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     ## Ahead of Time (AOT) Compilation
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  +  ## Ahead of Time (AOT) 提前编译
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       Angular applications can be compiled by developers at build-time. 
  
positive.code:     to a Module Factory, meaning you don't need to include the Angular compiler in your javascript bundle. 
      Ahead of Time compiled applications also benefit from decreased load time and increased performance.
  
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    ## Angular Module
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      Helps us organize an application into cohesive blocks of functionality.
      An Angular module identifies the components, directives, and pipes that are used by the application
      along with the list of external Angular modules that the application needs, such as `FormsModule`.
  
      Every Angular application has an application root module class. By convention the class is
      called `AppModule` and resides in a file named `app.component.ts`.
  
      See the [Angular Module](/docs/ts/latest/guide/ngmodule.html) chapter for details and examples.
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    ## Annotation
    ## 注解(Annotation)
  
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    ## Observable
  
    ## Observable
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      We can think of an observable as an array whose items arrive asynchronously over time. 
      Observables help us manage asynchronous data, such as data coming from a backend service.
      Observables are used within Angular itself, including Angular's event system and its http client service.
  
      一个`Observable`是一个数组，它包含的元素在一段时间内异步到达。`Observable`帮助我们管理异步数据，比如来自后台服务的数据。
      Angular自身使用了`Observable`，包括Angular的事件系统和它的http客户端服务。
  
      To use observables, Angular uses a third-party library called Reactive Extensions (RxJS).
      Observables are a proposed feature for ES 2016, the next version of JavaScript.
  
      为了利用`Observable`，Angular使用了名为Reactive Extensions (RxJS)的第三方包。
      在下个版本的JavaScript - ES 2016中，`Observable`是建议的功能之一。
  
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    ## Output
  