block includes
  include ../_util-fns
  - var _JavaScript = 'JavaScript';
  //- Double underscore means don't escape var, use !{__var}.
  - var __chaining_op = '<code>;</code> or <code>,</code>';
  - var __new_op = '<code>new</code>';
  - var __objectAsMap = 'object';

:marked
  Our Angular application manages what the user sees and can do, achieving this through the interaction of a Component class instance (the *component*) and its user-facing template.

  Many of us are familiar with the component/template duality from our experience with model-view-controller (MVC) or model-view-viewmodel (MVVM). In Angular,  the component plays the part of the controller/viewmodel, and the template represents the view.

  Let’s find out what it takes to write a template for our view. We’ll cover these basic elements of template syntax:

  * [HTML](#html)
  * [Interpolation](#interpolation)
  * [Template expressions](#template-expressions)
  * [Template statements](#template-statements)
  * [Binding syntax](#binding-syntax)
  * [Property binding](#property-binding)
  * [Attribute, class, and style bindings](#other-bindings)
  * [Event binding](#event-binding)
  * [Two-way data binding](#two-way)
  * [Two-way data binding with `NgModel`](#ngModel)
  * [Built-in directives](#directives)
    * [NgClass](#ngClass)
    * [NgStyle](#ngStyle)
    * [NgIf](#ngIf)
    * [NgSwitch](#ngSwitch)
    * [NgFor](#ngFor)
  * [* and &lt;template>](#star-template)
  * [Template reference variables](#ref-vars)
  * [Input and output properties](#inputs-outputs)
  * [Template expression operators](#expression-operators)
    * [pipe](#pipe)
    * [safe navigation operator (?.)](#safe-navigation-operator)

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

.l-main-section
:marked
  ## HTML
  HTML is the language of the Angular template. Our [QuickStart](../quickstart.html) application has a template that is pure HTML:

code-example(language="html" escape="html").
  <h1>My First Angular App</h1>

:marked
  Almost all HTML syntax is valid template syntax. The `<script>` element is a notable exception; it is forbidden, eliminating the risk of script injection attacks. (In practice, `<script>` is simply ignored.)

  Some legal HTML doesn’t make much sense in a template. The `<html>`, `<body>`, and `<base>` elements have no useful role in our repertoire. Pretty much everything else is fair game.

  We can extend the HTML vocabulary of our templates with components and directives that appear as new elements and attributes. In the following sections we are going to learn how to get and set DOM (Document Object Model) values dynamically through data binding.

  Let’s turn to the first form of data binding &mdash; interpolation &mdash; to see how much richer template HTML can be.

.l-main-section
:marked
  ## Interpolation
  We met the double-curly braces of interpolation, `{{` and `}}`, early in our Angular education.
+makeExample('template-syntax/ts/app/app.component.html', 'first-interpolation')(format=".")
:marked
  We use interpolation to weave calculated strings into the text between HTML element tags and within attribute assignments.

+makeExample('template-syntax/ts/app/app.component.html', 'title+image')(format=".")
:marked
  The material between the braces is often the name of a component property. Angular replaces that name with the
  string value of the corresponding component property. In the example above, Angular evaluates the `title` and `heroImageUrl` properties
  and "fills in the blanks", first displaying a bold application title and then a heroic image.

  More generally, the material between the braces is a **template expression** that Angular first **evaluates**
  and then **converts to a string**. The following interpolation illustrates the point by adding the two numbers within braces:
+makeExample('template-syntax/ts/app/app.component.html', 'sum-1')(format=".")
:marked
  The expression can invoke methods of the host component, as we do here with `getVal()`:
+makeExample('template-syntax/ts/app/app.component.html', 'sum-2')(format=".")
:marked
  Angular evaluates all expressions in double curly braces, converts the expression results to strings, and links them with neighboring literal strings. Finally,
  it assigns this composite interpolated result to an **element or directive property**.

  We appear to be inserting the result between element tags and assigning it to attributes.
  It's convenient to think so, and we rarely suffer for this mistake.
  Though this is not exactly true. Interpolation is a special syntax that Angular converts into a
  [property binding](#property-binding), and is explained below.

  But first, let's take a closer look at template expressions and statements.

<a id="template-expressions"></a>
.l-main-section
:marked
  ## Template expressions
  A template **expression** produces a value.
  Angular executes the expression and assigns it to a property of a binding target;
  the target might be an HTML element, a component, or a directive.

  We put a template expression within the interpolation braces when we wrote `{{1 + 1}}`.
  We’ll see template expressions again in the [property binding](#property-binding) section,
  appearing in quotes to the right of the `=` symbol as in `[property]="expression"`.

  We write template expressions in a language that looks like #{_JavaScript}.
  Many #{_JavaScript} expressions are legal template expressions, but not all.

  #{_JavaScript} expressions that have or promote side effects are prohibited,
  including:

  * assignments (`=`, `+=`, `-=`, ...)
  * !{__new_op}
  * chaining expressions with !{__chaining_op}
  * increment and decrement operators (`++` and `--`)

:marked
  Other notable differences from #{_JavaScript} syntax include:

block notable-differences
  :marked
    * no support for the bitwise operators `|` and `&`
    * new [template expression operators](#expression-operators), such as `|` and `?.`

h3#expression-context Expression context

block template-expressions-cannot
  :marked
    Perhaps more surprising, template expressions cannot refer to anything in
    the global namespace. They can’t refer to `window` or `document`. They
    can’t call `console.log` or `Math.max`. They are restricted to referencing
    members of the expression context.

:marked
  The *expression context* is typically the **component instance**, which is
  the source of binding values.

  When we see *title* wrapped in double-curly braces, `{{title}}`,
  we know that `title` is a property of the data-bound component.
  When we see *isUnchanged* in `[disabled]="isUnchanged"`,
  we know we are referring to that component's `isUnchanged` property.

  The component itself is usually the expression *context*, in which case
  the template expression usually references that component.

  The expression context can include objects other than the component.
  A [template reference variable](#ref-vars) is one such alternative context object.

:marked
  <a id="no-side-effects"></a>
  ### Expression guidelines
  Template expressions can make or break an application.
  Please follow these guidelines:

  * [No visible side effects](#no-visible-side-effects)
  * [Quick execution](#quick-execution)
  * [Simplicity](#simplicity)
  * [Idempotence](#idempotence)

  The only exceptions to these guidelines should be in specific circumstances that you thoroughly understand.

  #### No visible side effects

  A template expression should not change any application state other than the value of the
  target property.

  This rule is essential to Angular's "unidirectional data flow" policy.
  We should never worry that reading a component value might change some other displayed value.
  The view should be stable throughout a single rendering pass.

  #### Quick execution
  Angular executes template expressions more often than we think.
  They can be called after every keypress or mouse move.
  Expressions should finish quickly or the user experience may drag, especially on slower devices.
  Consider caching values computed from other values when the computation is expensive.

  #### Simplicity
  Although it's possible to write quite complex template expressions, we really shouldn't.

  A property name or method call should be the norm.
  An occasional Boolean negation (`!`) is OK.
  Otherwise, confine application and business logic to the component itself,
  where it will be easier to develop and test.

  #### Idempotence
  An [idempotent](https://en.wikipedia.org/wiki/Idempotence) expression is ideal because
  it is free of side effects and improves Angular's change detection performance.

  In Angular terms, an idempotent expression always returns *exactly the same thing* until
  one of its dependent values changes.
:marked
  Dependent values should not change during a single turn of the event loop.
  If an idempotent expression returns a string or a number, it returns the same string or number
  when called twice in a row. If the expression returns an object (including #{_an} `#{_Array}`),
  it returns the same object *reference* when called twice in a row.

.l-main-section#template-statements
:marked
  ## Template statements

  A template **statement** responds to an **event** raised by a binding target
  such as an element, component, or directive.

