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Template Syntax

The 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.

You may be familiar with the component/template duality from your 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.

This page is a comprehensive technical reference to the Angular template language. It explains basic principles of the template language and describes most of the syntax that you'll encounter elsewhere in the documentation.

Many code snippets illustrate the points and concepts, all of them available in the .

HTML in templates

HTML is the language of the Angular template. 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 ignored and a warning appears in the browser console. See the Security page for details.

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

You can extend the HTML vocabulary of your templates with components and directives that appear as new elements and attributes. In the following sections, you'll learn how to get and set DOM (Document Object Model) values dynamically through data binding.

Begin with the first form of data binding—interpolation—to see how much richer template HTML can be.

Interpolation and Template Expressions

Interpolation allows you to incorporate calculated strings into the text between HTML element tags and within attribute assignments. Template expressions are what you use to calculate those strings.

The interpolation live example demonstrates all of the syntax and code snippets described in this section.

Interpolation {{...}}

Interpolation refers to embedding expressions into marked up text. By default, interpolation uses as its delimiter the double curly braces, {{ and }}.

In the following snippet, {{ currentCustomer }} is an example of interpolation.

<h3>Current customer: {{ currentCustomer }}</h3>

The text between the braces is often the name of a component property. Angular replaces that name with the string value of the corresponding component property.

<p>{{title}}</p>
<div><img src="{{itemImageUrl}}"></div>

In the example above, Angular evaluates the title and itemImageUrl properties and fills in the blanks, first displaying some title text and then an image.

More generally, the text 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 two numbers:

<!-- "The sum of 1 + 1 is 2" -->
<p>The sum of 1 + 1 is {{1 + 1}}.</p>

The expression can invoke methods of the host component such as getVal() in the following example:

<!-- "The sum of 1 + 1 is not 4" -->
<p>The sum of 1 + 1 is not {{1 + 1 + getVal()}}.</p>

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.

You appear to be inserting the result between element tags and assigning it to attributes. However, interpolation is a special syntax that Angular converts into a property binding.

If you'd like to use something other than {{ and }}, you can configure the interpolation delimiter via the interpolation option in the Component metadata.

Template expressions

A template expression produces a value and appears within the double curly braces, {{ }}. Angular executes the expression and assigns it to a property of a binding target; the target could be an HTML element, a component, or a directive.

The interpolation braces in {{1 + 1}} surround the template expression 1 + 1. In the property binding, a template expression appears in quotes to the right of the = symbol as in [property]="expression".

In terms of syntax, template expressions are similar to JavaScript. Many JavaScript expressions are legal template expressions, with a few exceptions.

You can't use JavaScript expressions that have or promote side effects, including:

  • Assignments (=, +=, -=, ...)
  • Operators such as new, typeof, instanceof, etc.
  • Chaining expressions with ; or ,
  • The increment and decrement operators ++ and --
  • Some of the ES2015+ operators

Other notable differences from JavaScript syntax include:

Expression context

The expression context is typically the component instance. In the following snippets, the recommended within double curly braces and the itemImageUrl2 in quotes refer to properties of the AppComponent.

<h4>{{recommended}}</h4>
<img [src]="itemImageUrl2">

An expression may also refer to properties of the template's context such as a template input variable,

let customer, or a template reference variable, #customerInput.

<ul>
  <li *ngFor="let customer of customers">{{customer.name}}</li>
</ul>
<label>Type something:
  <input #customerInput>{{customerInput.value}}
</label>

The context for terms in an expression is a blend of the template variables, the directive's context object (if it has one), and the component's members. If you reference a name that belongs to more than one of these namespaces, the template variable name takes precedence, followed by a name in the directive's context, and, lastly, the component's member names.

The previous example presents such a name collision. The component has a customer property and the *ngFor defines a customer template variable.

The customer in {{customer.name}} refers to the template input variable, not the component's property.

Template expressions cannot refer to anything in the global namespace, except undefined. They can't refer to window or document. Additionally, they can't call console.log() or Math.max() and they are restricted to referencing members of the expression context.

Expression guidelines

When using template expressions follow these guidelines:

Simplicity

Although it's possible to write complex template expressions, it's a better practice to avoid them.

A property name or method call should be the norm, but an occasional Boolean negation, !, is OK. Otherwise, confine application and business logic to the component, where it is easier to develop and test.

Quick execution

Angular executes template expressions after every change detection cycle. Change detection cycles are triggered by many asynchronous activities such as promise resolutions, HTTP results, timer events, key presses and mouse moves.

Expressions should finish quickly or the user experience may drag, especially on slower devices. Consider caching values when their computation is expensive.

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. You should never worry that reading a component value might change some other displayed value. The view should be stable throughout a single rendering pass.

An idempotent 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.

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.

There is one exception to this behavior that applies to *ngFor. *ngFor has trackBy functionality that can deal with referential inequality of objects when iterating over them. See *ngFor with trackBy for details.

Template statements

A template statement responds to an event raised by a binding target such as an element, component, or directive. You'll see template statements in the event binding section, appearing in quotes to the right of the = symbol as in (event)="statement".

<button (click)="deleteHero()">Delete hero</button>

A template statement has a side effect. That's the whole point of an event. It's how you update application state from user action.

