angular-docs-cn/aio/content/guide/bootstrapping.md

# Launching your app with a root module

#### Prerequisites

A basic understanding of the following:
* [JavaScript Modules vs. NgModules](guide/ngmodule-vs-jsmodule).

<hr />

An NgModule describes how the application parts fit together.
Every application has at least one Angular module, the _root_ module,
which must be present for bootstrapping the application on launch.
By convention and by default, this NgModule is named `AppModule`.

When you use the [Angular CLI](cli) command `ng new` to generate an app, the default `AppModule` looks like the following:

```typescript
/* JavaScript imports */
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

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

/* the AppModule class with the @NgModule decorator */
@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

```

After the import statements is a class with the
**`@NgModule`** [decorator](guide/glossary#decorator '"Decorator" explained').

The `@NgModule` decorator identifies `AppModule` as an `NgModule` class.
`@NgModule` takes a metadata object that tells Angular how to compile and launch the application.

* **_declarations_**&mdash;this application's lone component.
* **_imports_**&mdash;import `BrowserModule` to have browser specific services such as DOM rendering, sanitization, and location.
* **_providers_**&mdash;the service providers.
* **_bootstrap_**&mdash;the _root_ component that Angular creates and inserts
into the `index.html` host web page.

The default application created by the Angular CLI only has one component, `AppComponent`, so it
is in both the `declarations` and the `bootstrap` arrays.

{@a declarations}

## The `declarations` array

The module's `declarations` array tells Angular which components belong to that module.
As you create more components, add them to `declarations`.

You must declare every component in exactly one `NgModule` class.
If you use a component without declaring it, Angular returns an
error message.

The `declarations` array only takes declarables. Declarables
are components, [directives](guide/attribute-directives) and [pipes](guide/pipes).
All of a module's declarables must be in the `declarations` array.
Declarables must belong to exactly one module. The compiler emits
an error if you try to declare the same class in more than one module.

These declared classes are visible within the module but invisible
to components in a different module unless they are exported from
this module and the other module imports this one.

An example of what goes into a declarations array follows:

```typescript
  declarations: [
    YourComponent,
    YourPipe,
    YourDirective
  ],
```

A declarable can only belong to one module, so only declare it in
one `@NgModule`. When you need it elsewhere,
import the module that has the declarable you need in it.


### Using directives with `@NgModule`

Use the `declarations` array for directives.
To use a directive, component, or pipe in a module, you must do a few things:

1. Export it from the file where you wrote it.
2. Import it into the appropriate module.
3. Declare it in the `@NgModule` `declarations` array.


Those three steps look like the following. In the file where you create your directive, export it.
The following example, named `ItemDirective` is the default directive structure that the CLI generates in its own file, `item.directive.ts`:

<code-example path="bootstrapping/src/app/item.directive.ts" region="directive" header="src/app/item.directive.ts"></code-example>

The key point here is that you have to export it so you can import it elsewhere. Next, import it
into the `NgModule`, in this example `app.module.ts`, with a JavaScript import statement:

<code-example path="bootstrapping/src/app/app.module.ts" region="directive-import" header="src/app/app.module.ts"></code-example>

And in the same file, add it to the `@NgModule` `declarations` array:

<code-example path="bootstrapping/src/app/app.module.ts" region="declarations" header="src/app/app.module.ts"></code-example>


Now you could use your `ItemDirective` in a component. This example uses `AppModule`, but you'd do it the same way for a feature module. For more about directives, see [Attribute Directives](guide/attribute-directives) and [Structural Directives](guide/structural-directives). You'd also use the same technique for [pipes](guide/pipes) and components.

Remember, components, directives, and pipes belong to one module only. You only need to declare them once in your app because you share them by importing the necessary modules. This saves you time and helps keep your app lean.

{@a imports}

## The `imports` array

The module's `imports` array appears exclusively in the `@NgModule` metadata object.
It tells Angular about other NgModules that this particular module needs to function properly.

<code-example
    path="bootstrapping/src/app/app.module.ts"
    region="imports"
    header="src/app/app.module.ts (excerpt)">
</code-example>

This list of modules are those that export components, directives, or pipes
that component templates in this module reference. In this case, the component is
`AppComponent`, which references components, directives, or pipes in `BrowserModule`,
`FormsModule`, or  `HttpClientModule`.
A component template can reference another component, directive,
or pipe when the referenced class is declared in this module or
the class was imported from another module.



{@a bootstrap-array}

## The `providers` array

The providers array is where you list the services the app needs. When
you list services here, they are available app-wide. You can scope
them when using feature modules and lazy loading. For more information, see
[Providers](guide/providers).

## The `bootstrap` array

The application launches by bootstrapping the root `AppModule`, which is
also referred to as an `entryComponent`.
Among other things, the bootstrapping process creates the component(s) listed in the `bootstrap` array
and inserts each one into the browser DOM.

Each bootstrapped component is the base of its own tree of components.
Inserting a bootstrapped component usually triggers a cascade of
component creations that fill out that tree.

While you can put more than one component tree on a host web page,
most applications have only one component tree and bootstrap a single root component.

This one root component is usually called `AppComponent` and is in the
root module's `bootstrap` array.



## More about Angular Modules

For more on NgModules you're likely to see frequently in apps,
see [Frequently Used Modules](guide/frequent-ngmodules).