Such [entry components](#q-entry-component-defined) can never be selected in another component's template.
  There's no harm in exporting them but no benefit either. 

  * Pure service modules that don't have public (exported) declarations. 
  For example, there is no point in re-exporting `HttpModule` because it doesn't export anything.
  It's only purpose is to add http service providers to the application as a whole.

.l-hr

a#q-reexport
a#q-re-export
.l-main-section
:marked
  ### Can I re-export classes and modules?

  Absolutely!

  Modules are a great way to selectively aggregate classes from other modules and
  re-export them in a consolidated, convenience module.

  A module can re-export entire modules which effectively re-exports all of their exported classes.
  Angular's own `BrowserModule` exports a couple of modules like this:
code-example.
  exports: [CommonModule, ApplicationModule]

:marked
  A module can export a combination of its own declarations, selected imported classes, and imported modules.
.l-sub-section
  :marked
    Don't bother re-exporting pure service modules.
    Pure service modules don't export [declarable](#q-declarable) classes that another module could use.
    For example, there is no point in re-exporting `HttpModule` because it doesn't export anything.
    It's only purpose is to add http service providers to the application as a whole.

.l-hr

a#q-for-root
.l-main-section
:marked
  ### What is the _forRoot_ method?

  The `forRoot` static method is a convention that makes it easy for developers to configure the module's provider(s).
  
  The `RouterModule.forRoot` method is a good example. 
  Apps pass a `Routes` object to `RouterModule.forRoot` in order to configure the app-wide `Router` service with routes. 
  `RouterModule.forRoot` returns a [ModuleWithProviders](../api/core/index/ModuleWithProviders-interface.html). 
  We add that result to the `imports` list of the root `AppModule`.

.alert.is-important
  :marked
    Only call and import a `.forRoot` result in the root application module, `AppModule`.
    Importing it in any other module, particularly in a lazy loaded module,
    is contrary to the intent and is likely to produce a runtime error.
:marked
  `RouterModule` also offers a `forChild` static method for configuring the routes of lazy loaded modules.

  **_forRoot_** and **_forChild_** are conventional names for methods that
  configure services in root and feature modules respectively.
  
  Angular doesn't recognize these names but Angular developers do.
  Follow this convention when you write similar modules with configurable service providers.

.l-hr

a#q-module-provider-visibility
.l-main-section
:marked
  ### Why is a service provided in a feature module visible everywhere?

  Providers listed in the `@NgModule.providers` of a bootstrapped module have **application scope**.
  Adding a service provider to `@NgModule.providers` effectively publishes the service to the entire application.

  When we import a module,
  Angular adds the module's service providers (the contents of its `providers` list)
  to the application _root injector_.

  This makes the provider visible to every class in the application that knows the provider's lookup token.

  This is by design. 
  Extensibility through module imports is a primary goal of the Angular module system.
  Merging module providers into the application injector
  makes it easy for a module library to enrich the entire application with new services.
  By adding the `HttpModule` once, every application component can make http requests. 
  
  However, this can feel like an unwelcome surprise if you are expecting the module's services 
  to be visible only to the components declared by that feature module.
  If the `HeroModule` provides the `HeroService` and the root `AppModule` imports `HeroModule`, 
  any class that knows the `HeroService` _type_ can inject that service, 
  not just the classes declared in the `HeroModule`.

.l-hr

a#q-lazy-loaded-module-provider-visibility
.l-main-section
:marked
  ### Why is a service provided in a lazy loaded module visible only to that module?
  
  Unlike providers of the modules loaded at launch, 
  providers of lazy loaded modules are *module-scoped*.

  When the Angular router lazy-loads a module, it creates a new execution context.
  That [context has its own injector](#q-why-child-injector "Why Angular creates a child injector") which is a direct child of the application injector.

  The router adds the lazy module's providers and the providers of its imported modules to this child injector.

  These providers are insulated from changes to application providers with the same lookup token.
  When the router creates a component within the lazy loaded context,
  Angular prefers service instances created from these providers to the service instances of the application root injector.

.l-hr
a#q-module-provider-duplicates
.l-main-section
:marked
  ### What if two modules provide the _same_ service?
  
  When two imported modules, loaded at the same time, list a provider with the same token, 
  the second module's provider "wins". That's because both providers are added to the same injector.