Responding to events is the other side of Angular's "unidirectional data flow". You're free to change anything, anywhere, during this turn of the event loop.

Like template expressions, template statements use a language that looks like JavaScript. The template statement parser differs from the template expression parser and specifically supports both basic assignment (=) and chaining expressions (with ; or ,).

However, certain JavaScript syntax is not allowed:

  • new
  • increment and decrement operators, ++ and --
  • operator assignment, such as += and -=
  • the bitwise operators | and &
  • the template expression operators

Statement context

As with expressions, statements can refer only to what's in the statement context such as an event handling method of the component instance.

The statement context is typically the component instance. The deleteHero in (click)="deleteHero()" is a method of the data-bound component.

<button (click)="deleteHero()">Delete hero</button>

The statement context may also refer to properties of the template's own context. In the following examples, the template $event object, a template input variable (let hero), and a template reference variable (#heroForm) are passed to an event handling method of the component.

<button (click)="onSave($event)">Save</button>
<button *ngFor="let hero of heroes" (click)="deleteHero(hero)">{{hero.name}}</button>
<form #heroForm (ngSubmit)="onSubmit(heroForm)"> ... </form>

Template context names take precedence over component context names. In deleteHero(hero) above, the hero is the template input variable, not the component's hero property.

Statement guidelines

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.

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

Binding syntax: an overview

Data-binding is a mechanism for coordinating what users see, specifically with application data values. While you could push values to and pull values from HTML, the application is easier to write, read, and maintain if you turn these tasks over to a binding framework. You simply declare bindings between binding sources, target HTML elements, and let the framework do the rest.

For a demonstration of the syntax and code snippets in this section, see the binding syntax example.

Angular provides many kinds of data-binding. Binding types can be grouped into three categories distinguished by the direction of data flow:

  • From the source-to-view
  • From view-to-source
  • Two-way sequence: view-to-source-to-view
Type Syntax Category
Interpolation Property Attribute Class Style
{{expression}}
        [target]="expression"
        bind-target="expression"
One-wayfrom data sourceto view target
Event
(target)="statement"
          on-target="statement"
One-wayfrom view targetto data source
Two-way
[(target)]="expression"
          bindon-target="expression"
Two-way

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

The target of a binding is the property or event inside the binding punctuation: [], () or [()].

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

Data-binding and HTML

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

<div class="special">Plain old HTML</div>
<img src="images/item.png">
<button disabled>Save</button>

With data-binding, you can control things like the state of a button:

<!-- Bind button disabled state to `isUnchanged` property -->
<button [disabled]="isUnchanged">Save</button>

Notice that the binding is to the disabled property of the button's DOM element, not the attribute. This applies to data-binding in general. Data-binding works with properties of DOM elements, components, and directives, not HTML attributes.

HTML attribute vs. DOM property

The distinction between an HTML attribute and a DOM property is key to understanding how Angular binding works. Attributes are defined by HTML. Properties are accessed from DOM (Document Object Model) nodes.

  • A few HTML attributes have 1:1 mapping to properties; for example, id.

  • Some HTML attributes don't have corresponding properties; for example, aria-*.

  • Some DOM properties don't have corresponding attributes; for example, textContent.

It is important to remember that HTML attribute and the DOM property are different things, even when they have the same name. In Angular, the only role of HTML attributes is to initialize element and directive state.

Template binding works with properties and events, not attributes.

When you write a data-binding, you're dealing exclusively with the DOM properties and events of the target object.

This general rule can help you build a mental model of attributes and DOM properties: Attributes initialize DOM properties and then they are done. Property values can change; attribute values can't.

There is one exception to this rule. Attributes can be changed by setAttribute(), which re-initializes corresponding DOM properties.

For more information, see the MDN Interfaces documentation which has API docs for all the standard DOM elements and their properties. Comparing the <td> attributes attributes to the <td> properties provides a helpful example for differentiation. In particular, you can navigate from the attributes page to the properties via "DOM interface" link, and navigate the inheritance hierarchy up to HTMLTableCellElement.

Example 1: an <input>

When the browser renders <input type="text" value="Sarah">, it creates a corresponding DOM node with a value property initialized to "Sarah".

<input type="text" value="Sarah">

When the user enters "Sally" into the <input>, the DOM element value property becomes "Sally". However, if you look at the HTML attribute value using input.getAttribute('value'), you can see that the attribute remains unchanged—it returns "Sarah".

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

To see attributes versus DOM properties in a functioning app, see the especially for binding syntax.

Example 2: a disabled button

The disabled attribute is another example. A button's disabled property is false by default so the button is enabled.

When you add the disabled attribute, its presence alone initializes the button's disabled property to true so the button is disabled.

<button disabled>Test Button</button>

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

To control the state of the button, set the disabled property,

Though you could technically set the [attr.disabled] attribute binding, the values are different in that the property binding requires to a boolean value, while its corresponding attribute binding relies on whether the value is null or not. Consider the following:

<input [disabled]="condition ? true : false">
<input [attr.disabled]="condition ? 'disabled' : null">

Generally, use property binding over attribute binding as it is more intuitive (being a boolean value), has a shorter syntax, and is more performant.

To see the disabled button example in a functioning app, see the especially for binding syntax. This example shows you how to toggle the disabled property from the component.