  We’ll see template statements in the [event binding](#event-binding) section,
  appearing in quotes to the right of the `=` symbol as in `(event)="statement"`.

  A template statement *has a side effect*.
  It's how we update application state from user input.
  There would be no point to responding to an event otherwise.

.l-sub-section
  :marked
    Responding to events is the other side of Angular's "unidirectional data flow".
    We're free to change anything, anywhere, during this turn of the event loop.

:marked
  Like template expressions, template *statements* use a language that looks like #{_JavaScript}.
  The template statement parser is different than the template expression parser and
  specifically supports both basic assignment (`=`) and chaining expressions 
  (with !{__chaining_op}).

  However, certain #{_JavaScript} syntax is not allowed:
  * !{__new_op}
  * increment and decrement operators, `++` and `--`
  * operator assignment, such as `+=` and `-=`
  * the bitwise operators `|` and `&`
  * the [template expression operators](#expression-operators)

:marked
  ### Statement context

  As with expressions, statements can refer only to what's in the statement context — typically the
  **component instance** to which we're binding the event.

block statement-context
  :marked
    Template statements cannot refer to anything in the global namespace. They
    can’t refer to `window` or `document`. They can’t call `console.log` or
    `Math.max`.

:marked
  The *onSave* in `(click)="onSave()"` is sure to be a method of the data-bound component instance.

  The statement context may include an object other than the component.
  A [template reference variable](#ref-vars) is one such alternative context object.
  We'll frequently see the reserved `$event` symbol in event binding statements,
  representing the "message" or "payload" of the raised event.

  ### Statement guidelines

  As with expressions, avoid writing complex template statements.
  A method call or simple property assignment should be the norm.

  Now that we have a feel for template expressions and statements,
  we’re ready to learn about the varieties of data binding syntax beyond interpolation.

.l-main-section
:marked
  <a id="binding-syntax"></a>
  ## Binding syntax: An overview
  Data binding is a mechanism for coordinating what users see with application data values.
  While we could push values to and pull values from HTML,
  the application is easier to write, read, and maintain if we turn these chores over to a binding framework.
  We simply declare bindings between binding sources and target HTML elements and let the framework do the work.

  Angular provides many kinds of data binding, and we’ll discuss each of them in this chapter.
  First we'll take a high-level view of Angular data binding and its syntax.

  We can group all bindings into three categories by the direction in which data flows.
  Each category has its distinctive syntax:
table
  tr
    th Data direction
    th Syntax
    th Binding type
  tr
    td One-way<br>from data source<br>to view target
    td
      code-example().
        {{expression}}
        [target] = "expression"
        bind-target = "expression"
    td.
      Interpolation<br>
      Property<br>
      Attribute<br>
      Class<br>
      Style
    tr
      td One-way<br>from view target<br>to data source
      td
        code-example().
          (target) = "statement"
          on-target = "statement"
      td Event
    tr
      td Two-way
      td
        code-example().
          [(target)] = "expression"
          bindon-target = "expression"
      td Two-way

:marked
  Binding types other than interpolation have a **target name** to the left of the equal sign,
  either surrounded by punctuation (`[]`, `()`) or preceded by a prefix (`bind-`, `on-`, `bindon-`).

  What is that target? Before we can answer that question, we must challenge ourselves to look at template HTML in a new way.

  ### A new mental model

  With all the power of data binding and our ability to extend the HTML vocabulary
  with custom markup, it is tempting to think of template HTML as *HTML Plus*.

  Well, it *is* HTML Plus.
  But it’s also significantly different than the HTML we’re used to.
  We really need a new mental model.

  In the normal course of HTML development, we create a visual structure with HTML elements, and
  we modify those elements by setting element attributes with string constants.

+makeExample('template-syntax/ts/app/app.component.html', 'img+button')(format=".")
:marked
  We still create a structure and initialize attribute values this way in Angular templates.

  Then we learn to create new elements with components that encapsulate HTML
  and drop them into our templates as if they were native HTML elements.
+makeExample('template-syntax/ts/app/app.component.html', 'hero-detail-1')(format=".")
:marked
  That’s HTML Plus.

  Now we start to learn about data binding. The first binding we meet might look like this:

+makeExample('template-syntax/ts/app/app.component.html', 'disabled-button-1')(format=".")
:marked
  We’ll get to that peculiar bracket notation in a moment. Looking beyond it,
  our intuition tells us that we’re binding to the button's `disabled` attribute and setting
  it to the current value of the component’s `isUnchanged` property.

  Our intuition is wrong! Our everyday HTML mental model is misleading us.
  In fact, once we start data binding, we are no longer working with HTML *attributes*. We aren't setting attributes.
  We are setting the *properties* of DOM elements, components, and directives.

.l-sub-section
  :marked
    ### HTML attribute vs. DOM property

    The distinction between an HTML attribute and a DOM property is crucial to understanding how Angular binding works.

    **Attributes are defined by HTML. Properties are defined by the DOM (Document Object Model).**

    * A few HTML attributes have 1:1 mapping to properties. `id` is one example.

    * Some HTML attributes don't have corresponding properties. `colspan` is one example.

    * Some DOM properties don't have corresponding attributes. `textContent` is one example.

    * Many HTML attributes appear to map to properties ... but not in the way we might think!

    That last category can be especially confusing ... until we understand this general rule:

    **Attributes *initialize* DOM properties and then they are done.
    Property values can change; attribute values can't.**

    For example, when the browser renders `<input type="text" value="Bob">`, it creates a
    corresponding DOM node with a `value` property *initialized* to "Bob".

    When the user enters "Sally" into the input box, the DOM element `value` *property* becomes "Sally".
    But the HTML `value` *attribute* remains unchanged as we discover if we ask the input element
    about that attribute: `input.getAttribute('value') // returns "Bob"`

    The HTML attribute `value` specifies the *initial* value; the DOM `value` property is the *current* value.

    The `disabled` attribute is another peculiar example. A button's `disabled` *property* is
    `false` by default so the button is enabled.
    When we add the `disabled` *attribute*, its presence alone initializes the  button's `disabled` *property* to `true`
    so the button is disabled.

    Adding and removing the `disabled` *attribute* disables and enables the button. The value of the *attribute* is irrelevant,
    which is why we cannot enable a button by writing `<button disabled="false">Still Disabled</button>`.

    Setting the button's `disabled` *property*  (say, with an Angular binding) disables or enables the button.
    The value of the *property* matters.

    **The HTML attribute and the DOM property are not the same thing, even when they have the same name.**

:marked
  This is so important, we’ll say it again.

  **Template binding works with *properties* and *events*, not *attributes*.**

.callout.is-helpful
  header A world without attributes
  :marked
    In the world of Angular, the only role of attributes is to initialize element and directive state.
    When we data bind, we're dealing exclusively with element and directive properties and events.
    Attributes effectively disappear.
:marked
  With this model firmly in mind, let's learn about binding targets.

  ### Binding targets
  The **target of a data binding** is something in the DOM.
  Depending on the binding type, the target can be an
  (element | component | directive) property, an
  (element | component | directive) event, or (rarely) an attribute name.
  The following table summarizes:

// If you update this table, UPDATE it in Dart & JS, too.
<div width="90%">
table
  tr
    th Binding type
    th Target
    th Examples
  tr
    td Property
    td.
      Element&nbsp;property<br>
      Component&nbsp;property<br>
      Directive&nbsp;property
    td
      +makeExample('template-syntax/ts/app/app.component.html', 'property-binding-syntax-1')(format=".")
  tr
    td Event
    td.
      Element&nbsp;event<br>
      Component&nbsp;event<br>
      Directive&nbsp;event
    td
      +makeExample('template-syntax/ts/app/app.component.html', 'event-binding-syntax-1')(format=".")
  tr
    td Two-way
    td.
      Event and property
    td
      +makeExample('template-syntax/ts/app/app.component.html', '2-way-binding-syntax-1')(format=".")
  tr
    td Attribute
    td.
      Attribute
      (the&nbsp;exception)
    td
      +makeExample('template-syntax/ts/app/app.component.html', 'attribute-binding-syntax-1')(format=".")
  tr
    td Class
    td.
      <code>class</code> property
    td
      +makeExample('template-syntax/ts/app/app.component.html', 'class-binding-syntax-1')(format=".")
  tr
    td Style
    td.
      <code>style</code> property
    td
      +makeExample('template-syntax/ts/app/app.component.html', 'style-binding-syntax-1')(format=".")
</div>

:marked
  Let’s descend from the architectural clouds and look at each of these binding types in concrete detail.