  When Angular looks to inject a service for that token, 
  it creates and delivers the instance created by the second provider.

  _Every_ class that injects this service gets the instance created by the second provider.
  Even classes declared within the first module get the instance created by the second provider.
  _This can be an unwelcome surprise_.

  If Module A provides a service for token 'X' and imports a module B
  that also provides a service for token 'X', then Module A's service definition "wins".

  The service provided by the root `AppModule` takes precedence over services provided by imported modules.
  The `AppModule` always wins.

.l-hr

a#q-component-scoped-providers
.l-main-section
:marked
  ### How do I restrict service scope to a module?

  When a module is loaded at application launch,
  its `@NgModule.providers` have ***application-wide scope***.
  They are visible throughout the application as discussed [above](#application-scoped-providers).
  
  Imported providers are easily replaced by providers from another imported module.
  Such replacement may be by design. It could be unintentional and have adverse consequences.

.alert.is-important
  :marked
    As a general rule, import modules with providers _exactly once_, preferably in the application's _root module_.
    That's also usually the best place to configure, wrap, and override them.

:marked
  Suppose a module requires a customized `HttpBackend` that adds a special header for all Http requests.
  If another module elsewhere in the application also customizes `HttpBackend`
  or merely imports the `HttpModule`, it could override this module's `HttpBackend` provider, 
  losing the special header. The server will reject http requests from this module.

.alert.is-important
  :marked
    Avoid this problem by importing the `HttpModule` only in the `AppModule`, the application _root module_.

:marked
  If you must guard against this kind of "provider corruption", *don't rely on a launch-time module's `providers`.*

  Load the module lazily if you can. 
  Angular gives a [lazy-loaded module](#q-lazy-loaded-module-provider-visibility) its own child injector.
  The module's providers are visible only within the component tree created with this injector.

  If you must load the module eagerly, when the application starts,
  ***provide the service in a component instead.***
  
  Continuing with the same example, suppose the components of a module truly require a private, custom `HttpBackend`.

  Create a "top component" that acts as the root for all of the module's components. 
  Add the custom `HttpBackend` provider to the top component's `providers` list rather than the module's `providers`.
  Recall that Angular creates a child injector for each component instance and populates the injector
  with the component's own providers.
  
  When a child of this component _asks_ for the `HttpBackend` service,
  Angular provides the local `HttpBackend` service, 
  not the version provided in the application root injector.
  Child components will make proper http requests no matter what other modules do to `HttpBackend`.
  
  Be sure to create module components as children of this module's top component.

  You can embed the child components in the top component's template.
  Alternatively, make the top component a routing host by giving it a `<router-outlet>`.
  Define child routes and let the router load module components into that outlet.

.l-hr

a#q-root-component-or-module
.l-main-section
:marked
  ### Should I add providers to the root _AppModule_ or the root _AppComponent_?

  Most apps launch with an initial set of service providers. 
  Should we register those providers on the root `AppModule` (`@NgModule.providers`) or
  the root `AppComponent` (`@Component.providers`)?

  **_List such providers in the root_ `AppModule` _unless you have a compelling reason to do otherwise_**.
  
  Angular registers all startup module providers with the application root injector.
  The services created from root injector providers are available to the entire application. 
  They are _application-scoped_.
  
  Certain services (e.g., the `Router`) only work when registered in the application root injector.

  By contrast, Angular registers `AppComponent` providers with the `AppComponent`'s own injector.
  `AppComponent`services are available to that component and its component tree. 
  They are _component-scoped_.

  The `AppComponent`'s injector is a _child_ of the root injector, one down in the injector hierarchy.
  That is _almost_ the entire application for apps that don't use the router.
  But "almost" isn't good enough for routed applications.

  `AppComponent` services don't exist at the root level where routing operates. 
  Lazy loaded modules can't reach them.
  In this sample applications, if we had registered `UserService` in the `AppComponent`, 
  the `HeroComponent` couldn't inject it.
  The application  would fail the moment a user navigated to "Heroes".

  We _can_ register a service in `AppComponent` providers if the app doesn't use routing.
  We _should_ register a service in `AppComponent` providers if the service must be hidden
  from components outside the `AppComponent` tree. 
  
  These are special cases.
  When in doubt, register with the `AppModule`.

.l-hr
  
a#q-why-it-is-bad
.l-main-section
:marked
  ### Why is it bad if _SharedModule_ provides a service to a lazy loaded module?

