angular-cn/aio/content/guide/displaying-data.md

# Displaying data in views

Angular [components](guide/glossary#component) form the data structure of your application.
The HTML [template](guide/glossary#template) associated with a component provides the means to display that data in the context of a web page.
Together, a component's class and template form a [view](guide/glossary#view) of your application data.

The process of combining data values with their representation on the page is called [data binding](guide/glossary#data-binding).
You display your data to a user (and collect data from the user) by *binding* controls in the HTML template to the data properties of the component class.

In addition, you can add logic to the template by including [directives](guide/glossary#directive), which tell Angular how to modify the page as it is rendered.

Angular defines a *template language* that expands HTML notation with syntax that allows you to define various kinds of data binding and logical directives.
When the page is rendered, Angular interprets the template syntax to update the HTML according to your logic and current data state.
Before you read the complete [template syntax guide](guide/template-syntax), the exercises on this page give you a quick demonstration of how template syntax works.

In this demo, you'll create a component with a list of heroes.
You'll display the list of hero names and conditionally show a message below the list.
The final UI looks like this:

<div class="lightbox">
  <img src="generated/images/guide/displaying-data/final.png" alt="Final UI">
</div>

<div class="alert is-helpful">

The <live-example></live-example> demonstrates all of the syntax and code snippets described in this page.

</div>

{@a interpolation}

## Showing component properties with interpolation
The easiest way to display a component property is to bind the property name through interpolation.
With interpolation, you put the property name in the view template, enclosed in double curly braces: `{{myHero}}`.

Use the CLI command [`ng new displaying-data`](cli/new) to create a workspace and app named `displaying-data`.

Delete the <code>app.component.html</code> file. It is not needed for this example.

Then modify the <code>app.component.ts</code> file by
changing the template and the body of the component.

When you're done, it should look like this:

<code-example path="displaying-data/src/app/app.component.1.ts" header="src/app/app.component.ts"></code-example>

You added two properties to the formerly empty component: `title` and `myHero`.

The template displays the two component properties using double curly brace
interpolation:

<code-example path="displaying-data/src/app/app.component.1.ts" header="src/app/app.component.ts (template)" region="template"></code-example>

<div class="alert is-helpful">

The template is a multi-line string within ECMAScript 2015 backticks (<code>\`</code>).
The backtick (<code>\`</code>)&mdash;which is *not* the same character as a single
quote (`'`)&mdash;allows you to compose a string over several lines, which makes the
HTML more readable.

</div>

Angular automatically pulls the value of the `title` and `myHero` properties from the component and
inserts those values into the browser. Angular updates the display
when these properties change.

<div class="alert is-helpful">

More precisely, the redisplay occurs after some kind of asynchronous event related to
the view, such as a keystroke, a timer completion, or a response to an HTTP request.

</div>

Notice that you don't call **new** to create an instance of the `AppComponent` class.
Angular is creating an instance for you. How?

The CSS `selector` in the `@Component` decorator specifies an element named `<app-root>`.
That element is a placeholder in the body of your `index.html` file:

<code-example path="displaying-data/src/index.html" header="src/index.html (body)" region="body"></code-example>

When you bootstrap with the `AppComponent` class (in <code>main.ts</code>), Angular looks for a `<app-root>`
in the `index.html`, finds it, instantiates an instance of `AppComponent`, and renders it
inside the `<app-root>` tag.

Now run the app. It should display the title and hero name:

<div class="lightbox">
  <img src="generated/images/guide/displaying-data/title-and-hero.png" alt="Title and Hero">
</div>

The next few sections review some of the coding choices in the app.


## Choosing the template source

The `@Component` metadata tells Angular where to find the component's template.
You can store your component's template in one of two places.

* You can define the template *inline* using the `template` property of the `@Component` decorator. An inline template is useful for a small demo or test.
* Alternatively, you can define the template in a separate HTML file and link to that file in the `templateUrl` property of the `@Component` decorator. This configuration is typical for anything more complex than a small test or demo, and is the default when you generate a new component.

In either style, the template data bindings have the same access to the component's properties.
Here the app uses inline HTML because the template is small and the demo is simpler without the additional HTML file.

<div class="alert is-helpful">

  By default, the Angular CLI command [`ng generate component`](cli/generate) generates components with a template file.
  You can override that by adding the "-t" (short for `inlineTemplate=true`) option:

  <code-example hideCopy language="sh" class="code-shell">
    ng generate component hero -t
  </code-example>

</div>


## Initialization

The following example uses variable assignment to initialize the components.

<code-example path="displaying-data/src/app/app-ctor.component.1.ts" region="class"></code-example>

 You could instead declare and initialize the properties using a constructor.
 This app uses more terse "variable assignment" style simply for brevity.


{@a ngFor}

## Add logic to loop through data

The `*ngFor` directive (predefined by Angular) lets you loop through data. The following example uses the directive to show all of the values in an array property.

To display a list of heroes, begin by adding an array of hero names to the component and redefine `myHero` to be the first name in the array.


<code-example path="displaying-data/src/app/app.component.2.ts" header="src/app/app.component.ts (class)" region="class"></code-example>


Now use the Angular `ngFor` directive in the template to display each item in the `heroes` list.