.l-main-section
:marked
  ## Property binding
  We write a template **property binding** when we want to set a property of a view element to the value of
  a [template expression](#template-expressions).

  The most common property binding sets an element property to a component property value. An example is
  binding the `src` property of an image element to a component’s `heroImageUrl` property:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-1')(format=".")
:marked
  Another example is disabling a button when the component says that it `isUnchanged`:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-2')(format=".")
:marked
  Another is setting a property of a directive:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-3')(format=".")
:marked
  Yet another is setting the model property of a custom component (a great way
  for parent and child components to communicate):
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-4')(format=".")
:marked
  ### One-way *in*
  People often describe property binding as *one-way data binding* because it flows a value in one direction,
  from a component’s data property into a target element property.

  We cannot use property binding to pull values *out* of the target element.
  We can't bind to a property of the target element to read it. We can only set it.

.l-sub-section
  :marked
    Nor can we use property binding to *call* a method on the target element.

    If the element raises events we can listen to them with an [event binding](#event-binding).

    If we must read a target element property or call one of its methods,
    we'll need a different technique.
    See the API reference for
    [ViewChild](../api/core/index/ViewChild-decorator.html) and
    [ContentChild](../api/core/index/ContentChild-decorator.html).

:marked
  ### Binding target
  An element property between enclosing square brackets identifies the target property. The target property in the following code is the image element’s `src` property.

+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-1')(format=".")
:marked
  Some people prefer the `bind-` prefix alternative, known as the *canonical form*:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-5')(format=".")
:marked
  The target name is always the name of a property, even when it appears to be the name of something else. We see `src` and may think it’s the name of an attribute. No. It’s the name of an image element property.

  Element properties may be the more common targets,
  but Angular looks first to see if the name is a property of a known directive,
  as it is in the following example:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-3')(format=".")

.l-sub-section
  :marked
    Technically, Angular is matching the name to a directive [input](#inputs-outputs),
    one of the property names listed in the directive’s `inputs` array or a property decorated with `@Input()`.
    Such inputs map to the directive’s own properties.
:marked
  If the name fails to match a property of a known directive or element, Angular reports an “unknown directive” error.

  ### Avoid side effects
  As we've already discussed, evaluation of a template expression should have no visible side effects. The expression language itself does its part to keep us safe. We can’t assign a value to anything in a property binding expression nor use the increment and decrement operators.

  Of course, our expression might invoke a property or method that has side effects. Angular has no way of knowing that or stopping us.

  The expression could call something like `getFoo()`. Only we know what `getFoo()` does.
  If `getFoo()` changes something and we happen to be binding to that something, we risk an unpleasant experience. Angular may or may not display the changed value. Angular may detect the change and throw a warning error. Our general advice: stick to data properties and to methods that return values and do no more.

  ### Return the proper type
  The template expression should evaluate to the type of value expected by the target property.
  Return a string if the target property expects a string.
  Return a number if the target property expects a number.
  Return an object if the target property expects an object.

  The `hero` property of the `HeroDetail` component expects a `Hero` object, which is exactly what we’re sending in the property binding:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-4')(format=".")

block dart-type-exceptions
  //- N/A

:marked
  ### Remember the brackets
  The brackets tell Angular to evaluate the template expression.
  If we forget the brackets, Angular treats the string as a constant and *initializes the target property* with that string.
  It does *not* evaluate the string!

  Don't make the following mistake:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-6')(format=".")

block dart-type-exception-example
  //- N/A

a(id="one-time-initialization")
:marked
  ### One-time string initialization
  We *should* omit the brackets when all of the following are true:
  * The target property accepts a string value.
  * The string is a fixed value that we can bake into the template.
  * This initial value never changes.

  We routinely initialize attributes this way in standard HTML, and it works
  just as well for directive and component property initialization.
  The following example initializes the `prefix` property of the `HeroDetailComponent` to a fixed string,
  not a template expression. Angular sets it and forgets about it.
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-7')(format=".")
:marked
  The `[hero]` binding, on the other hand, remains a live binding to the component's `currentHero` property.

  ### Property binding or interpolation?
  We often have a choice between interpolation and property binding. 
  The following binding pairs do the same thing:
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-vs-interpolation')(format=".")
:marked
  Interpolation is a convenient alternative for property binding in many cases.
  In fact, Angular  translates those interpolations into the corresponding property bindings
  before rendering the view.

  There is no technical reason to prefer one form to the other.
  We lean toward readability, which tends to favor interpolation.
  We suggest establishing coding style rules and choosing the form that
  both conforms to the rules and feels most natural for the task at hand.


:marked
  #### Content security
  Imagine the following *malicious content*.
+makeExample('template-syntax/ts/app/app.component.ts', 'evil-title')(format=".")    
:marked
  Fortunately, Angular data binding is on alert for dangerous HTML.
  It *sanitizes* the values before displaying them.
  It **will not** allow HTML with script tags to leak into the browser, neither with interpolation
  nor property binding.
+makeExample('template-syntax/ts/app/app.component.html', 'property-binding-vs-interpolation-sanitization')(format=".")    
:marked
  Interpolation handles the script tags differently than property binding but both approaches render the
  content harmlessly.
figure.image-display
  img(src='/resources/images/devguide/template-syntax/evil-title.png' alt="evil title made safe" width='500px')

.l-main-section
:marked
  <a id="other-bindings"></a>
  ## Attribute, class, and style bindings
  The template syntax provides specialized one-way bindings for scenarios less well suited to property binding.

  ### Attribute binding
  We can set the value of an attribute directly with an **attribute binding**.
.l-sub-section
  :marked
    This is the only exception to the rule that a binding sets a target property. This is the only binding that creates and sets an attribute.

:marked
  We have stressed throughout this chapter that setting an element property with a property binding is always preferred to setting the attribute with a string. Why does Angular offer attribute binding?

  **We must use attribute binding when there is no element property to bind.**

  Consider the [ARIA](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA),
  [SVG](https://developer.mozilla.org/en-US/docs/Web/SVG), and
  table span attributes. They are pure attributes.
  They do not correspond to element properties, and they do not set element properties.
  There are no property targets to bind to.

  We become painfully aware of this fact when we try to write something like this:
code-example(language="html").
  &lt;tr>&lt;td colspan="{{1 + 1}}">Three-Four&lt;/td>&lt;/tr>
:marked
  We get this error:
code-example(format="nocode").
  Template parse errors:
  Can't bind to 'colspan' since it isn't a known native property
:marked
  As the message says, the `<td>` element does not have a `colspan` property.
  It has the "colspan" *attribute*, but
  interpolation and property binding can set only *properties*, not attributes.

  We need attribute bindings to create and bind to such attributes.

  Attribute binding syntax resembles property binding.
  Instead of an element property between brackets, we start with the prefix **`attr`**,
  followed by a dot (`.`) and the name of the attribute. We then set the attribute
  value, using an expression that resolves to a string.

  Here we bind `[attr.colspan]` to a calculated value:
+makeExample('template-syntax/ts/app/app.component.html', 'attrib-binding-colspan')(format=".")
:marked
  Here's how the table renders:
  <table border="1px">
    <tr><td colspan="2">One-Two</td></tr>
    <tr><td>Five</td><td>Six</td></tr>
   </table>

  One of the primary use cases for attribute binding
  is to set ARIA attributes, as in this example:
+makeExample('template-syntax/ts/app/app.component.html', 'attrib-binding-aria')(format=".")
:marked
  ### Class binding

  We can add and remove CSS class names from an element’s `class` attribute with
  a **class binding**.