  This question arose earlier when we discussed the importance
  of keeping providers out of the [_SharedModule_](#no-shared-module-providers).

  Suppose we had listed the `UserService` in the module's `providers` (which we did not).
  Suppose every module imports this `SharedModule` (which they all do).

  When the app starts, Angular eagerly loads the `AppModule` and the `ContactModule`.

  Both instances of the imported `SharedModule` would provide the `UserService`. 
  Angular registers one of them in the root app injector (see [above](#q-reimport)).
  Then some component injects `UserService`, Angular finds it in the app root injector,
  and delivers the app-wide singleton `UserService`. No problem. 

  Now consider the `HeroModule` _which is lazy loaded!_

  When the router lazy loads the `HeroModule`, it creates a child injector and registers the `UserService`
  provider with that child injector. The child injector is _not_ the root injector.
  
  When Angular creates a lazy `HeroComponent`, it must inject a `UserService`.
  This time it finds a `UserService` provider in the lazy module's _child injector_
  and creates a _new_ instance of the `UserService`.
  This is an entirely different `UserService` instance
  than the app-wide singleton version that Angular injected in one of the eagerly loaded components.

  That's almost certainly a mistake.
.l-sub-section
  :marked
    Prove it for yourself.
    Run the <live-example>live example</live-example>.
    Modify the `SharedModule` so that it provides the `UserService` rather than the `CoreModule`.
    Then toggle between the "Contact" and "Heroes" links a few times.
    The username goes bonkers as the Angular creates a new `UserService` instance each time.

.l-hr

a#q-why-child-injector
.l-main-section
:marked
  ### Why does lazy loading create a child injector?
  
  Angular adds `@NgModule.providers` to the application root injector ... unless the module is lazy loaded.
  Then it creates a _child injector_ and adds the module's providers to the child injector.

  This means that a module behaves differently depending on whether it is loaded during application start
  or lazy loaded later. Neglecting that difference can lead to [adverse consequences](#q-why-it-is-bad).

  Why doesn't Angular add lazy loaded providers to the app root injector as it does for eagerly loaded modules? 
  Why the inconsistency?
  
  The answer is grounded in a fundamental characteristic of the Angular dependency injection system.
  An injector can add providers _until it is first used_. 
  Once an injector starts creating and delivering services, its provider list is frozen. No new providers allowed.

  When an applications starts, Angular first configures the root injector with the providers of all eagerly loaded modules
  _before_ creating its first component and injecting any of the provided services. 
  Once the application begins, the app root injector is closed to new providers.

  Time passes. Application logic triggers lazy loading of a module.
  Angular must add the lazy loaded module's providers to an injector _somewhere_. 
  It can't added them to the app root injector because that injector is closed to new providers. 
  So Angular creates a new child injector for the lazy loaded module context.

.l-hr

a#q-is-it-loaded
.l-main-section
:marked
  ### How can I tell if a module or service was previously loaded?

  Some modules and its services should only be loaded once by the root `AppModule`.
  Importing the module a second time by lazy loading a module could [produce errant behavior](#q-why-it-is-bad) 
  that may be difficult to detect and diagnose.

  We can guard against that danger by writing a constructor that attempts to inject the module or service
  from the root app injector. If the injection succeeds, the class has been loaded a second time.
  We can throw an error or take other remedial action.

  Certain Angular modules (such as `BrowserModule`) implements such a guard 
  as does this sample's [_CoreModule_ constructor](#prevent-reimport).

.l-hr

a#q-entry-component-defined
.l-main-section
:marked
  ### What is an _entry component_?

  Any component that Angular loads _imperatively_ by type is an _entry component_,
  
  A component loaded _declaratively_ via its selector is _not_ an entry component.

  Most application components are loaded declaratively. 
  Angular uses the component's selector to locate the element in the template. 
  It then creates the HTML representation of the component and inserts it into the DOM at the selected element.
  These are not entry components.

  A few components are only loaded dynamically and are _never_ referenced in a component template. 

  The bootstrapped root `AppComponent` is an _entry component_.
  True, its selector matches an element tag in `index.html`.
  But `index.html` is not a component template and the `AppComponent` 
  selector doesn't match an element in any component template.
  
