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NgModules

NgModules help organize an application into cohesive blocks of functionality.

An NgModule is a class adorned with the @NgModule decorator function. @NgModule takes a metadata object that tells Angular how to compile and run module code. It identifies the module's own components, directives, and pipes, making some of them public so external components can use them. @NgModule may add service providers to the application dependency injectors. And there are many more options covered here.

Before reading this page, read the The Root Module page, which introduces NgModules and the essentials of creating and maintaining a single root AppModule for the entire application.

This page covers NgModules in greater depth.

Live examples

This page explains NgModules 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 the sample:

A minimal NgModule app
The first contact module
The revised contact module
Just before adding SharedModule
The final version

Frequently asked questions (FAQs)

This page covers NgModule concepts in a tutorial fashion.

The companion NgModule FAQs guide offers answers to specific design and implementation questions. Read this page before reading those FAQs.

{@a angular-modularity}

Angular modularity

Modules are a great way to organize an application and extend it with capabilities from external libraries.

Many Angular libraries are modules (such as FormsModule, HttpModule, and RouterModule). Many third-party libraries are available as NgModules (such as Material Design, Ionic, AngularFire2).

NgModules 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. Services 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 NgModule is a class decorated with @NgModule metadata. The metadata do the following:

Declare which components, directives, and pipes belong to the module.
Make some of those classes public so that other component templates can use them.
Import other modules with the components, directives, and pipes needed by the components in this module.
Provide services at the application level that any application component can use.

Every Angular app has at least one module class, the root module. You bootstrap that module to launch the application.

The root module is all you need in a simple application with a few components. As the app grows, you refactor the root module into feature modules that represent collections of related functionality. You then import these modules into the root module.

Later in this page, you'll read about this process. For now, you'll start with the root module.

{@a root-module}

The root AppModule

Every Angular app has a root module class. By convention, the root module class is called AppModule and it exists in a file named app.module.ts.

The AppModule from the QuickStart seed on the Setup page is as minimal as possible:

The @NgModule decorator defines the metadata for the module. This page takes an intuitive approach to understanding the metadata and fills in details as it progresses.

The metadata imports a single helper module, BrowserModule, which 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 module's component templates.

The declarations list identifies the application's only component, the root component, the top of the app's rather bare component tree.

The example AppComponent simply displays a data-bound title:

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}

Bootstrapping in main.ts

You launch the application by bootstrapping the AppModule in the main.ts file.

Angular offers a variety of bootstrapping options targeting multiple platforms. This page describes two options, both targeting the browser.

Compile just-in-time (JIT)

In the first, dynamic option, the Angular compiler compiles the application in the browser and then launches the app.

The samples in this page demonstrate the dynamic bootstrapping approach.

Try the live example.

Compile ahead-of-time (AOT)

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.

Because the entire application was pre-compiled, Angular doesn't ship the Angular compiler to the browser and doesn't compile in the browser.

The application code downloaded to the browser is much smaller than the dynamic equivalent and it's 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 is imported 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 you'll look at main.ts.

{@a declarations}

Declare directives and components

As the app evolves, the first addition is a HighlightDirective, an attribute directive that sets the background color of the attached element.

Update the AppComponent template to attach the directive to the title:

If you ran the app now, Angular wouldn't recognize the highlight attribute and would ignore it. You must declare the directive in AppModule.

Import the HighlightDirective class and add it to the module's declarations like this:

Refactor the title into its own TitleComponent. The component's template binds to the component's title and subtitle properties like this:

Rewrite the AppComponent to display the new TitleComponent in the <app-title> element, using an input binding to set the subtitle.

Angular won't recognize the <app-title> tag until you declare it in AppModule. Import the TitleComponent class and add it to the module's declarations:

{@a providers}

Service providers

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

The Dependency Injection page describes the Angular hierarchical dependency-injection system and how to configure that system with 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.

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.

Update the TitleComponent class with a constructor that injects the UserService and sets the component's user property from the service.

You've defined and used the service. Now to provide it for all components to use, add it to a providers property in the AppModule metadata:

{@a imports}

Import supporting modules

In the revised TitleComponent, an *ngIf directive guards the message. There is no message if there is no user.

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 you imported it earlier. The initial version of AppModule imports BrowserModule.

Importing BrowserModule made all of its public components, directives, and pipes visible to the component templates in AppModule.

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. The net effect is that an importer of BrowserModule gets CommonModule directives automatically.

Many familiar Angular directives don't belong to CommonModule. For example, NgModel and RouterLink belong to Angular's FormsModule and RouterModule respectively. You must import those modules before you can use their directives.

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

Add the _ContactComponent_

Angular forms are a great way to manage user data entry.

The ContactComponent presents a "contact editor," implemented with Angular forms in the template-driven form style.