  Class binding syntax resembles property binding.
  Instead of an element property between brackets, we start with the prefix `class`,
  optionally followed by a dot (`.`) and the name of a CSS class: `[class.class-name]`.

  The following examples show how to add and remove the application's "special" class
  with class bindings.  Here's how we set the attribute without binding:
+makeExample('template-syntax/ts/app/app.component.html', 'class-binding-1')(format=".")
:marked
  We can replace that with a binding to a string of the desired class names; this is an all-or-nothing, replacement binding.
+makeExample('template-syntax/ts/app/app.component.html', 'class-binding-2')(format=".")

:marked
  Finally, we can bind to a specific class name.
  Angular adds the class when the template expression evaluates to #{_truthy}.
  It removes the class when the expression is #{_falsey}.
+makeExample('template-syntax/ts/app/app.component.html', 'class-binding-3')(format=".")

.l-sub-section
  :marked
    While this is a fine way to toggle a single class name,
    we generally prefer the [NgClass directive](#ngClass) for managing multiple class names at the same time.

:marked
  ### Style binding

  We can set inline styles with a **style binding**.

  Style binding syntax resembles property binding.
  Instead of an element property between brackets, we start with the prefix `style`,
  followed by a dot (`.`) and the name of a CSS style property: `[style.style-property]`.

+makeExample('template-syntax/ts/app/app.component.html', 'style-binding-1')(format=".")
:marked
  Some style binding styles have unit extension. Here we conditionally set the font size in  “em” and “%” units .
+makeExample('template-syntax/ts/app/app.component.html', 'style-binding-2')(format=".")

.l-sub-section
  :marked
    While this is a fine way to set a single style,
    we generally prefer the [NgStyle directive](#ngStyle) when setting several inline styles at the same time.

.l-sub-section
  :marked
    Note that a _style property_ name can be written in either
    [dash-case](glossary.html#dash-case), as shown above, or
    [camelCase](glossary.html#camelcase), such as `fontSize`.

block style-property-name-dart-diff
  //- N/A

.l-main-section
:marked
  ## Event binding
  The bindings we’ve met so far flow data in one direction: **from a component to an element**.

  Users don’t just stare at the screen. They enter text into input boxes. They pick items from lists.
  They click buttons. Such user actions may result in a flow of data in the opposite direction:
  **from an element to a component**.

  The only way to know about a user action is to listen for certain events such as
  keystrokes, mouse movements, clicks, and touches.
  We declare our interest in user actions through Angular event binding.

  Event binding syntax consists of a **target event** within parentheses on the left of an equal sign, and a quoted
  [template statement](#template-statements) on the right.
  The following event binding listens for the button’s click event, calling
  the component's `onSave()` method whenever a click occurs:
+makeExample('template-syntax/ts/app/app.component.html', 'event-binding-1')(format=".")
:marked
  ### Target event
  A **name between parentheses** &mdash; for example, `(click)` &mdash;
  identifies the target event. In the following example, the target is the button’s click event.
+makeExample('template-syntax/ts/app/app.component.html', 'event-binding-1')(format=".")
:marked
  Some people prefer the `on-` prefix alternative, known as the **canonical form**:
+makeExample('template-syntax/ts/app/app.component.html', 'event-binding-2')(format=".")
:marked
  Element events may be the more common targets, but Angular looks first to see if the name matches an event property
  of a known directive, as it does in the following example:
+makeExample('template-syntax/ts/app/app.component.html', 'event-binding-3')(format=".")

.l-sub-section
  :marked
    The `myClick` directive is further described in the section
    on [aliasing input/output properties](#aliasing-io).

:marked
  If the name fails to match an element event or an output property of a known directive,
  Angular reports an “unknown directive” error.

  ### *$event* and event handling statements
  In an event binding, Angular sets up an event handler for the target event.

  When the event is raised, the handler executes the template statement.
  The template statement typically involves a receiver, which performs an action
  in response to the event, such as storing a value from the HTML control
  into a model.

  The binding conveys information about the event, including data values, through
  an **event object named `$event`**.

  The shape of the event object is determined by the target event.
  If the target event is a native DOM element event, then `$event` is a
  [DOM event object]( https://developer.mozilla.org/en-US/docs/Web/Events),
  with properties such as `target` and `target.value`.

  Consider this example:
+makeExample('template-syntax/ts/app/app.component.html', 'without-NgModel')(format=".")
:marked
  This code sets the input box `value` property by binding to the `firstName` property. To listen for changes to the value, the code binds to the input box's `input` event.
  When the user makes changes, the `input` event is raised, and the binding executes the statement within a context that includes the DOM event object, `$event`.

  To update the `firstName` property, the changed text is retrieved by following the path `$event.target.value`.

  If the event belongs to a directive (recall that components are directives), `$event` has whatever shape the directive decides to produce.

  <a id="eventemitter"></a>
  <a id="custom-event"></a>
  ### Custom events with *EventEmitter*

  Directives typically raise custom events with an Angular [EventEmitter](../api/core/index/EventEmitter-class.html).
  The directive creates an `EventEmitter` and exposes it as a property.
  The directive calls `EventEmitter.emit(payload)` to fire an event, passing in a message payload, which can be anything.
  Parent directives listen for the event by binding to this property and accessing the payload through the `$event` object.

  Consider a `HeroDetailComponent` that presents hero information and responds to user actions.
  Although the `HeroDetailComponent` has a delete button it doesn't know how to delete the hero itself.
  The best it can do is raise an event reporting the user's delete request.

  Here are the pertinent excerpts from that `HeroDetailComponent`:
+makeExample('template-syntax/ts/app/hero-detail.component.ts',
'template-1', 'HeroDetailComponent.ts (template)')(format=".")
+makeExample('template-syntax/ts/app/hero-detail.component.ts',
'deleteRequest', 'HeroDetailComponent.ts (delete logic)')(format=".")

:marked
  The component defines a `deleteRequest` property that returns an `EventEmitter`.
  When the user clicks *delete*, the component invokes the `delete()` method, 
  telling the `EventEmitter` to emit a `Hero` object.

  Now imagine a hosting parent component that binds to the `HeroDetailComponent`'s `deleteRequest` event.

+makeExample('template-syntax/ts/app/app.component.html',
'event-binding-to-component')(format=".")
:marked
  When the `deleteRequest` event fires, Angular calls the parent component's `deleteHero` method,
  passing the *hero-to-delete* (emitted by `HeroDetail`) in the `$event` variable.

  ### Template statements have side effects
  The `deleteHero` method has a side effect: it deletes a hero.
  Template statement side effects are not just OK, but expected.

  Deleting the hero updates the model, perhaps triggering other changes
  including queries and saves to a remote server.
  These changes percolate through the system and are ultimately displayed in this and other views.

//
  :marked
    ### Event bubbling and propagation [TODO: reinstate this section when it becomes true]
    Angular invokes the event-handling statement if the event is raised by the current element or one of its child elements.
  +makeExample('template-syntax/ts/app/app.component.html', 'event-binding-bubbling')(format=".")
  :marked
    Many DOM events, both [native](https://developer.mozilla.org/en-US/docs/Web/Guide/Events/Overview_of_Events_and_Handlers ) and [custom](https://developer.mozilla.org/en-US/docs/Web/Guide/Events/Creating_and_triggering_events ), bubble up their ancestor tree of DOM elements until an event handler along the way prevents further propagation.

  .l-sub-section
    :marked
      `EventEmitter` events don’t bubble.

  :marked
    The result of an event binding statement determines whether
    [event propagation](https://developer.mozilla.org/en-US/docs/Web/API/Document_Object_Model/Examples#Example_5:_Event_Propagation)
    continues or stops with the current element.