  Angular loads `AppComponent` dynamically either because we listed it _by type_ in `@NgModule.bootstrap` 
  or because we boostrapped it imperatively with the module's `ngDoBootstrap` method.

  Components in route definitions are also _entry components_.
  A route definition refers to a component by its _type_. 
  The router ignores a routed component's selector (if it even has one) and 
  loads the component dynamically into a `RouterOutlet`. 

  The compiler can't discover these _entry components_ by looking for them in other component templates. 
  We must tell it about them ... by adding them to the `entryComponents` list.
  
  Angular automatically adds two kinds of components to the module's `entryComponents`:
  1. the component in the `@NgModule.bootstrap` list
  1. components referenced in router configuration

  We don't have to mention these components explicitly although it does not harm to do so.

.l-hr

a#q-bootstrap_vs_entry_component
.l-main-section
:marked
  ### What's the difference between a _bootstrap_ component and an _entry component_?

  A bootstrapped component _is_ an [entry component](#entry-component-defined).
  It's an entry component that Angular loads into the DOM during the bootstrap (application launch) process.
  Other entry components are loaded dynamically by other means such as with the router.

  The `@NgModule.bootstrap` property tells the compiler _both_ that this is an entry component _and_
  that it should generate code to bootstrap the application with this component.

  There is no need to list a component in both the `bootstrap` and `entryComponent` lists 
  although it is harmless to do so.

.l-hr

a#q-when-entry-components
.l-main-section
:marked
  ### When do I add components to _entryComponents_?
  
  Most application developers won't need to add components to the `entryComponents`.

  Angular adds certain components to _entry components_ automatically.
  Components listed in `@NgModule.bootstrap` are added automatically.
  Components referenced in router configuration are added automatically.
  These two mechanisms account for almost all entry components.
  
  If your app happens to bootstrap or dynamically load a component _by type_ in some other manner, 
  you'll have to add it to `entryComponents` explicitly.

  Although it's harmless to add components to this list,
  it's best to add only the components that are truly _entry components_.
  Don't include components that [are referenced](#q-template-reference) 
  in the templates of other components.

.l-hr

a#q-why-entry-components
.l-main-section
:marked
  ### Why does Angular need _entryComponents_?
  _Entry components_ are also declared.
  Why doesn't the Angular compiler generate code for every component in `@NgModule.declarations`?
  Then we wouldn't need entry components.
  
  The reason is _tree shaking_. For production apps we want to load the smallest, fastest code possible.
  The code should contain only the classes that we actually need.
  It should exclude a component that's never used, whether or not that component is declared.

  In fact, many libraries declare and export components we'll never use.
  The _tree shaker_ will drop these components from the final code package
  if we don't reference them. 

  If the [Angular compiler](#angular-compilar) generated code for every declared component, 
  it would defeat the purpose of the tree shaker.
  
  Instead, the compiler adopts a recursive strategy that generates code only for the components we use.
  
  It starts with the entry components, 
  then it generates code for the declared components it [finds](#q-template-reference) in an entry component's template,
  then for the declared components it discovers in the templates of previously compiled components,
  and so on. At the end of the process, it has generated code for every  entry component
  and every component reachable from an entry component.

  If a component isn't an _entry component_ or wasn't found in a template, 
  the compiler omits it. 


.l-hr

a#q-module-recommendations
.l-main-section
:marked
  #### What kinds of modules should I have and how should I use them?

  Every app is different and developers have varying levels of experience and comfort with the available choices.
  Some suggestions and guidelines appear to have wide appeal.

.alert.is-important
  :marked
    The following is preliminary guidance based on early experience using Angular modules in a few applications.
    Read with appropriate caution and reflection.

:marked
  #### _SharedModule_
  Create a `SharedModule` with the components, directives, and pipes that you use
  everywhere in your app. This module should consist entirely of `declarations`
  most of them exported. 
  
  It may re-export other [widget modules](#widget-feature-module) such as `CommonModule`,
  `FormsModule` and modules with the UI controls that you use most widely.
  
  It should ***not*** have `providers` for reasons [explained earlier](#q-why-it-is-bad).
  Nor should any of its imported or re-exported modules have `providers`.
  Know what you're doing and why if you deviate from this guideline.

  Import the `SharedModule` in your _feature_ modules, 
  both those loaded when the app starts and those you lazy load later.