Angular form styles

You can write Angular form components in template-driven or reactive style.

The following sample imports the FormsModule from @angular/forms because the ContactComponent is written in template-driven style. Modules with components written in the reactive style import the ReactiveFormsModule.

The ContactComponent selector matches an element named <app-contact>. Add an element with that name to the AppComponent template, just below the <app-title>:

Form components are often complex. The ContactComponent has its own ContactService and custom pipe (called Awesome), and an alternative version of the HighlightDirective.

To make it manageable, place all contact-related material in an src/app/contact folder and break the component into three constituent HTML, TypeScript, and css files:

In the middle of the component template, notice the two-way data binding [(ngModel)]. ngModel is the selector for the NgModel directive.

Although NgModel is an Angular directive, the Angular compiler won't recognize it for the following reasons:

AppModule doesn't declare NgModel.
NgModel wasn't imported via BrowserModule.

Even if Angular somehow recognized ngModel, ContactComponent wouldn't behave like an Angular form because form features such as validation aren't yet available.

Import the FormsModule

Add the FormsModule to the AppModule metadata's imports list.

Now [(ngModel)] binding will work and the user input will be validated by Angular forms, once you declare the new component, pipe, and directive.

Do not add NgModel—or the FORMS_DIRECTIVES—to the AppModule metadata's declarations. These directives belong to the FormsModule.

Components, directives, and pipes belong to one module only.

Never re-declare classes that belong to another module.

{@a declare-pipe}

Declare the contact component, directive, and pipe

The application won't compile until you declare the contact component, directive, and pipe. Update the declarations in the AppModule accordingly:

{@a import-name-conflict}

There are two directives with the same name, both called HighlightDirective.

To work around this, create an alias for the contact version using the as JavaScript import keyword.

This solves the immediate issue of referencing both directive types in the same file but leaves another issue unresolved. You'll learn more about that issue later in this page, in Resolve directive conflicts.

Provide the _ContactService_

The `ContactComponent` displays contacts retrieved by the `ContactService`, which Angular injects into its constructor.

You have to provide that service somewhere. The ContactComponent could provide it, but then the service would be scoped to this component only. You want to share this service with other contact-related components that you'll surely add later.

In this app, add ContactService to the AppModule metadata's providers list:

Now you can inject ContactService (like UserService) into any component in the application.

{@a application-scoped-providers}

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.

You might expect Angular to offer a module-scoping mechanism to enforce this design. It doesn't. NgModule instances, unlike components, don't have their own injectors so they can't have their own provider scopes.

This omission is intentional. NgModules are designed primarily to extend an application, to enrich the entire app with the module's capabilities.

In practice, service scoping is rarely an issue. Non-contact components can't accidentally inject the ContactService. To inject ContactService, you must first import its type. Only Contact components should import the ContactService type.

Read more in the How do I restrict service scope to a module? section of the NgModule FAQs page.

Run the app

Everything is in place to run the application with its contact editor.

The app file structure looks like this:

app

<div class='file'>
  app.component.ts
</div>

<div class='file'>
  app.module.ts
</div>

<div class='file'>
  highlight.directive.ts
</div>

<div class='file'>
  title.component.(html|ts)
</div>

<div class='file'>
  user.service.ts
</div>

<div class='file'>
  contact
</div>

<div class='children'>

  <div class='file'>
    awesome.pipe.ts
  </div>

  <div class='file'>
    contact.component.(css|html|ts)
  </div>

  <div class='file'>
    contact.service.ts
  </div>

  <div class='file'>
    highlight.directive.ts
  </div>

</div>

Try the example:

{@a resolve-conflicts}

Resolve directive conflicts

An issue arose earlier when you declared the contact's HighlightDirective because you already had a HighlightDirective class at the application level.

The selectors of the two directives both highlight the attached element with a different color.

Both directives are declared in this module so both directives are active.

When the two directives compete to color the same element, the directive that's declared later wins because its DOM changes overwrite the first. In this case, the contact's HighlightDirective makes the application title text blue when it should stay gold.

The issue is that two different classes are 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 from Angular's perspective, two different classes, defined in different files, that have the same name are not duplicates. Angular keeps both directives and they take turns modifying the same HTML element.

At least the app still compiles. If you 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.

To eliminate component and directive conflicts, create feature modules that insulate the declarations in one module from the declarations in another.

{@a feature-modules}

Feature modules

This application isn't big yet, but it's already experiencing structural issues.

The root AppModule grows larger with each new application class.
There are conflicting directives. The HighlightDirective in the contact re-colors the work done by the HighlightDirective declared in AppModule. Also, it colors the application title text when it should color only the ContactComponent.
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.

You can resolve these issues with feature modules.

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.

The modules have the following significant technical differences:

You boot the root module to launch the app; you import a feature module to extend the app.
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, user workflow, facility (forms, http, routing), or collection of related utilities.