    Event propagation stops if the binding statement returns a falsey value (as does a method with no return value).
    Clicking the button in the next example triggers a save;
    the click doesn't make it to the outer `<div>` so the div's save handler is not called.
  +makeExample('template-syntax/ts/app/app.component.html', 'event-binding-no-propagation')(format=".")
  :marked
    Propagation continues if the statement returns a truthy value. In the next example, the click is heard by both the button
    and the outer `<div>`, causing a double save.
  +makeExample('template-syntax/ts/app/app.component.html', 'event-binding-propagation')(format=".")

#two-way
.l-main-section
:marked
  ## Two-way binding
  We often want to both display a data property and update that property when the user makes changes.

  On the element side that takes a combination of setting a specific element property
  and listening for an element change event.

  Angular offers a special _two-way data binding_ syntax for this purpose, **`[(x)]`**.
  The `[(x)]` syntax combines the brackets 
  of _property binding_, `[x]`, with the parentheses of _event binding_, `(x)`.

.callout.is-important
  header [( )] = banana in a box
  :marked
    Visualize a *banana in a box* to remember that the parentheses go _inside_ the brackets.

:marked
  The `[(x)]` syntax is easy to demonstrate when the element has a settable property called `x`
  and a corresponding event named `xChange`. 
  Here's a `SizerComponent` that fits the pattern.
  It has a `size` value property and a companion `sizeChange` event:

+makeExample('app/sizer.component.ts')

:marked
  The initial `size` is an input value from a property binding.
  Clicking the buttons increases or decreases the `size`, within min/max values constraints,
  and then raises (_emits_) the `sizeChange` event with the adjusted size.

  Here's an example in which the `AppComponent.fontSizePx` is two-way bound to the `SizerComponent`:

+makeExcerpt('app/app.component.html', 'two-way-1', '')

:marked
  The `AppComponent.fontSizePx` establishes the initial `SizerComponent.size` value.
  Clicking the buttons updates the `AppComponent.fontSizePx` via the two-way binding.
  The revised `AppComponent.fontSizePx` value flows through to the _style_ binding, making the displayed text bigger or smaller.
  Try it in the <live-example></live-example>.

  The two-way binding syntax is really just syntactic sugar for a _property_ binding and an _event_ binding.
  Angular _desugars_ the `SizerComponent` binding into this:

+makeExcerpt('app/app.component.html', 'two-way-2', '')

:marked
  The `$event` variable contains the payload of the `SizerComponent.sizeChange` event.
  Angular assigns the `$event` value to the `AppComponent.fontSizePx` when the user clicks the buttons.

  Clearly the two-way binding syntax is a great convenience compared to separate property and event bindings.

  We'd like to use two-way binding with HTML form elements like `<input>` and `<select>`.
  Sadly, no native HTML element follows the `x` value and `xChange` event pattern.

  Fortunately, the Angular [_NgModel_](#ngModel) directive is a bridge that enables two-way binding to form elements.

a#ngModel
.l-main-section
:marked
  ## Two-way binding with NgModel
  When developing data entry forms, we often want to both display a data property and update that property when the user makes changes.

  Two-way data binding with the `NgModel` directive makes that easy. Here's an example:
+makeExample('template-syntax/ts/app/app.component.html', 'NgModel-1')(format=".")


+ifDocsFor('ts|js')
  .callout.is-important
    header FormsModule is Required to use ngModel
    :marked
      Before we can use the `ngModel` directive in a two-way data binding,
      we must import the `FormsModule` and add it to the Angular module's `imports` list.
      Learn more about the `FormsModule` and `ngModel` in the
      [Forms](../guide/forms.html#ngModel) chapter.
  :marked
    Here's how to import the `FormsModule` to make `[(ngModel)]` available.
  +makeExample('template-syntax/ts/app/app.module.1.ts', '', 'app.module.ts (FormsModule import)')

:marked
  ### Inside `[(ngModel)]`
  Looking back at the `firstName` binding, it's important to note that
  we could have achieved the same result with separate bindings to
  the `<input>` element's  `value` property and `input` event.
+makeExample('template-syntax/ts/app/app.component.html', 'without-NgModel')(format=".")
:marked
  That’s cumbersome. Who can remember which element property to set and which element event emits user changes?
  How do we extract the currently displayed text from the input box so we can update the data property?
  Who wants to look that up each time?

  That `ngModel` directive hides these onerous details behind its own  `ngModel` input and `ngModelChange` output properties.
+makeExample('template-syntax/ts/app/app.component.html', 'NgModel-3')(format=".")
.l-sub-section
  :marked
    The `ngModel` data property sets the element's value property and the `ngModelChange` event property
    listens for changes to the element's value.
    
    The details are specific to each kind of element and therefore the `NgModel` directive only works for specific form elements,
    such as the input text box, that are supported by a [ControlValueAccessor](../api/forms/index/ControlValueAccessor-interface.html).

    We can't apply `[(ngModel)]` to a custom component until we write a suitable *value accessor*,
    a technique that is beyond the scope of this chapter.
    That's something we might want to do for an Angular component or a WebComponent whose API we can't control.
    
    It's completely unnecessary for an Angular component that we _do_ control ... because we can name the value and event properties
    to suit Angular's basic [two-way binding syntax](#two-way) and skip `NgModel` altogether.

:marked
  Separate `ngModel` bindings is an improvement over binding to the element's native properties. We can do better.

  We shouldn't have to mention the data property twice. Angular should be able to capture the component’s data property and set it
  with a single declaration &mdash; which it can with the `[(ngModel)]` syntax:
+makeExample('template-syntax/ts/app/app.component.html', 'NgModel-1')(format=".")
:marked
  Is `[(ngModel)]` all we need? Is there ever a reason to fall back to its expanded form?

  The `[(ngModel)]` syntax can only _set_ a data-bound property.
  If we need to do something more or something different, we need to write the expanded form ourselves.

  Let's try something silly like forcing the input value to uppercase:
+makeExample('template-syntax/ts/app/app.component.html', 'NgModel-4')(format=".")
:marked
  Here are all variations in action, including the uppercase version:
figure.image-display
    img(src='/resources/images/devguide/template-syntax/ng-model-anim.gif' alt="NgModel variations")

.l-main-section
:marked
  <a id="directives"></a>
  ## Built-in directives

  Earlier versions of Angular included over seventy built-in directives.
  The community contributed many more, and countless private directives
  have been created for internal applications.

  We don’t need many of those directives in Angular.
  Quite often we can achieve the same results with the more capable and expressive Angular binding system.
  Why create a directive to handle a click when we can write a simple binding such as this?
+makeExample('template-syntax/ts/app/app.component.html', 'event-binding-1')(format=".")
:marked
  We still benefit from directives that simplify complex tasks.
  Angular still ships with built-in directives; just not as many.
  We'll write our own directives, just not as many.

  This segment reviews some of the most frequently used built-in directives.

<a id="ngClass"></a>
.l-main-section
:marked
  ### NgClass

  We typically control how elements appear
  by adding and removing CSS classes dynamically.
  We can bind to `NgClass` to add or remove several classes simultaneously.

  A [class binding](#class-binding) is a good way to add or remove a *single* class.
+makeExample('template-syntax/ts/app/app.component.html', 'class-binding-3a')(format=".")
:marked
  The `NgClass` directive may be the better choice
  when we want to add or remove *many* CSS classes at the same time.

  A good way to apply `NgClass` is by binding it to a key:value control !{__objectAsMap}. Each key of the object is a CSS class name; its value is `true` if the class should be added, `false` if it should be removed.

:marked
  Consider a component method such as `setClasses` that manages the state of three CSS classes:
+makeExample('template-syntax/ts/app/app.component.ts', 'setClasses')(format=".")
:marked
  Now we can add an `NgClass` property binding that calls `setClasses`
  and sets the element's classes accordingly:
+makeExample('template-syntax/ts/app/app.component.html', 'NgClass-1')(format=".")