  #### _CoreModule_
  Create a `CoreModule` with `providers` for the singleton services you load when the application starts.
  
  Import `CoreModule` in the root `AppModule` only.
  Never import `CoreModule` in any module other than the root `AppModule`.

  Consider making `CoreModule` a [pure services module](#service-feature-module) with no `declarations`.

.l-sub-section
  :marked
    This chapter sample departs from that advice by declaring and exporting two components that are
    only used within the root `AppComponent` declared by `AppModule`. 
    Someone following this guideline strictly would have declared these components in the `AppModule` instead.

:marked
  #### Feature Modules
  Create _Feature Modules_ around specific application business domains, user workflows, and utility collections.
  
  Feature modules tend to fall into one of these four groups:
    * [Domain Feature Modules](#domain-feature-module)
    * [Routed Feature Modules](#routed-feature-module)
    * [Service Feature Modules](#service-feature-module)
    * [Widget Feature Modules](#widget-feature-module)

.l-sub-section
  :marked
    Real world modules are often hybrids that knowingly deviate from the following guidelines.
    They are guidelines, not laws.
    Follow them until you have a good reason to do otherwise.

table
  tr
    th(style="vertical-align: top") Feature Module
    th(style="vertical-align: top") Guidelines
  tr
    td(style="vertical-align: top")<a id="domain-feature-module"></a>Domain
    td
      :marked
        Domain Feature Modules deliver a user experience **dedicated to a particular application domain**
        like editing a customer or placing an order.
        
        They typically have a top component that acts as the feature root.
        Private, supporting sub-components descend from it.

        Domain feature module consist mostly of _declarations_.
        Only the top component is exported.

        Domain feature modules rarely have _providers_.
        When they do, the lifetime of the provided services
        should be the same as the lifetime of the module.
        
        Do not provide application-wide singleton services in a domain feature module.

        Domain feature modules are typically imported _exactly once_ by a larger feature module.
        
        They might be imported by the root `AppModule` of a small application that lacks routing.

      .l-sub-section
        :marked
          For an example, see this chapter's first version of the [_ContactModule_](#contact-module-v1)
          before we introduced routing.
  tr
    td(style="vertical-align: top")<a id="routed-feature-module"></a>Routed
    td
      :marked
        _Routed Feature Modules_ are _Domain Feature modules_ 
        whose top components are the **targets of router navigation routes**.

        All lazy loaded modules are routed feature modules by nature.

        This chapter's `ContactModule`, `HeroModule` and `CrisisModule` are routed feature modules.

        Routed Feature Modules _should not export anything_. 
        They don't have to because none of their components ever appear in the template of an external component.

        Routed Feature Modules are _never imported_.

        Routed Feature Modules rarely have _providers_ for reasons [explained earlier](#q-why-it-is-bad).
        When they do, the lifetime of the provided services
        should be the same as the lifetime of the module.
        
        Do not provide application-wide singleton services in a routed feature module
        or in a module that the routed module imports.

  tr
    td(style="vertical-align: top")<a id="service-feature-module"></a>Service
    td
      :marked
        _Service Modules_ **provide utility services** such as data access and messaging.

        Ideally they consist entirely of _providers_ and have no _declarations_.
        The `CoreModule` and Angular's `HttpModule` are good examples.

        Service Modules should _only_ be imported by the root `AppModule`.
        
        Do **not** import them in other feature modules.
        Know what you're doing and why if you deviate from this guideline.
  tr
    td(style="vertical-align: top")<a id="widget-feature-module"></a>Widget
    td
      :marked
        A _Widget Module_ makes **components, directives, and pipes** available to external modules.
        
        `CommonModule` and `SharedModule` are widget modules.
        Many third party UI component libraries are widget modules.

        A Widget Module should consist entirely of _declarations_, most of them exported. 

        A Widget Module should rarely have _providers_. 
        Know what you're doing and why if you deviate from this guideline.

        Import Widget Modules in any module whose component templates need the widgets.