Binding types and targets

The target of a data-binding is something in the DOM. Depending on the binding type, the target can be a property (element, component, or directive), an event (element, component, or directive), or sometimes an attribute name. The following table summarizes the targets for the different binding types.

Type Target Examples
Property Element property Component property Directive property src, hero, and ngClass in the following:
<img [src]="heroImageUrl">
<app-hero-detail [hero]="currentHero"></app-hero-detail>
<div [ngClass]="{'special': isSpecial}"></div>
Event Element event Component event Directive event click, deleteRequest, and myClick in the following:
<button (click)="onSave()">Save</button>
<app-hero-detail (deleteRequest)="deleteHero()"></app-hero-detail>
<div (myClick)="clicked=$event" clickable>click me</div>
Two-way Event and property
<input [(ngModel)]="name">
Attribute Attribute (the exception)
<button [attr.aria-label]="help">help</button>
Class class property
<div [class.special]="isSpecial">Special</div>
Style style property
<button [style.color]="isSpecial ? 'red' : 'green'">

Property binding [property]

Use property binding to set properties of target elements or directive @Input() decorators. For an example demonstrating all of the points in this section, see the property binding example.

One-way in

Property binding flows a value in one direction, from a component's property into a target element property.

You can't use property binding to read or pull values out of target elements. Similarly, you cannot use property binding to call a method on the target element. If the element raises events, you can listen to them with an event binding.

If you must read a target element property or call one of its methods, see the API reference for ViewChild and ContentChild.

Examples

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 itemImageUrl property:

<img [src]="itemImageUrl">

Here's an example of binding to the colSpan property. Notice that it's not colspan, which is the attribute, spelled with a lowercase s.

<!-- Notice the colSpan property is camel case -->
<tr><td [colSpan]="2">Span 2 columns</td></tr>

For more details, see the MDN HTMLTableCellElement documentation.

Another example is disabling a button when the component says that it isUnchanged:

<!-- Bind button disabled state to `isUnchanged` property -->
<button [disabled]="isUnchanged">Disabled Button</button>

Another is setting a property of a directive:

<p [ngClass]="classes">[ngClass] binding to the classes property making this blue</p>

Yet another is setting the model property of a custom component—a great way for parent and child components to communicate:

<app-item-detail [childItem]="parentItem"></app-item-detail>

Binding targets

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.

<img [src]="itemImageUrl">

There's also the bind- prefix alternative:

<img bind-src="itemImageUrl">

In most cases, the target name is the name of a property, even when it appears to be the name of an attribute. So in this case, src is the name of the <img> 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:

<p [ngClass]="classes">[ngClass] binding to the classes property making this blue</p>

Technically, Angular is matching the name to a directive @Input(), 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.

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

Though the target name is usually the name of a property, there is an automatic attribute-to-property mapping in Angular for several common attributes. These include class/className, innerHtml/innerHTML, and tabindex/tabIndex.

Avoid side effects

Evaluation of a template expression should have no visible side effects. The expression language itself, or the way you write template expressions, helps to a certain extent; you can't assign a value to anything in a property binding expression nor use the increment and decrement operators.

For example, you could have an expression that invoked a property or method that had side effects. The expression could call something like getFoo() where only you know what getFoo() does. If getFoo() changes something and you happen to be binding to that something, Angular may or may not display the changed value. Angular may detect the change and throw a warning error. As a best practice, stick to properties and to methods that return values and avoid side effects.

Return the proper type

The template expression should evaluate to the type of value that the target property expects. Return a string if the target property expects a string, a number if it expects a number, an object if it expects an object, and so on.

In the following example, the childItem property of the ItemDetailComponent expects a string, which is exactly what you're sending in the property binding:

<app-item-detail [childItem]="parentItem"></app-item-detail>

You can confirm this by looking in the ItemDetailComponent where the @Input type is set to a string:

@Input() childItem: string;

As you can see here, the parentItem in AppComponent is a string, which the ItemDetailComponent expects:

parentItem = 'lamp';

Passing in an object

The previous simple example showed passing in a string. To pass in an object, the syntax and thinking are the same.

In this scenario, ListItemComponent is nested within AppComponent and the item property expects an object.

<app-list-item [items]="currentItem"></app-list-item>

The item property is declared in the ListItemComponent with a type of Item and decorated with @Input():

@Input() items: Item[];

In this sample app, an Item is an object that has two properties; an id and a name.

export class Item {
  id: number;
  name: string;
}

While a list of items exists in another file, mock-items.ts, you can specify a different item in app.component.ts so that the new item will render:

currentItem = [{
  id: 21,
  name: 'phone'
}];

You just have to make sure, in this case, that you're supplying an object because that's the type of item and is what the nested component, ListItemComponent, expects.

In this example, AppComponent specifies a different item object (currentItem) and passes it to the nested ListItemComponent. ListItemComponent was able to use currentItem because it matches what an Item object is according to item.ts. The item.ts file is where ListItemComponent gets its definition of an item.

Remember the brackets

The brackets, [], tell Angular to evaluate the template expression. If you omit the brackets, Angular treats the string as a constant and initializes the target property with that string:

<app-item-detail childItem="parentItem"></app-item-detail>

Omitting the brackets will render the string parentItem, not the value of parentItem.