<a id="ngStyle"></a>
.l-main-section
:marked
  ### NgStyle
  We can set inline styles dynamically, based on the state of the component.
  Binding to `NgStyle` lets us set many inline styles simultaneously.

  A [style binding](#style-binding) is an easy way to set a *single* style value.
+makeExample('template-syntax/ts/app/app.component.html', 'NgStyle-1')(format=".")
:marked
  The `NgStyle` directive may be the better choice
  when we want to set *many* inline styles at the same time.

  We apply `NgStyle` by binding it to a key:value control !{__objectAsMap}.
  Each key of the object is a style name; its value is whatever is appropriate for that style.

  Consider a component method such as `setStyles` that returns an object defining three styles:
+makeExample('template-syntax/ts/app/app.component.ts', 'setStyles')(format=".")
:marked
  Now we just add an `NgStyle` property binding that calls `setStyles`
  and sets the element's styles accordingly:
+makeExample('template-syntax/ts/app/app.component.html', 'NgStyle-2')(format=".")

<a id="ngIf"></a>
.l-main-section
:marked
  ### NgIf
  We can add an element subtree (an element and its children) to the DOM  by binding an `NgIf` directive to a #{_truthy} expression.
+makeExample('template-syntax/ts/app/app.component.html', 'NgIf-1')(format=".")

.alert.is-critical
  :marked
    Don't forget the asterisk (`*`) in front of `ngIf`.
    For more information, see [\* and &lt;template>](#star-template).
:marked
  Binding to a #{_falsey} expression removes the element subtree from the DOM.
+makeExample('template-syntax/ts/app/app.component.html', 'NgIf-2')(format=".")

block dart-no-truthy-falsey
  //- N/A

:marked
  #### Visibility and NgIf are not the same
  We can show and hide an element subtree (the element and its children) with a
  [class](#class-binding) or [style](#style-binding) binding:
+makeExample('template-syntax/ts/app/app.component.html', 'NgIf-3')(format=".")
:marked
  Hiding a subtree is quite different from excluding a subtree with `NgIf`.

  When we hide the element subtree, it remains in the DOM.
  Components in the subtree are preserved, along with their state.
  Angular may continue to check for changes even to invisible properties.
  The subtree may tie up substantial memory and computing resources.

  When `NgIf` is `false`, Angular physically removes the element subtree from the DOM.
  It destroys components in the subtree, along with their state, potentially freeing up substantial resources and
  resulting in better performance for the user.

  The show/hide technique is probably fine for small element trees.
  We should be wary when hiding large trees; `NgIf` may be the safer choice. Always measure before leaping to conclusions.

<a id="ngSwitch"></a>
.l-main-section
:marked
  ### NgSwitch
  We bind to `NgSwitch` when we want to display *one* element tree (an element and its children)
  from a *set* of possible element trees, based on some condition.
  Angular puts only the *selected* element tree into the DOM.

  Here’s an example:
+makeExample('template-syntax/ts/app/app.component.html', 'NgSwitch')(format=".")
:marked
  We bind the parent `NgSwitch` directive to an expression returning a *switch value*.
  The value is a string in this example, but it can be a value of any type.

  In this example, the parent `NgSwitch` directive controls a set of child `<span>` elements.
  A `<span>` is either pegged to a *match value* expression or marked as the default.

  **At any particular moment, at most one of these *spans* is in the DOM.**

  If the *span*’s *match value* equals the switch value, Angular adds the `<span>` to the DOM.
  If none of the *spans* is a match, Angular adds the default *span* to the DOM.
  Angular removes and destroys all other *spans*.
.l-sub-section
  :marked
    We could substitute any element for the *span* in this example.
    That element could be a `<div>` with a vast subtree of its own elements.
    Only the matching `<div>` and its subtree would appear in the DOM;
    the others would be removed.
:marked
  Three collaborating directives are at work here:
  1. `ngSwitch`: bound to an expression that returns the switch value
  1. `ngSwitchCase`: bound to an expression returning a match value
  1. `ngSwitchDefault`: a marker attribute on the default element

.alert.is-critical
  :marked
    **Do *not*** put the asterisk (`*`) in front of `ngSwitch`. Use the property binding instead.

    **Do** put the asterisk (`*`) in front of `ngSwitchCase` and `ngSwitchDefault`.
    For more information, see [\* and &lt;template>](#star-template).

<a id="ngFor"></a>
.l-main-section
:marked
  ### NgFor
  `NgFor` is a _repeater_ directive &mdash; a way to customize data display.

  Our goal is to present a list of items. We define a block of HTML that defines how a single item should be displayed.
  We tell Angular to use that block as a template for rendering each item in the list.

  Here is an example of `NgFor` applied to a simple `<div>`:
+makeExample('template-syntax/ts/app/app.component.html', 'NgFor-1')(format=".")
:marked
  We can also apply an `NgFor` to a component element, as in this example:
+makeExample('template-syntax/ts/app/app.component.html', 'NgFor-2')(format=".")

.alert.is-critical
  :marked
    Don't forget the asterisk (`*`) in front of `ngFor`.
    For more information, see [\* and &lt;template>](#star-template).
:marked
  The text assigned to `*ngFor` is the instruction that guides the repeater process.

<a id="ngForMicrosyntax"></a>
:marked
  #### NgFor microsyntax
  The string assigned to `*ngFor` is not a [template expression](#template-expressions).
  It’s a *microsyntax* &mdash; a little language of its own that Angular interprets. In this example, the string `"let hero of heroes"` means:

  > *Take each hero in the `heroes` #{_array}, store it in the local `hero` variable, and make it available to the templated HTML for each iteration.*

  Angular translates this instruction into a new set of elements and bindings.

  In the two previous examples, the `ngFor` directive iterates over the `heroes` #{_array} returned by the parent component’s `heroes` property,
  stamping out instances of the element to which it is applied.
  Angular creates a fresh instance of the template for each hero in the array.

  The `let` keyword before `hero` creates a template input variable called `hero`.

.alert.is-critical
  :marked
     A template input variable is **not** the same as a [template reference variable](#ref-vars)!

:marked
  We use this variable within the template to access a hero’s properties,
  as we’re doing in the interpolation.
  We can also pass the variable in a binding to a component element,
  as we're doing with `hero-detail`.

:marked
  #### NgFor with index
  The `ngFor` directive supports an optional `index` that increases from 0 to the length of the array for each iteration.
  We can capture the index in a template input variable and use it in our template.

  The next example captures the index in a variable named `i`, using it to stamp out rows like "1 - Hercules Son of Zeus".
+makeExample('template-syntax/ts/app/app.component.html', 'NgFor-3')(format=".")
.l-sub-section
  :marked
    Learn about other special *index-like* values such as `last`, `even`, and `odd` in the [NgFor API reference](../api/common/index/NgFor-directive.html).

:marked
  #### NgForTrackBy
  The `ngFor` directive has the potential to perform poorly, especially with large lists.
  A small change to one item, an item removed, or an item added can trigger a cascade of DOM manipulations.

  For example, we could refresh the list of heroes by re-querying the server.
  The refreshed list probably contains most, if not all, of the previously displayed heroes.

  *We* know this because the `id` of each hero hasn't changed.
  But Angular sees only a fresh list of new object references.
  It has no choice but to tear down the old list, discard those DOM elements, and re-build a new list with new DOM elements.

  Angular can avoid this churn if we give it a *tracking* function that tells it what we know:
  that two objects with the same `hero.id` are the same *hero*. Here is such a function:
+makeExample('template-syntax/ts/app/app.component.ts', 'trackByHeroes')(format=".")
:marked
  Now set the `NgForTrackBy` directive to that *tracking* function.
+makeExample('template-syntax/ts/app/app.component.html', 'NgForTrackBy-2')(format=".")
:marked
  The *tracking* function doesn't eliminate all DOM changes.
  Angular may have to update the DOM element if the same-hero *properties* have changed.
  But if the properties haven't changed &mdash; and most of the time they will not have changed &mdash;
  Angular can leave those DOM elements alone. The list UI will be smoother and more responsive.