:marked
  The following table summarizes the key characteristics of each _Feature Module_ group.
.l-sub-section
  :marked
    Real world modules are often hybrids that knowingly deviate from these guidelines.
table
  tr
    th Feature Module
    th Declarations 
    th Providers 
    th Exports
    th Imported By
    th Examples
  tr
    td Domain
    td Yes
    td Rare
    td Top Component
    td Feature, <code>AppModule</code>
    td <code>ContactModule</code> (before routing)
  tr
    td Routed
    td Yes
    td Rare
    td None
    td Nobody
    td <code>ContactModule</code>, <code>HeroModule</code>, <code>CrisisModule</code>
  tr
    td Service
    td No
    td Yes
    td No
    td <code>AppModule</code>
    td <code>HttpModule</code>, <code>CoreModule</code>
  tr
    td Widget
    td Yes
    td Rare
    td Yes
    td Feature
    td <code>CommonModule</code>, <code>SharedModule</code>

.l-hr

a#q-ng-vs-js-modules
.l-main-section
:marked
  ### What's the difference between Angular and JavaScript Modules?

  Angular and JavaScript are two different yet complementary module systems.

  In modern JavaScript, [every file is a _module_](http://exploringjs.com/es6/ch_modules.html).
  Within each file we write an `export` statement to make parts of the module public:

code-example(format='.').
  export class AppComponent { ... }

:marked
  Then we `import` a part in another module:

code-example(format='.').
  import { AppComponent }  from './app.component';

:marked
  This kind of modularity is a feature of the _JavaScript language_.

  An _Angular Module_ is a feature of _Angular_ itself.

  Angular's `NgModule` also has `imports` and `exports` and they serve a similar purpose. 

  We _import_ other Angular modules so we can use their exported classes in component templates.
  We _export_ this Angular module's classes so they can be imported and used by components of _other_ modules.
  
  The Angular module classes differ from JavaScript module class in three key respects:

  1. An Angular module bounds [_declarable classes_](#q-declarables) only. 
  Declarables are the only classes that matter to the [Angular compiler](#angular-compiler).

  1. Instead of defining all member classes in one giant file (as in a JavaScript module),
     we list the module's classes in the `@NgModule.declarations` list.

  1. An Angular module can only export the [_declarable classes_](#q-declarables) 
  it owns or imports from other modules.
  It doesn't declare or export any other kind of class.

  The Angular Module is also special in another way.
  Unlike JavaScript modules, an Angular module can extend the _entire_ application with services
  by adding providers to the `@NgModule.providers` list.

.alert.is-important
  :marked
    The provided services do not belong to the module nor are they scoped to the declared classes.
    They are available _everywhere_.

:marked
  Here's an _Angular Module_ class with imports, exports, and declarations.
+makeExample('ngmodule/ts/app/contact/contact.module.2.ts', 'class')(format=".")
:marked
  Of course we use _JavaScript_ modules to write _Angular_ modules as seen in the complete `contact.module.ts` file:
+makeExample('ngmodule/ts/app/contact/contact.module.2.ts', '', 'app/contact/contact.module.ts')(format=".")

.l-hr

a#q-template-reference
.l-main-section
h4.
  How does Angular find components, directives, and pipes in a template?<br>What is a <i><b>template reference</b></i>?
:marked
  The [Angular compiler](#q-angular-compiler) looks inside component templates 
  for other components, directives, and pipes. When it finds one, that's a "template reference".

  The Angular compiler finds a component or directive in a template when it can match the **selector** of that 
  component or directive to some HTML in that template. 

  The compiler finds a pipe if the pipe's **name** appears within the pipe syntax of the template HTML.

  Angular only matches selectors and pipe names for classes that are declared by this module
  or exported by a module that this module imports.

.l-hr

a#q-angular-compiler  
.l-main-section
:marked
  ### What is the Angular Compiler?

  The _Angular Compiler_ converts the application code we write into highly performant JavaScript code.
  The `@NgModule` metadata play an important role in guiding the compilation process.

  The code we write is not immediately executable.
  Consider **components**.
  Components have templates that contain custom elements, attribute directives, Angular binding declarations, 
  and some peculiar syntax that clearly isn't native HTML.
  
  The _Angular Compiler_ reads the template markup, 
  combines it with the corresponding component class code, and emits _component factories_.
  
  A component factory creates a pure, 100% JavaScript representation
  of the component that incorporates everything described in its `@Component` metadata: 
  the HTML, the binding instructions, the attached styles ... everything.

  Because **directives** and **pipes** appear in component templates, 
  the _Angular Compiler_ incorporates them into compiled component code too.

  `@NgModule` metadata tells the _Angular Compiler_ what components to compile for this module and 
  how to link this module with other modules.