One-time string initialization

You 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 you can put directly into the template.
  • This initial value never changes.

You 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 StringInitComponent to a fixed string, not a template expression. Angular sets it and forgets about it.

<app-string-init prefix="This is a one-time initialized string."></app-string-init>

The [item] binding, on the other hand, remains a live binding to the component's currentItem property.

Property binding vs. interpolation

You often have a choice between interpolation and property binding. The following binding pairs do the same thing:

<p><img src="{{itemImageUrl}}"> is the <i>interpolated</i> image.</p>
<p><img [src]="itemImageUrl"> is the <i>property bound</i> image.</p>

<p><span>"{{interpolationTitle}}" is the <i>interpolated</i> title.</span></p>
<p>"<span [innerHTML]="propertyTitle"></span>" is the <i>property bound</i> title.</p>

Interpolation is a convenient alternative to property binding in many cases. When rendering data values as strings, there is no technical reason to prefer one form to the other, though readability tends to favor interpolation. However, when setting an element property to a non-string data value, you must use property binding.

Content security

Imagine the following malicious content.

evilTitle = 'Template <script>alert("evil never sleeps")</script> Syntax';

In the component template, the content might be used with interpolation:

<p><span>"{{evilTitle}}" is the <i>interpolated</i> evil title.</span></p>

Fortunately, Angular data binding is on alert for dangerous HTML. In the above case, the HTML displays as is, and the Javascript does not execute. Angular does not allow HTML with script tags to leak into the browser, neither with interpolation nor property binding.

In the following example, however, Angular sanitizes the values before displaying them.

<!--
 Angular generates a warning for the following line as it sanitizes them
 WARNING: sanitizing HTML stripped some content (see http://g.co/ng/security#xss).
-->
 <p>"<span [innerHTML]="evilTitle"></span>" is the <i>property bound</i> evil title.</p>

Interpolation handles the <script> tags differently than property binding but both approaches render the content harmlessly. The following is the browser output of the evilTitle examples.

"Template  Syntax" is the interpolated evil title.
"Template alert("evil never sleeps")Syntax" is the property bound evil title.

Attribute, class, and style bindings

The template syntax provides specialized one-way bindings for scenarios less well-suited to property binding.

To see attribute, class, and style bindings in a functioning app, see the especially for this section.

Attribute binding

Set the value of an attribute directly with an attribute binding. This is the only exception to the rule that a binding sets a target property and the only binding that creates and sets an attribute.

Usually, setting an element property with a property binding is preferable to setting the attribute with a string. However, sometimes there is no element property to bind, so attribute binding is the solution.

Consider the ARIA and SVG. They are purely attributes, don't correspond to element properties, and don't set element properties. In these cases, there are no property targets to bind to.

Attribute binding syntax resembles property binding, but instead of an element property between brackets, start with the prefix attr, followed by a dot (.), and the name of the attribute. You then set the attribute value, using an expression that resolves to a string, or remove the attribute when the expression resolves to null.

One of the primary use cases for attribute binding is to set ARIA attributes, as in this example:

<!-- create and set an aria attribute for assistive technology -->
<button [attr.aria-label]="actionName">{{actionName}} with Aria</button>

colspan and colSpan

Notice the difference between the colspan attribute and the colSpan property.

If you wrote something like this:

<tr><td colspan="{{1 + 1}}">Three-Four</td></tr>

You'd get this error:

Template parse errors:
  Can't bind to 'colspan' since it isn't a known native property

As the message says, the <td> element does not have a colspan property. This is true because colspan is an attribute—colSpan, with a capital S, is the corresponding property. Interpolation and property binding can set only properties, not attributes.

Instead, you'd use property binding and write it like this:

<!-- Notice the colSpan property is camel case -->
<tr><td [colSpan]="1 + 1">Three-Four</td></tr>

Class binding

Add and remove CSS class names from an element's class attribute with a class binding.

Here's how to set the attribute without binding in plain HTML:

<!-- standard class attribute setting -->
<div class="item clearance special">Item clearance special</div>

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

You can replace that with a binding to a string of the desired class names; this is an all-or-nothing, replacement binding.

<h3>Overwrite all existing classes with a new class:</h3>
<div class="item clearance special" [attr.class]="resetClasses">Reset all classes at once</div>

You can also add append a class to an element without overwriting the classes already on the element:

<h3>Add a class:</h3>
<div class="item clearance special" [class.item-clearance]="itemClearance">Add another class</div>

Finally, you 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 falsy.

<h3>toggle the "special" class on/off with a property:</h3>
<div [class.special]="isSpecial">The class binding is special.</div>

<h3>binding to class.special overrides the class attribute:</h3>
<div class="special" [class.special]="!isSpecial">This one is not so special.</div>

<h3>Using the bind- syntax:</h3>
<div bind-class.special="isSpecial">This class binding is special too.</div>

While this technique is suitable for toggling a single class name, consider the NgClass directive when managing multiple class names at the same time.

Style binding

You can set inline styles with a style binding.

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

<button [style.color]="isSpecial ? 'red': 'green'">Red</button>
<button [style.background-color]="canSave ? 'cyan': 'grey'" >Save</button>

Some style binding styles have a unit extension. The following example conditionally sets the font size in “em” and “%” units.

