Angular Module FAQs

FAQs

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

The Angular Modules chapter covers the concepts and takes you step by step from the most elementary @NgModule to a multi-faceted sample with lazy loaded modules.

This chapter answers the questions many developers ask about Angular Module design and implementation.

Declarations

• What classes should I add to declarations?
• What is a declarable?
• What classes should I not add to declarations?
• Why list the same component in multiple NgModule properties?
• What does "Can't bind to 'x' since it isn't a known property of 'y'" mean?

Imports

• What should I import?
• Should I import BrowserModule or CommonModule?
• What if I import the same module twice?

Exports

• What should I export?
• What should I not export?
• Can I re-export imported classes and modules?
• What is the forRoot method?

Service Providers

• Why is a service provided in a feature module visible everywhere?
• Why is a service provided in a lazy loaded module visible only to that module?
• What if two modules provide the same service?
• How do I restrict service scope to a module?
• Should I add app-wide providers to the root AppModule or the root AppComponent?
• Should I add other providers to a module or a component?
• Why is it bad if SharedModule provides a service to a lazy loaded module?
• Why does lazy loading create a child injector?
• How can I tell if a module or service was previously loaded?

Entry Components

• What is an entry component?
• What is the difference between a bootstrap component and an entry component?
• When do I add components to entryComponents?
• Why does Angular need entryComponents?

General

• What kinds of modules should I have and how should I use them?
• What's the difference between Angular and JavaScript Modules?
• What is a "template reference"?
• How does Angular find components, directives, and pipes in a template?
• What is the Angular Compiler?
• Can you summarize the NgModule API?

What classes should I add to declarations?

Add declarable classes — components, directives, and pipes — 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.

What is a declarable?

Declarables are the class types — components, directives, and pipes — that you can add to a module's declarations list. They're the only classes that you can add to declarations.

What classes should I not add to declarations?

Only 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.

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.

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.

What should I import?

Import modules whose public (exported) declarable classes 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 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 in the root AppModule.

Should I import BrowserModule or CommonModule?

The root application module (AppModule) of almost every browser application should import BrowserModule from @angular/platform-browser.

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.

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.

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'.

What should I export?

Export 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 — components, directives, and pipes — 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.

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 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.

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:

exports: [CommonModule, ApplicationModule]

A module can export a combination of its own declarations, selected imported classes, and imported modules.

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. We add that result to the imports list of the root AppModule.

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.

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.

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 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.

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.

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 available for injection throughout the application.

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.

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.

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 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.

Should I add app-wide providers to the root AppModule or the root AppComponent?

Lazy-loaded modules and their components can inject AppModule services; they cannot inject AppComponent services.

Register a service in AppComponent providers only if the service must be hidden from components outside the AppComponent tree. This is a rare exceptional use case.

More generally, prefer registering providers in modules to registering in components.

Discussion:

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. AppComponentservices are available only 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 the Angular Module Chapter 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".

Should I add other providers to a module or a component?

In general, prefer registering feature-specific providers in modules (@NgModule.providers) to registering in components (@Component.providers).

Register a provider with a component when you must limit the scope of a service instance to that component and its component tree. Apply the same reasoning to registering a provider with a directive.

For example, a hero editing component that needs a private copy of a caching hero service should register the HeroService with the HeroEditorComponent. Then each new instance of the HeroEditorComponent gets its own cached service instance. The changes that editor makes to heroes in its service do not touch the hero instances elsewhere in the application.

Always register application-wide services with the root AppModule, not the root AppComponent.

Why is it bad if SharedModule provides a service to a lazy loaded module?

This question arose in the Angular Module chapter when we discussed the importance of keeping providers out of the SharedModule.

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). 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.

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.

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.

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 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 Angular Module chapter sample's CoreModule constructor.

constructor (@Optional() @SkipSelf() parentModule: CoreModule) {
  if (parentModule) {
    throw new Error(
      'CoreModule is already loaded. Import it in the AppModule only');
  }
}

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
2. components referenced in router configuration

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

What's the difference between a bootstrap component and an entry component?

A bootstrapped component is an entry component. 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.

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 in the templates of other components.

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 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 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.

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.

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 such as CommonModule, FormsModule and modules with the UI controls that you use most widely.

It should not have providers for reasons explained earlier. 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 with no declarations.

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
• Routed Feature Modules
• Service Feature Modules
• Widget Feature Modules

The following table summarizes the key characteristics of each Feature Module group.

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. Within each file we write an export statement to make parts of the module public:

export class AppComponent { ... }

Then we import a part in another module:

import { AppComponent }  from './app.component';

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 only. Declarables are the only classes that matter to the Angular compiler.

2. 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.

3. An Angular module can only export the declarable classes 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.

Here's an Angular Module class with imports, exports, and declarations.

@NgModule({
  imports:      [ CommonModule, FormsModule ],
  declarations: [ ContactComponent, HighlightDirective, AwesomePipe ],
  exports:      [ ContactComponent ],
  providers:    [ ContactService ]
})
export class ContactModule { }

Of course we use JavaScript modules to write Angular modules as seen in the complete contact.module.ts file:

import { NgModule }           from '@angular/core';
import { CommonModule }       from '@angular/common';
import { FormsModule }        from '@angular/forms';

import { AwesomePipe }        from './awesome.pipe';

import
       { ContactComponent }   from './contact.component';
import { ContactService }     from './contact.service';
import { HighlightDirective } from './highlight.directive';

@NgModule({
  imports:      [ CommonModule, FormsModule ],
  declarations: [ ContactComponent, HighlightDirective, AwesomePipe ],
  exports:      [ ContactComponent ],
  providers:    [ ContactService ]
})
export class ContactModule { }

How does Angular find components, directives, and pipes in a template?
What is a template reference?

The 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.

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.

NgModule API

The following chart summarizes the NgModule metadata properties.