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        In Angular, there are two types of modules:
  -     - [Angular modules](#angular-module). 
  +
  +     在Angular里有两种模块：
  +
  +     - [Angular modules](#angular-module).
        See the [Angular Module](/docs/ts/latest/guide/ngmodule.html) chapter for details and examples.
  +
  +     - [Angular 模块](#angular-module).到[Angular Module](/docs/ts/latest/guide/ngmodule.html)章节查看详情和例子。
  +
        - ES2015 modules as described in this section.
  +
  +     - 本节描述的ES2015模块。
   
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       Angular apps are modular.
  
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    ## 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.
  
      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.
  
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    ## Output
  
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    ## Ahead of Time (AOT) Compilation
  
    ## Ahead of Time (AOT) 提前编译
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      Angular applications can be compiled by developers at build-time. 
  