<button [style.font-size.em]="isSpecial ? 3 : 1" >Big</button>
<button [style.font-size.%]="!isSpecial ? 150 : 50" >Small</button>

This technique is suitable for setting a single style, but consider the NgStyle directive when setting several inline styles at the same time.

Note that a style property name can be written in either dash-case, as shown above, or camelCase, such as fontSize.

Event binding (event)

Event binding allows you to listen for certain events such as keystrokes, mouse movements, clicks, and touches. For an example demonstrating all of the points in this section, see the event binding example.

Angular event binding syntax consists of a target event name within parentheses on the left of an equal sign, and a quoted template statement on the right. The following event binding listens for the button's click events, calling the component's onSave() method whenever a click occurs:

Target event

As above, the target is the button's click event.

<button (click)="onSave($event)">Save</button>

Alternatively, use the on- prefix, known as the canonical form:

<button on-click="onSave($event)">on-click Save</button>

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:

<h4>myClick is an event on the custom ClickDirective:</h4>
<button (myClick)="clickMessage=$event" clickable>click with myClick</button>
{{clickMessage}}

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. This information can include data values such as an event object, string, or number named $event.

The target event determines the shape of the $event object. If the target event is a native DOM element event, then $event is a DOM event object, with properties such as target and target.value.

Consider this example:

<input [value]="currentItem.name"
       (input)="currentItem.name=$event.target.value" >
without NgModel

This code sets the <input> value property by binding to the name property. To listen for changes to the value, the code binds to the input event of the <input> element. 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 name 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 produces.

Custom events with EventEmitter

Directives typically raise custom events with an Angular EventEmitter. 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 an ItemDetailComponent that presents item information and responds to user actions. Although the ItemDetailComponent has a delete button, it doesn't know how to delete the hero. It can only raise an event reporting the user's delete request.

Here are the pertinent excerpts from that ItemDetailComponent:

<img src="{{itemImageUrl}}" [style.display]="displayNone">
<span [style.text-decoration]="lineThrough">{{ item.name }}
</span>
<button (click)="delete()">Delete</button>
// This component makes a request but it can't actually delete a hero.
@Output() deleteRequest = new EventEmitter<Item>();

delete() {
  this.deleteRequest.emit(this.item);
  this.displayNone = this.displayNone ? '' : 'none';
  this.lineThrough = this.lineThrough ? '' : 'line-through';
}

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 an Item object.

Now imagine a hosting parent component that binds to the deleteRequest event of the ItemDetailComponent.

<app-item-detail (deleteRequest)="deleteItem($event)" [item]="currentItem"></app-item-detail>

When the deleteRequest event fires, Angular calls the parent component's deleteItem() method, passing the item-to-delete (emitted by ItemDetail) in the $event variable.

Template statements have side effects

Though template expressions shouldn't have side effects, template statements usually do. The deleteItem() method does have a side effect: it deletes an item.

Deleting an item updates the model, and depending on your code, triggers other changes including queries and saving to a remote server. These changes propagate through the system and ultimately display in this and other views.

Two-way binding [(...)]

Two-way binding gives your app a way to share data between a component class and its template.

For a demonstration of the syntax and code snippets in this section, see the two-way binding example.

Basics of two-way binding

Two-way binding does two things:

  1. Sets a specific element property.
  2. Listens for an element change event.

Angular offers a special two-way data binding syntax for this purpose, [()]. The [()] syntax combines the brackets of property binding, [], with the parentheses of event binding, ().

[( )] = banana in a box

Visualize a banana in a box to remember that the parentheses go inside the brackets.

The [()] 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 this pattern. It has a size value property and a companion sizeChange event:

import { Component, Input, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'app-sizer',
  templateUrl: './sizer.component.html',
  styleUrls: ['./sizer.component.css']
})
export class SizerComponent {


  @Input()  size: number | string;
  @Output() sizeChange = new EventEmitter<number>();

  dec() { this.resize(-1); }
  inc() { this.resize(+1); }

  resize(delta: number) {
    this.size = Math.min(40, Math.max(8, +this.size + delta));
    this.sizeChange.emit(this.size);
  }

}

The initial size is an input value from a property binding. Clicking the buttons increases or decreases the size, within min/max value constraints, and then raises, or emits, the sizeChange event with the adjusted size.

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

<app-sizer [(size)]="fontSizePx"></app-sizer>
<div [style.font-size.px]="fontSizePx">Resizable Text</div>

The AppComponent.fontSizePx establishes the initial SizerComponent.size value.

fontSizePx = 16;

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.

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:

<app-sizer [size]="fontSizePx" (sizeChange)="fontSizePx=$event"></app-sizer>

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.

Two-way binding in forms

The two-way binding syntax is a great convenience compared to separate property and event bindings. It would be convenient to use two-way binding with HTML form elements like <input> and <select>. However, no native HTML element follows the x value and xChange event pattern.

For more on how to use two-way binding in forms, see Angular NgModel.

Built-in directives

Angular offers two kinds of built-in directives: attribute directives and structural directives. This segment reviews some of the most common built-in directives, classified as either attribute directives or structural directives and has its own built-in directives example.