While you can do everything within the root module, feature modules help you 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 shares.

In the next section, you'll carve the contact functionality out of the root module and into a dedicated feature module.

{@a contact-module-v1}

Make Contact a feature module

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

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

AppModule is the only existing class that changes. But you do add one new file.

Add the ContactModule

Here's the new ContactModule:

You copy from AppModule the contact-related import statements and @NgModule properties that concern the contact, and paste them into ContactModule.

You import the FormsModule because the contact component needs it.

Modules don't inherit access to the components, directives, or pipes that are declared in other modules. What AppModule imports is irrelevant to ContactModule and vice versa. Before ContactComponent can bind with [(ngModel)], its ContactModule must import FormsModule.

You also replaced BrowserModule by CommonModule, for reasons explained in the Should I import BrowserModule or CommonModule? section of the NgModule FAQs page.

You declare the contact component, directive, and pipe in the module declarations.

You 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.

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.
Delete 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 along with the previous version.

Improvements

There's a lot to like in the revised AppModule.

It does not change as the Contact domain grows.
It only changes when you add new modules.
It's simpler:
- Fewer import statements.
- No FormsModule import.
- No contact-specific declarations.
- No ContactService provider.
- No HighlightDirective conflict.

Try this ContactModule version of the sample.

Try the live example.

{@a lazy-load}

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 this page doesn't discuss every line of code.

Examine and download the complete source for this version from the live example.

Some facets of the current application merit discussion are as follows:

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 app-component-template} The new AppComponent template has a title, three links, and a <router-outlet>.

The <app-contact> element is gone; you're routing to the Contact page now.

The AppModule has changed modestly:

Some file names bear a .3 extension that indicates a difference with prior or future versions. The significant differences will be explained in due course.

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 the AppRoutingModule to the module imports. The AppRoutingModule is a routing module that handles the app's routing concerns.

App routing

The router is the subject of the Routing & Navigation page, so this section skips many of the details and concentrates on the intersection of NgModules and routing.

The app-routing.module.ts file defines three routes.

The first route redirects the empty URL (such as http://host.com/) to another route whose path is contact (such as http://host.com/contact).

The contact route isn't defined here. It's defined in the Contact feature's own routing module, contact-routing.module.ts. It's standard practice for feature modules with routing components to define their own routes. You'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:

A lazy-loaded module location is a string, not a type. In this app, the string identifies both the module file and the module class, the latter separated from the former by a #.

RouterModule.forRoot

The forRoot static class method of the RouterModule with the provided configuration and added to the imports array provides the routing concerns for the module.

The returned AppRoutingModule class is a Routing Module containing both the RouterModule directives and the dependency-injection providers that produce a configured Router.

This AppRoutingModule is intended for the app root module only.

Never call RouterModule.forRoot in a feature-routing module.

Back in the root AppModule, add the AppRoutingModule to its imports list, and the app is ready to navigate.

Routing to a feature module

The src/app/contact folder holds a new file, contact-routing.module.ts. It defines the contact route mentioned earlier and provides a ContactRoutingModule as follows:

This time you pass the route list to the forChild method of the RouterModule. The route list is only responsible for providing additional routes and is intended for feature modules.

Always call RouterModule.forChild in a feature-routing module.

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 if you write a similar module that has both shared declarables and services.

ContactModule has changed in two small but important ways.

It imports the ContactRoutingModule object from contact-routing.module.ts.
It no longer exports ContactComponent.

Now that you navigate to ContactComponent with the router, there's no reason to make it public. Also, ContactComponent doesn't need a selector. No template will ever again reference this ContactComponent. It's gone from the AppComponent template.

{@a hero-module}

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. Its file structure is as follows:

hero

<div class='file'>
  hero-detail.component.ts
</div>

<div class='file'>
  hero-list.component.ts
</div>

<div class='file'>
  hero.component.ts
</div>

<div class='file'>
  hero.module.ts
</div>

<div class='file'>
  hero-routing.module.ts
</div>

<div class='file'>
  hero.service.ts
</div>

<div class='file'>
  highlight.directive.ts
</div>

This is the child routing scenario familiar to readers of the Child routing component section of the Routing & Navigation page. 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.

Yet another HighlightDirective colors elements in yet a different shade. In the next section, Shared modules, you'll resolve the repetition and inconsistencies.

The HeroModule is a feature module like any other.

It imports the FormsModule because the HeroDetailComponent template binds with [(ngModel)]. It imports the HeroRoutingModule from hero-routing.module.ts just as ContactModule and CrisisModule do.

The CrisisModule is much the same.

Try the live example.

{@a shared-module}

Shared modules

The app is shaping up. But it carries three different versions of the HighlightDirective. And the many files cluttering the app folder level could be better organized.