  Here is an illustration of the `NgForTrackBy` effect.
figure.image-display
  img(src='/resources/images/devguide/template-syntax/ng-for-track-by-anim.gif' alt="NgForTrackBy")

<a id="star-template"></a>
<a id="structural-directive"></a>
.l-main-section
:marked
  ## * and &lt;template&gt;
  When we reviewed the `NgFor`, `NgIf`, and `NgSwitch` built-in directives, we called out an oddity of the syntax: the asterisk (`*`) that appears before the directive names.

  The `*` is a bit of syntactic sugar that makes it easier to read and write directives that modify HTML layout
  with the help of templates.
  `NgFor`, `NgIf`, and `NgSwitch` all add and remove element subtrees that are wrapped in `<template>` tags.

  We didn't see the `<template>` tags because the `*` prefix syntax allowed us to skip those tags and
  focus directly on the HTML element that we are including, excluding, or repeating.

  In this section we go under the hood and see how
  Angular strips away the `*` and expands the HTML into the `<template>` tags for us.

:marked
  ### Expanding `*ngIf`
  We can do what Angular does ourselves and expand the `*` prefix syntax to template syntax. Here's some code with `*ngIf`:
+makeExample('template-syntax/ts/app/app.component.html', 'Template-1')(format=".")
:marked
  The `currentHero` is referenced twice, first as the true/false condition for `NgIf` and
  again as the actual hero passed into the `HeroDetailComponent`.

  The first expansion step transports the `ngIf` (without the `*` prefix) and its contents
  into an expression assigned to a `template` directive.
+makeExample('template-syntax/ts/app/app.component.html', 'Template-2a')(format=".")
:marked
  The next (and final) step unfolds the HTML into a `<template>` tag and `[ngIf]` [property binding](#property-binding):
+makeExample('template-syntax/ts/app/app.component.html', 'Template-2')(format=".")
:marked
  Notice that the `[hero]="currentHero"` binding remains on the child `<hero-detail>`
  element inside the template.

block remember-the-brackets
  .callout.is-critical
    header Remember the brackets!
    :marked
      Don’t make the mistake of writing `ngIf="currentHero"`!
      That syntax assigns the *string* value `"currentHero"` to `ngIf`.
      In JavaScript a non-empty string is a truthy value, so `ngIf` would always be
      `true` and Angular would always display the `hero-detail`
      … even when there is no `currentHero`!

:marked
  ### Expanding `*ngSwitch`
  A similar transformation applies to `*ngSwitch`. We can unfold the syntax ourselves.
  Here's an example, first with `*ngSwitchCase` and `*ngSwitchDefault` and then again with `<template>` tags:
+makeExample('template-syntax/ts/app/app.component.html', 'NgSwitch-expanded')(format=".")
:marked
  The `*ngSwitchCase` and `*ngSwitchDefault` expand in exactly the same manner as `*ngIf`,
  wrapping their former elements in `<template>` tags.

  Now we can see why the `ngSwitch` itself is not prefixed with an asterisk (*).
  It does not define content. It's job is to control a collection of templates.

  In this case, it governs two sets of `ngSwitchCase` and `NgSwitchDefault` directives.
  We should expect it to display the values of the selected template twice,
  once for the (*) prefixed version and once for the expanded template version.
  That's exactly what we see in this example:
figure.image-display
    img(src='/resources/images/devguide/template-syntax/ng-switch-anim.gif' alt="NgSwitch")
:marked
  ### Expanding `*ngFor`
  The `*ngFor` undergoes a similar transformation. We begin with an `*ngFor` example:
+makeExample('template-syntax/ts/app/app.component.html', 'Template-3a')(format=".")
:marked
  Here's the same example after transporting the `ngFor` to the `template` directive:
+makeExample('template-syntax/ts/app/app.component.html', 'Template-3')(format=".")
:marked
  And here it is expanded further into a `<template>` tag wrapping the original `<hero-detail>` element:
+makeExample('template-syntax/ts/app/app.component.html', 'Template-4')(format=".")
:marked
  The `NgFor` code is a bit more complex than `NgIf` because a repeater has more moving parts to configure.
  In this case, we have to remember to create and assign the `NgForOf` directive that identifies the list and the `NgForTrackBy` directive.
  Using the `*ngFor` syntax is much easier than writing out this expanded HTML ourselves.

<a id="ref-vars"></a>
.l-main-section
:marked
  ## Template reference variables

  A **template reference variable** is a reference to a DOM element or directive within a template.

  It can be used with native DOM elements but also with Angular components &mdash; in fact, it will work with any custom web component.

:marked
  ### Referencing a template reference variable

  We can refer to a template reference variable _anywhere_ in the current template.
.l-sub-section
  :marked
    Do not define the same variable name more than once in the same template.
    The runtime value will be unpredictable.

:marked
  Here are two other examples of creating and consuming a Template reference variable:
+makeExample('template-syntax/ts/app/app.component.html', 'ref-phone')(format=".")
:marked
  The hash (`#`) prefix to "phone" means that we're defining a `phone` variable.
.l-sub-section
  :marked
    Folks who don't like using the `#` character can use its canonical alternative,
    the `ref-` prefix. For example, we can declare the our `phone` variable using
    either `#phone` or `ref-phone`.

:marked
  ### How a variable gets its value

  Angular sets the variable's value to the element on which it was defined.
  We defined these variables on the `input` elements.
  We’re passing those `input` element objects across to the
  button elements, where they're used in arguments to the `call` methods in the event bindings.

:marked
  ### NgForm and template reference variables
  Let's look at one final example: a form, the poster child for template reference variables.

  The HTML for a form can be quite involved, as we saw in the [Forms](forms.html) chapter.
  The following is a *simplified* example &mdash; and it's not simple at all.
+makeExample('template-syntax/ts/app/app.component.html', 'ref-form')(format=".")
:marked
  A template reference variable, `theForm`, appears three times in this example, separated
  by a large amount of HTML.
+makeExample('template-syntax/ts/app/app.component.html', 'ref-form-a')(format=".")
:marked
  What is the value of `theForm`?

  It would be the [HTMLFormElement](https://developer.mozilla.org/en-US/docs/Web/API/HTMLFormElement)
  if Angular hadn't taken it over.
  It's actually `ngForm`, a reference to the Angular built-in `NgForm` directive that wraps the native `HTMLFormElement`
  and endows it with additional superpowers such as the ability to
  track the validity of user input.

  This explains how we can disable the submit button by checking `theForm.form.valid`
  and pass an object with rich information to the parent component's `onSubmit` method.

<a id="inputs-outputs"></a>
.l-main-section
:marked
  ## Input and output properties
  So far, we’ve focused mainly on binding to component members within template expressions and statements
  that appear on the *right side of the binding declaration*.
  A member in that position is a data binding **source**.

  This section concentrates on binding to **targets**, which are directive
  properties on the *left side of the binding declaration*.
  These directive properties must be declared as **inputs** or **outputs**.

.alert.is-important
  :marked
    Remember: All **components** are **directives**.
:marked
.l-sub-section
  :marked
    We're drawing a sharp distinction between a data binding **target** and a data binding **source**.

    The *target* of a binding is to the *left* of the `=`.
    The *source* is on the *right* of the `=`.

    The *target* of a binding is the property or event inside the binding punctuation: `[]`, `()` or `[()]`.
    The *source* is either inside quotes (`" "`) or within an interpolation (`{{}}`).

    Every member of a **source** directive is automatically available for binding.
    We don't have to do anything special to access a directive member in a template expression or statement.