For more detail, including how to build your own custom directives, see Attribute Directives and Structural Directives.

Built-in attribute directives

Attribute directives listen to and modify the behavior of other HTML elements, attributes, properties, and components. You usually apply them to elements as if they were HTML attributes, hence the name.

Many NgModules such as the RouterModule and the FormsModule define their own attribute directives. The most common attribute directives are as follows:

  • NgClass—adds and removes a set of CSS classes.
  • NgStyle—adds and removes a set of HTML styles.
  • NgModel—adds two-way data binding to an HTML form element.

NgClass

Add or remove several CSS classes simultaneously with ngClass.

<!-- toggle the "special" class on/off with a property -->
<div [ngClass]="isSpecial ? 'special' : ''">This div is special</div>

To add or remove a single class, use class binding rather than NgClass.

Consider a setCurrentClasses() component method that sets a component property, currentClasses, with an object that adds or removes three classes based on the true/false state of three other component properties. 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.

currentClasses: {};
setCurrentClasses() {
  // CSS classes: added/removed per current state of component properties
  this.currentClasses =  {
    'saveable': this.canSave,
    'modified': !this.isUnchanged,
    'special':  this.isSpecial
  };
}

Adding an ngClass property binding to currentClasses sets the element's classes accordingly:

<div [ngClass]="currentClasses">This div is initially saveable, unchanged, and special.</div>

Remember that in this situation you'd call setCurrentClasses(), both initially and when the dependent properties change.

NgStyle

Use NgStyle to set many inline styles simultaneously and dynamically, based on the state of the component.

Without NgStyle

For context, consider setting a single style value with style binding, without NgStyle.

<div [style.font-size]="isSpecial ? 'x-large' : 'smaller'">
  This div is x-large or smaller.
</div>

However, to set many inline styles at the same time, use the NgStyle directive.

The following is a setCurrentStyles() method that sets a component property, currentStyles, with an object that defines three styles, based on the state of three other component properties:

currentStyles: {};
setCurrentStyles() {
  // CSS styles: set per current state of component properties
  this.currentStyles = {
    'font-style':  this.canSave      ? 'italic' : 'normal',
    'font-weight': !this.isUnchanged ? 'bold'   : 'normal',
    'font-size':   this.isSpecial    ? '24px'   : '12px'
  };
}

Adding an ngStyle property binding to currentStyles sets the element's styles accordingly:

<div [ngStyle]="currentStyles">
  This div is initially italic, normal weight, and extra large (24px).
</div>

Remember to call setCurrentStyles(), both initially and when the dependent properties change.

[(ngModel)]: Two-way binding

The NgModel directive allows you to display a data property and update that property when the user makes changes. Here's an example:

<label for="example-ngModel">[(ngModel)]:</label>
<input [(ngModel)]="currentItem.name" id="example-ngModel">

Import FormsModule to use ngModel

Before using the ngModel directive in a two-way data binding, you must import the FormsModule and add it to the NgModule's imports list. Learn more about the FormsModule and ngModel in Forms.

Remember to import the FormsModule to make [(ngModel)] available as follows:

import { FormsModule } from '@angular/forms'; // <--- JavaScript import from Angular
/* . . . */
@NgModule({
/* . . . */

  imports: [
    BrowserModule,
    FormsModule // <--- import into the NgModule
  ],
/* . . . */
})
export class AppModule { }

You could achieve the same result with separate bindings to the <input> element's value property and input event:

<label for="without">without NgModel:</label>
<input [value]="currentItem.name" (input)="currentItem.name=$event.target.value" id="without">

To streamline the syntax, the ngModel directive hides the details behind its own ngModel input and ngModelChange output properties:

<label for="example-change">(ngModelChange)="...name=$event":</label>
<input [ngModel]="currentItem.name" (ngModelChange)="currentItem.name=$event" id="example-change">

The ngModel data property sets the element's value property and the ngModelChange event property listens for changes to the element's value.

NgModel and value accessors

The details are specific to each kind of element and therefore the NgModel directive only works for an element supported by a ControlValueAccessor that adapts an element to this protocol. Angular provides value accessors for all of the basic HTML form elements and the Forms guide shows how to bind to them.

You can't apply [(ngModel)] to a non-form native element or a third-party custom component until you write a suitable value accessor. For more information, see the API documentation on DefaultValueAccessor.

You don't need a value accessor for an Angular component that you write because you can name the value and event properties to suit Angular's basic two-way binding syntax and skip NgModel altogether. The sizer in the Two-way Binding section is an example of this technique.

Separate ngModel bindings are an improvement over binding to the element's native properties, but you can streamline the binding with a single declaration using the [(ngModel)] syntax:

<label for="example-ngModel">[(ngModel)]:</label>
<input [(ngModel)]="currentItem.name" id="example-ngModel">

This [(ngModel)] syntax can only set a data-bound property. If you need to do something more, you can write the expanded form; for example, the following changes the <input> value to uppercase:

<input [ngModel]="currentItem.name" (ngModelChange)="setUppercaseName($event)" id="example-uppercase">

Here are all variations in action, including the uppercase version:

Built-in structural directives

Structural directives are responsible for HTML layout. They shape or reshape the DOM's structure, typically by adding, removing, and manipulating the host elements to which they are attached.

