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  • What is the purpose of NgModule?

  • How do you decide to create a new NgModule?

  • What are the attributes that you can define in an NgModule annotation?

  • What is the difference between a module's forRoot() and forChild() methods and why do you need it?

  • What would you have in a shared module?

  • What would you not put shared module?

  • What module would you put a singleton service whose instance will be shared throughout the application (e.g. ExceptionService andLoggerService)?

  • What is the purpose of exports in a NgModule?

  • 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.

    Prove it for yourself. Run the 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.

    https://angular.io/docs/ts/latest/cookbook/ngmodule-faq.html#!#q-what-to-export