angular-cn/aio/content/guide/ngmodule-vs-jsmodule.md

# JavaScript Modules vs. NgModules

#### Prerequisites
A basic understanding of [JavaScript/ECMAScript modules](https://hacks.mozilla.org/2015/08/es6-in-depth-modules/).

<hr>

JavaScript and Angular use modules to organize code, and
though they organize it differently, Angular apps rely on both.

## JavaScript modules

In JavaScript, modules are individual files with JavaScript code in them. To make what’s in them available, you write an export statement, usually after the relevant code, like this:

```typescript
export class AppComponent { ... }
```

Then, when you need that file’s code in another file, you import it like this:

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

JavaScript modules help you namespace, preventing accidental global variables.

## NgModules

<!-- KW-- perMisko: let's discuss. This does not answer the question why it is different. Also, last sentence is confusing.-->
NgModules are classes decorated with `@NgModule`. The `@NgModule` decorator’s `imports` array tells Angular what other NgModules the current module needs. The modules in the `imports` array are different than JavaScript modules because they are NgModules rather than regular JavaScript modules. Classes with an `@NgModule` decorator are by convention kept in their own files, but what makes them an `NgModule` isn’t being in their own file, like JavaScript modules; it’s the presence of `@NgModule` and its metadata.

The `AppModule` generated from the [Angular CLI](cli) demonstrates both kinds of modules in action:

```typescript
/* These are JavaScript import statements. Angular doesn’t know anything about these. */
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

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

/* The @NgModule decorator lets Angular know that this is an NgModule. */
@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [     /* These are NgModule imports. */
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }
```


The NgModule classes differ from JavaScript module in the following key ways:

* An NgModule bounds [declarable classes](guide/ngmodule-faq#q-declarable) only.
Declarables are the only classes that matter to the [Angular compiler](guide/ngmodule-faq#q-angular-compiler).
* Instead of defining all member classes in one giant file as in a JavaScript module,
you list the module's classes in the `@NgModule.declarations` list.
* An NgModule can only export the [declarable classes](guide/ngmodule-faq#q-declarable)
it owns or imports from other modules. It doesn't declare or export any other kind of class.
* Unlike JavaScript modules, an NgModule can extend the _entire_ application with services
by adding providers to the `@NgModule.providers` list.

<hr />

## More on NgModules

For more information on NgModules, see:
* [Bootstrapping](guide/bootstrapping).
* [Frequently used modules](guide/frequent-ngmodules).
* [Providers](guide/providers).