    We have *limited* access to members of a **target** directive.
    We can only bind to properties that are explicitly identified as *inputs* and *outputs*.
:marked
  In the following example, `iconUrl` and `onSave` are members of a component
  that are referenced within quoted syntax to the right of the `=`.
+makeExample('template-syntax/ts/app/app.component.html', 'io-1')(format=".")
:marked
  They are *neither inputs nor outputs* of the component. They are data sources for their bindings.

  Now look at `HeroDetailComponent` when it is the **target of a binding**.
+makeExample('template-syntax/ts/app/app.component.html', 'io-2')(format=".")
:marked
  Both `HeroDetailComponent.hero` and `HeroDetailComponent.deleteRequest` are on the **left side** of binding declarations.
  `HeroDetailComponent.hero` is inside brackets; it is the target of a property binding.
  `HeroDetailComponent.deleteRequest` is inside parentheses; it is the target of an event binding.

  ### Declaring input and output properties
  Target properties must be explicitly marked as inputs or outputs.

  When we peek inside `HeroDetailComponent`, we see that these properties are marked
  with decorators as input and output properties.
+makeExample('template-syntax/ts/app/hero-detail.component.ts', 'input-output-1')(format=".")

:marked
.l-sub-section
  :marked
    Alternatively, we can identify members in the `inputs` and `outputs` #{_array}s
    of the directive metadata, as in this example:
  +makeExample('template-syntax/ts/app/hero-detail.component.ts', 'input-output-2')(format=".")
  <br>
  :marked
    We can specify an input/output property either with a decorator or in a metadata #{_array}.
    Don't do both!
:marked
  ### Input or output?

  *Input* properties usually receive data values.
  *Output* properties expose event producers, such as `EventEmitter` objects.

  The terms _input_ and _output_ reflect the perspective of the target directive.
figure.image-display
    img(src='/resources/images/devguide/template-syntax/input-output.png' alt="Inputs and outputs")
:marked
  `HeroDetailComponent.hero` is an **input** property from the perspective of `HeroDetailComponent`
  because data flows *into* that property from a template binding expression.

  `HeroDetailComponent.deleteRequest` is an **output** property from the perspective of `HeroDetailComponent`
  because events stream *out* of that property and toward the handler in a template binding statement.

h3#aliasing-io Aliasing input/output properties
:marked
  Sometimes we want the public name of an input/output property to be different from the internal name.

  This is frequently the case with [attribute directives](attribute-directives.html).
  Directive consumers expect to bind to the name of the directive.
  For example, when we apply a directive with a `myClick` selector to a `<div>` tag,
  we expect to bind to an event property that is also called `myClick`.
+makeExample('template-syntax/ts/app/app.component.html', 'myClick')(format=".")
:marked
  However, the directive name is often a poor choice for the name of a property within the directive class.
  The directive name rarely describes what the property does.
  The `myClick` directive name is not a good name for a property that emits click messages.

  Fortunately, we can have a public name for the property that meets conventional expectations,
  while using a different name internally.
  In the example immediately above, we are actually binding *through the* `myClick` *alias* to
  the directive's own `clicks` property.

  We can specify the alias for the property name by passing it into the input/output decorator like this:

+makeExample('template-syntax/ts/app/click.directive.ts', 'output-myClick')(format=".")

.l-sub-section
  :marked
    We can also alias property names in the `inputs` and `outputs` #{_array}s.
    We write a colon-delimited (`:`) string with
    the directive property name on the *left* and the public alias on the *right*:
  +makeExample('template-syntax/ts/app/click.directive.ts', 'output-myClick2')(format=".")

<a id="expression-operators"></a>
.l-main-section
:marked
  ## Template expression operators
  The template expression language employs a subset of #{_JavaScript} syntax supplemented with a few special operators
  for specific scenarios. We'll cover two of these operators: _pipe_ and _safe navigation operator_.

:marked
  <a id="pipe"></a>
  ### The pipe operator ( | )
  The result of an expression might require some transformation before we’re ready to use it in a binding.  For example, we might want to display a number as a currency, force text to uppercase, or filter a list and sort it.

  Angular [pipes](./pipes.html) are a good choice for small transformations such as these.
  Pipes are simple functions that accept an input value and return a transformed value.
  They're easy to apply within template expressions, using the **pipe operator (`|`)**:
+makeExample('template-syntax/ts/app/app.component.html', 'pipes-1')(format=".")
:marked
  The pipe operator passes the result of an expression on the left to a pipe function on the right.

  We can chain expressions through multiple pipes:
+makeExample('template-syntax/ts/app/app.component.html', 'pipes-2')(format=".")
:marked
  And we can also [apply parameters](./pipes.html#parameterizing-a-pipe) to a pipe:
+makeExample('template-syntax/ts/app/app.component.html', 'pipes-3')(format=".")

block json-pipe
  :marked
    The `json` pipe is particularly helpful for debugging our bindings:
  +makeExample('template-syntax/ts/app/app.component.html', 'pipes-json')(format=".")
  :marked
    The generated output would look something like this
  code-example(language="json").
    { "firstName": "Hercules", "lastName": "Son of Zeus",
      "birthdate": "1970-02-25T08:00:00.000Z",
      "url": "http://www.imdb.com/title/tt0065832/",
      "rate": 325, "id": 1 }

:marked
  <a id="safe-navigation-operator"></a>
  ### The safe navigation operator ( ?. ) and null property paths

  The Angular **safe navigation operator (`?.`)** is a fluent and convenient way to guard against null and undefined values in property paths.
  Here it is, protecting against a view render failure if the `currentHero` is null.
+makeExample('template-syntax/ts/app/app.component.html', 'safe-2')(format=".")

block dart-safe-nav-op
  //- N/A
  
:marked
  Let’s elaborate on the problem and this particular solution.

  What happens when the following data bound `title` property is null?
+makeExample('template-syntax/ts/app/app.component.html', 'safe-1')(format=".")
:marked
  The view still renders but the displayed value is blank; we see only "The title is" with nothing after it.
  That is reasonable behavior. At least the app doesn't crash.

  Suppose the template expression involves a property path, as in this next example
  where we’re displaying the `firstName` of a null hero.

code-example(language="html").
  The null hero's name is {{nullHero.firstName}}

block null-deref-example
  :marked
    JavaScript throws a null reference error, and so does Angular:
  code-example(format="nocode").
    TypeError: Cannot read property 'firstName' of null in [null].

:marked
  Worse, the *entire view disappears*.

  We could claim that this is reasonable behavior if we believed that the `hero` property must never be null.
  If it must never be null and yet it is null,
  we've made a programming error that should be caught and fixed.
  Throwing an exception is the right thing to do.

  On the other hand, null values in the property path may be OK from time to time,
  especially when we know the data will arrive eventually.

  While we wait for data, the view should render without complaint, and
  the null property path should display as blank just as the `title` property does.

  Unfortunately, our app crashes when the `currentHero` is null.

  We could code around that problem with [NgIf](#ngIf).
+makeExample('template-syntax/ts/app/app.component.html', 'safe-4')(format=".")

block safe-op-alt
  :marked
    Or we could try to chain parts of the property path with `&&`, knowing that the expression bails out
    when it encounters the first null.
  +makeExample('template-syntax/ts/app/app.component.html', 'safe-5')(format=".")

:marked
  These approaches have merit but can be cumbersome, especially if the property path is long.
  Imagine guarding against a null somewhere in a long property path such as `a.b.c.d`.

  The Angular safe navigation operator (`?.`) is a more fluent and convenient way to guard against nulls in property paths.
  The expression bails out when it hits the first null value.
  The display is blank, but the app keeps rolling without errors.
+makeExample('template-syntax/ts/app/app.component.html', 'safe-6')(format=".")
:marked
  It works perfectly with long property paths such as `a?.b?.c?.d`.

.l-main-section
:marked
  ## Summary
  We’ve completed our survey of template syntax. Now it's time to put that knowledge to work as we write our own components and directives.