Add a SharedModule to hold the common components, directives, and pipes and share them with the modules that need them.

Create an src/app/shared folder.
Move the AwesomePipe and HighlightDirective from src/app/contact to src/app/shared.
Delete the HighlightDirective classes from src/app/ and src/app/hero.
Create a SharedModule class to own the shared material.
Update other feature modules to import SharedModule.

Here is the SharedModule:

Note the following:

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

If you review the application, you may notice 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.

You 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 listing it among its imports.

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 no-shared-module-providers}

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 the 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. You don't want each module to have its own separate instance. Yet there is a real danger of that happening

if the SharedModule provides the UserService.

Do not specify app-wide singleton providers in a shared module. A lazy-loaded module that imports that shared module makes its own copy of the service.

{@a core-module}

The Core module

At the moment, the root folder is cluttered with the UserService and TitleComponent that only appear in the root AppComponent. You didn't include them in the SharedModule for reasons just explained.

Instead, gather them in a single CoreModule that you import once when the app starts and never import anywhere else.

Perform the following steps:

Create an src/app/core folder.

Move the UserService and TitleComponent from src/app/ to src/app/core.
Create a CoreModule class to own the core material.
Update the AppRoot module to import CoreModule.

Most of this work is familiar. The interesting part is the CoreModule.

You're importing some extra symbols from the Angular core library that you're not using yet. They'll become relevant later in this page.

The @NgModule metadata should be familiar. You declare the TitleComponent because this module owns it and you export it because AppComponent (which is in AppModule) displays the title in its template. TitleComponent needs the Angular NgIf directive that you 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.

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 you decide to relocate the UserService file to the src/app/core folder.

Real-world apps have more to worry about. They can have several single-use components (such as spinners, message toasts, and modal dialogs) that appear only in the AppComponent template. You 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—and therefore require JavaScript import statements to import their symbols—no other component or module should define or re-create the services themselves. Their providers aren't shared.

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

Cleanup

Having refactored to a CoreModule and a SharedModule, it's time to clean up the other modules.

A trimmer AppModule

Here is the updated AppModule paired with version 3 for comparison:

AppModule now has the following qualities:

A little smaller because many src/app/root classes have moved to other modules.
Stable because you'll add future components and providers to other modules, not this one.
Delegated 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:

Notice the following:

The AwesomePipe and HighlightDirective are gone.
The imports include SharedModule instead of CommonModule and FormsModule.
The new version is leaner and cleaner.

{@a core-for-root}

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, which is a simple object with the following properties:

ngModule: the CoreModule class
providers: the configured providers

The root AppModule imports the CoreModule and adds the providers to the AppModule providers.

More precisely, Angular accumulates all imported providers before appending the items listed in @NgModule.providers. This sequence ensures that whatever you add explicitly to the AppModule providers takes precedence over the providers of imported modules.

Add a CoreModule.forRoot method that configures the core UserService.

You've extended the core UserService with an optional, injected UserServiceConfig. If a UserServiceConfig exists, the UserService sets the user name from that config.

Here's CoreModule.forRoot that takes a UserServiceConfig object:

Lastly, call it within the imports list of the AppModule.

The app displays "Miss Marple" as the user instead of the default "Sherlock Holmes".

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 can produce a runtime error.

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

{@a prevent-reimport}

Prevent reimport of the CoreModule

Only the root AppModule should import the CoreModule. Bad things happen if a lazy-loaded module imports it.

You could hope that no developer makes that mistake. Or you can guard against it and fail fast by adding the following CoreModule constructor.

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 you 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.

Conclusion

You made it! You can examine and download the complete source for this final version from the live example.

Frequently asked questions

Now that you understand NgModules, you may be interested in the companion NgModule FAQs page with its ready answers to specific design and implementation questions.

50 KiB Raw Blame History

NgModules

Live examples

Frequently asked questions (FAQs)

Angular modularity

The root AppModule

Bootstrapping in main.ts

Compile just-in-time (JIT)

Compile ahead-of-time (AOT)

Declare directives and components

Service providers

Import supporting modules

Add the _ContactComponent_

Angular form styles

Import the FormsModule

Declare the contact component, directive, and pipe

Provide the _ContactService_

Application-scoped providers

Run the app

Resolve directive conflicts

Feature modules

Make Contact a feature module

Add the ContactModule

Refactor the AppModule

Improvements

Lazy-loading modules with the router

App routing

RouterModule.forRoot

Routing to a feature module

Lazy-loaded routing to a module

Shared modules

Re-exporting other modules

Why TitleComponent isn't shared

Why UserService isn't shared

The Core module

Why bother?

Cleanup

A trimmer AppModule

A trimmer ContactModule

Configure core services with CoreModule.forRoot

Prevent reimport of the CoreModule

Conclusion

Frequently asked questions

50 KiB

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