This section is an introduction to the common built-in structural directives:

  • NgIf—conditionally creates or destroys subviews from the template.
  • NgFor—repeat a node for each item in a list.
  • NgSwitch—a set of directives that switch among alternative views.

The deep details of structural directives are covered in the Structural Directives guide, which explains the following:

NgIf

You can add or remove an element from the DOM by applying an NgIf directive to a host element. Bind the directive to a condition expression like isActive in this example.

<app-item-detail *ngIf="isActive" [item]="item"></app-item-detail>

Don't forget the asterisk (*) in front of ngIf. For more information on the asterisk, see the asterisk (*) prefix section of Structural Directives.

When the isActive expression returns a truthy value, NgIf adds the ItemDetailComponent to the DOM. When the expression is falsy, NgIf removes the ItemDetailComponent from the DOM, destroying that component and all of its sub-components.

Show/hide vs. NgIf

Hiding an element is different from removing it with NgIf. For comparison, the following example shows how to control the visibility of an element with a class or style binding.

<!-- isSpecial is true -->
<div [class.hidden]="!isSpecial">Show with class</div>
<div [class.hidden]="isSpecial">Hide with class</div>

<p>ItemDetail is in the DOM but hidden</p>
<app-item-detail [class.hidden]="isSpecial"></app-item-detail>

<div [style.display]="isSpecial ? 'block' : 'none'">Show with style</div>
<div [style.display]="isSpecial ? 'none'  : 'block'">Hide with style</div>

When you hide an element, that element and all of its descendants remain in the DOM. All components for those elements stay in memory and Angular may continue to check for changes. You could be holding onto considerable computing resources and degrading performance unnecessarily.

NgIf works differently. When NgIf is false, Angular removes the element and its descendants from the DOM. It destroys their components, freeing up resources, which results in a better user experience.

If you are hiding large component trees, consider NgIf as a more efficient alternative to showing/hiding.

For more information on NgIf and ngIfElse, see the API documentation about NgIf.

Guard against null

Another advantage of ngIf is that you can use it to guard against null. Show/hide is best suited for very simple use cases, so when you need a guard, opt instead for ngIf. Angular will throw an error if a nested expression tries to access a property of null.

The following shows NgIf guarding two <div>s. The currentCustomer name appears only when there is a currentCustomer. The nullCustomer will not be displayed as long as it is null.

<div *ngIf="currentCustomer">Hello, {{currentCustomer.name}}</div>
<div *ngIf="nullCustomer">Hello, <span>{{nullCustomer}}</span></div>

See also the safe navigation operator below.

NgFor

NgFor is a repeater directive—a way to present a list of items. You define a block of HTML that defines how a single item should be displayed and then you tell Angular to use that block as a template for rendering each item in the list. The text assigned to *ngFor is the instruction that guides the repeater process.

The following example shows NgFor applied to a simple <div>. (Don't forget the asterisk (*) in front of ngFor.)

<div *ngFor="let item of items">{{item.name}}</div>

You can also apply an NgFor to a component element, as in the following example.

<app-item-detail *ngFor="let item of items" [item]="item"></app-item-detail>
*ngFor microsyntax

The string assigned to *ngFor is not a template expression. Rather, it's a microsyntax—a little language of its own that Angular interprets. The string "let item of items" means:

Take each item in the items array, store it in the local item looping variable, and make it available to the templated HTML for each iteration.

Angular translates this instruction into an <ng-template> around the host element, then uses this template repeatedly to create a new set of elements and bindings for each item in the list. For more information about microsyntax, see the Structural Directives guide.

Template input variables

The let keyword before item creates a template input variable called item. The ngFor directive iterates over the items array returned by the parent component's items property and sets item to the current item from the array during each iteration.

Reference item within the ngFor host element as well as within its descendants to access the item's properties. The following example references item first in an interpolation and then passes in a binding to the item property of the <app-item-detail> component.

<div *ngFor="let item of items">{{item.name}}</div>
<!-- . . . -->
  <app-item-detail *ngFor="let item of items" [item]="item"></app-item-detail>

For more information about template input variables, see Structural Directives.

*ngFor with index

The index property of the NgFor directive context returns the zero-based index of the item in each iteration. You can capture the index in a template input variable and use it in the template.

The next example captures the index in a variable named i and displays it with the item name.

<div *ngFor="let item of items; let i=index">{{i + 1}} - {{item.name}}</div>

NgFor is implemented by the NgForOf directive. Read more about the other NgForOf context values such as last, even, and odd in the NgForOf API reference.

*ngFor with trackBy

If you use NgFor with large lists, a small change to one item, such as removing or adding an item, can trigger a cascade of DOM manipulations. For example, re-querying the server could reset a list with all new item objects, even when those items were previously displayed. In this case, Angular sees only a fresh list of new object references and has no choice but to replace the old DOM elements with all new DOM elements.

You can make this more efficient with trackBy. Add a method to the component that returns the value NgFor should track. In this case, that value is the hero's id. If the id has already been rendered, Angular keeps track of it and doesn't re-query the server for the same id.

trackByItems(index: number, item: Item): number { return item.id; }

In the microsyntax expression, set trackBy to the trackByItems() method.