<code-example path="displaying-data/src/app/app.component.2.ts" header="src/app/app.component.ts (template)" region="template"></code-example>


This UI uses the HTML unordered list with `<ul>` and `<li>` tags. The `*ngFor`
in the `<li>` element is the Angular "repeater" directive.
It marks that `<li>` element (and its children) as the "repeater template":


<code-example path="displaying-data/src/app/app.component.2.ts" header="src/app/app.component.ts (li)" region="li"></code-example>

<div class="alert is-important">

Don't forget the leading asterisk (\*) in `*ngFor`. It is an essential part of the syntax.
Read more about `ngFor` and `*` in the [ngFor section](guide/built-in-directives#ngfor) of the [Built-in directives](guide/built-in-directives) page.

</div>

Notice the `hero` in the `ngFor` double-quoted instruction;
it is an example of a template input variable. Read
more about template input variables in the [microsyntax](guide/built-in-directives#microsyntax) section of
the [Built-in directives](guide/built-in-directives) page.

Angular duplicates the `<li>` for each item in the list, setting the `hero` variable
to the item (the hero) in the current iteration. Angular uses that variable as the
context for the interpolation in the double curly braces.

<div class="alert is-helpful">

In this case, `ngFor` is displaying an array, but `ngFor` can
repeat items for any [iterable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols) object.

</div>

Now the heroes appear in an unordered list.

<div class="lightbox">
  <img src="generated/images/guide/displaying-data/hero-names-list.png" alt="After ngfor">
</div>


## Creating a class for the data

The app's code defines the data directly inside the component, which isn't best practice.
In a simple demo, however, it's fine.

At the moment, the binding is to an array of strings.
In real applications, most bindings are to more specialized objects.

To convert this binding to use specialized objects, turn the array
of hero names into an array of `Hero` objects. For that you'll need a `Hero` class:

<code-example language="sh" class="code-shell">
  ng generate class hero
</code-example>

This command creates the following code.


<code-example path="displaying-data/src/app/hero.ts" header="src/app/hero.ts"></code-example>

You've defined a class with a constructor and two properties: `id` and `name`.

It might not look like the class has properties, but it does.
The declaration of the constructor parameters takes advantage of a TypeScript shortcut.

Consider the first parameter:


<code-example path="displaying-data/src/app/hero.ts" header="src/app/hero.ts (id)" region="id"></code-example>

That brief syntax does a lot:

* Declares a constructor parameter and its type.
* Declares a public property of the same name.
* Initializes that property with the corresponding argument when creating an instance of the class.


### Using the Hero class

After importing the `Hero` class, the `AppComponent.heroes` property can return a _typed_ array
of `Hero` objects:


<code-example path="displaying-data/src/app/app.component.3.ts" header="src/app/app.component.ts (heroes)" region="heroes"></code-example>



Next, update the template.
At the moment it displays the hero's `id` and `name`.
Fix that to display only the hero's `name` property.


<code-example path="displaying-data/src/app/app.component.3.ts" header="src/app/app.component.ts (template)" region="template"></code-example>


The display looks the same, but the code is clearer.

{@a ngIf}

## Conditional display with NgIf

Sometimes an app needs to display a view or a portion of a view only under specific circumstances.

Let's change the example to display a message if there are more than three heroes.

The Angular `ngIf` directive inserts or removes an element based on a _truthy/falsy_ condition.
To see it in action, add the following paragraph at the bottom of the template:


<code-example path="displaying-data/src/app/app.component.ts" header="src/app/app.component.ts (message)" region="message"></code-example>


<div class="alert is-important">

Don't forget the leading asterisk (\*) in `*ngIf`. It is an essential part of the syntax.
Read more about `ngIf` and `*` in the [ngIf section](guide/built-in-directives#ngIf) of the [Built-in directives](guide/built-in-directives) page.

</div>


The template expression inside the double quotes,
`*ngIf="heroes.length > 3"`, looks and behaves much like TypeScript.
When the component's list of heroes has more than three items, Angular adds the paragraph
to the DOM and the message appears.
If there are three or fewer items, Angular omits the paragraph, so no message appears.

For more information, see [template expression operators](guide/interpolation#template-expressions).


<div class="alert is-helpful">

Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in larger projects when conditionally including or excluding
big chunks of HTML with many data bindings.

</div>

Try it out. Because the array has four items, the message should appear.
Go back into <code>app.component.ts</code> and delete or comment out one of the elements from the heroes array.
The browser should refresh automatically and the message should disappear.


## Summary
Now you know how to use:

* **Interpolation** with double curly braces to display a component property.
* **ngFor** to display an array of items.
* A TypeScript class to shape the **model data** for your component and display properties of that model.
* **ngIf** to conditionally display a chunk of HTML based on a boolean expression.

Here's the final code:

<code-tabs>

  <code-pane header="src/app/app.component.ts" path="displaying-data/src/app/app.component.ts" region="final">

  </code-pane>

  <code-pane header="src/app/hero.ts" path="displaying-data/src/app/hero.ts">

  </code-pane>

  <code-pane header="src/app/app.module.ts" path="displaying-data/src/app/app.module.ts">

  </code-pane>

  <code-pane header="main.ts" path="displaying-data/src/main.ts">

  </code-pane>

</code-tabs>