angular-cn/aio/content/guide/component-overview.md

# Angular Components Overview

Components are the main building block for Angular applications. Each component consists of:

* An HTML template that declares what renders on the page
* A Typescript class that defines behavior
* A CSS selector that defines how the component is used in a template
* Optionally, CSS styles applied to the template

This topic describes how to create and configure an Angular component.

<div class="alert is-helpful">

To view or download the example code used in this topic, see the <live-example></live-example>.

</div>

## Prerequisites

To create a component, verify that you have met the following prerequisites:

1. Install the Angular CLI.
1. Create an Angular project.
   If you don't have a project, you can create one using `ng new <project-name>`, where `<project-name>` is the name of your Angular application.

## Creating a component

The easiest way to create a component is with the Angular CLI. You can also create a component manually.

### Creating a component using the Angular CLI

To create a component using the Angular CLI:

1. From a terminal window, navigate to the directory containing your application.
1. Run the `ng generate component <component-name>` command, where `<component-name>` is the name of your new component.

By default, this command creates the following:

* A folder named after the component
* A component file, `<component-name>.component.ts`
* A template file, `<component-name>.component.html`
* A CSS file, `<component-name>.component.css`
* A testing specification file, `<component-name>.component.spec.ts`

Where `<component-name>` is the name of your component.

<div class="alert is-helpful">

You can change how `ng generate component` creates new components.
For more information, see [ng generate component](cli/generate#component-command) in the Angular CLI documentation.

</div>

### Creating a component manually

Although the Angular CLI is the easiest way to create an Angular component, you can also create a component manually.
This section describes how to create the core component file within an existing Angular project.

To create a new component manually:

1. Navigate to your Angular project directory.
1. Create a new file, `<component-name>.component.ts`.
1. At the top of the file, add the following import statement.

   <code-example
        path="component-overview/src/app/component-overview/component-overview.component.ts"
        region="import">
   </code-example>

1. After the `import` statement, add a `@Component` decorator.

   <code-example
        path="component-overview/src/app/component-overview/component-overview.component.ts"
        region="decorator-skeleton">
   </code-example>

1. Choose a CSS selector for the component.

   <code-example
        path="component-overview/src/app/component-overview/component-overview.component.ts"
        region="selector">
   </code-example>

   For more information on choosing a selector, see [Specifying a component's selector](#specifying-a-components-css-selector).

1. Define the HTML template that the component uses to display information.
   In most cases, this template is a separate HTML file.

   <code-example
        path="component-overview/src/app/component-overview/component-overview.component.ts"
        region="templateUrl">
   </code-example>

   For more information on defining a component's template, see [Defining a component's template](#defining-a-components-template).

1. Select the styles for the component's template.
   In most cases, you define the styles for your component's template in a separate file.

   <code-example
        path="component-overview/src/app/component-overview/component-overview.component.ts"
        region="decorator">
   </code-example>

1. Add a `class` statement that includes the code for the component.

   <code-example
        path="component-overview/src/app/component-overview/component-overview.component.ts"
        region="class">
   </code-example>

## Specifying a component's CSS selector

Every component requires a CSS _selector_. A selector instructs Angular to instantiate this component wherever it finds the corresponding tag in template HTML. For example, consider a component `hello-world.component.ts` that defines its selector as `app-hello-world`. This selector instructs Angular to instantiate this component any time the tag `<app-hello-world>` appears in a template.

Specify a component's selector by adding a `selector` statement to the `@Component` decorator.

<code-example
    path="component-overview/src/app/component-overview/component-overview.component.ts"
    region="selector">
</code-example>

## Defining a component's template

A template is a block of HTML that tells Angular how to render the component in your application.
You can define a template for your component in one of two ways: by referencing an external file, or directly within the component.

To define a template as an external file, add a `templateUrl` property to the `@Component` decorator.

<code-example
    path="component-overview/src/app/component-overview/component-overview.component.ts"
    region="templateUrl">
</code-example>

To define a template within the component, add a `template` property to the `@Component` decorator that contains the HTML you want to use.

<code-example
    path="component-overview/src/app/component-overview/component-overview.component.1.ts"
    region="template">
</code-example>

If you want your template to span multiple lines, you can use backticks (<code> ` </code>).
For example:

<code-example
    path="component-overview/src/app/component-overview/component-overview.component.2.ts"
    region="templatebacktick">
</code-example>

<div class="alert is-helpful">

An Angular component requires a template defined using `template` or `templateUrl`. You cannot have both statements in a component.

</div>

## Declaring a component's styles

You can declare component styles uses for its template in one of two ways: by referencing an external file, or directly within the component.

To declare the styles for a component in a separate file, add a `styleUrls` property to the `@Component` decorator.

<code-example
    path="component-overview/src/app/component-overview/component-overview.component.ts"
    region="decorator">
</code-example>

To declare the styles within the component, add a `styles` property to the `@Component` decorator that contains the styles you want to use.

<code-example
    path="component-overview/src/app/component-overview/component-overview.component.3.ts"
    region="styles">
</code-example>

The `styles` property takes an array of strings that contain the CSS rule declarations.


## Next steps

* For an architectural overview of components, see [Introduction to components and templates](guide/architecture-components).
* For additional options you can use when creating a component, see [Component](api/core/Component) in the API Reference.
* For more information on styling components, see [Component styles](guide/component-styles).
* For more information on templates, see [Template syntax](guide/template-syntax).