<div *ngFor="let item of items; trackBy: trackByItems">
  ({{item.id}}) {{item.name}}
</div>

Here is an illustration of the trackBy effect. "Reset items" creates new items with the same item.ids. "Change ids" creates new items with new item.ids.

  • With no trackBy, both buttons trigger complete DOM element replacement.
  • With trackBy, only changing the id triggers element replacement.

Built-in directives use only public APIs; that is, they do not have special access to any private APIs that other directives can't access.

The NgSwitch directives

NgSwitch is like the JavaScript switch statement. It displays one element from among several possible elements, based on a switch condition. Angular puts only the selected element into the DOM.

NgSwitch is actually a set of three, cooperating directives: NgSwitch, NgSwitchCase, and NgSwitchDefault as in the following example.

<div [ngSwitch]="currentItem.feature">
  <app-stout-item    *ngSwitchCase="'stout'"    [item]="currentItem"></app-stout-item>
  <app-device-item   *ngSwitchCase="'slim'"     [item]="currentItem"></app-device-item>
  <app-lost-item     *ngSwitchCase="'vintage'"  [item]="currentItem"></app-lost-item>
  <app-best-item     *ngSwitchCase="'bright'"   [item]="currentItem"></app-best-item>
<!-- . . . -->
  <app-unknown-item  *ngSwitchDefault           [item]="currentItem"></app-unknown-item>
</div>

NgSwitch is the controller directive. Bind it to an expression that returns the switch value, such as feature. Though the feature value in this example is a string, the switch value can be of any type.

Bind to [ngSwitch]. You'll get an error if you try to set *ngSwitch because NgSwitch is an attribute directive, not a structural directive. Rather than touching the DOM directly, it changes the behavior of its companion directives.

Bind to *ngSwitchCase and *ngSwitchDefault. The NgSwitchCase and NgSwitchDefault directives are structural directives because they add or remove elements from the DOM.

  • NgSwitchCase adds its element to the DOM when its bound value equals the switch value and removes its bound value when it doesn't equal the switch value.

  • NgSwitchDefault adds its element to the DOM when there is no selected NgSwitchCase.

The switch directives are particularly useful for adding and removing component elements. This example switches among four item components defined in the item-switch.components.ts file. Each component has an item input property which is bound to the currentItem of the parent component.

Switch directives work as well with native elements and web components too. For example, you could replace the <app-best-item> switch case with the following.

<div *ngSwitchCase="'bright'"> Are you as bright as {{currentItem.name}}?</div>

Template reference variables (#var)

A template reference variable is often a reference to a DOM element within a template. It can also refer to a directive (which contains a component), an element, TemplateRef, or a web component.

For a demonstration of the syntax and code snippets in this section, see the template reference variables example.

Use the hash symbol (#) to declare a reference variable. The following reference variable, #phone, declares a phone variable on an <input> element.

<input #phone placeholder="phone number" />

You can refer to a template reference variable anywhere in the component's template. Here, a <button> further down the template refers to the phone variable.

<input #phone placeholder="phone number" />

<!-- lots of other elements -->

<!-- phone refers to the input element; pass its `value` to an event handler -->
<button (click)="callPhone(phone.value)">Call</button>

How a reference variable gets its value

In most cases, Angular sets the reference variable's value to the element on which it is declared. In the previous example, phone refers to the phone number <input>. The button's click handler passes the <input> value to the component's callPhone() method.

The NgForm directive can change that behavior and set the value to something else. In the following example, the template reference variable, itemForm, appears three times separated by HTML.

<form #itemForm="ngForm" (ngSubmit)="onSubmit(itemForm)">
  <label for="name"
    >Name <input class="form-control" name="name" ngModel required />
  </label>
  <button type="submit">Submit</button>
</form>

<div [hidden]="!itemForm.form.valid">
  <p>{{ submitMessage }}</p>
</div>

The reference value of itemForm, without the ngForm attribute value, would be the HTMLFormElement. There is, however, a difference between a Component and a Directive in that a Component will be referenced without specifying the attribute value, and a Directive will not change the implicit reference (that is, the element).

However, with NgForm, itemForm is a reference to the NgForm directive with the ability to track the value and validity of every control in the form.

The native <form> element doesn't have a form property, but the NgForm directive does, which allows disabling the submit button if the itemForm.form.valid is invalid and passing the entire form control tree to the parent component's onSubmit() method.

Template reference variable considerations

A template reference variable (#phone) is not the same as a template input variable (let phone) such as in an *ngFor. See Structural Directives for more information.

The scope of a reference variable is the entire template. So, don't define the same variable name more than once in the same template as the runtime value will be unpredictable.

Alternative syntax

You can use the ref- prefix alternative to #. This example declares the fax variable as ref-fax instead of #fax.

<input ref-fax placeholder="fax number" />
<button (click)="callFax(fax.value)">Fax</button>

@Input() and @Output() properties

@Input() and @Output() allow Angular to share data between the parent context and child directives or components. An @Input() property is writable while an @Output() property is observable.

Consider this example of a child/parent relationship:

<parent-component>
  <child-component></child-component>
</parent-component>

Here, the <child-component> selector, or child directive, is embedded within a <parent-component>, which serves as the child's context.

@Input() and @Output() act as the API, or application programming interface, of the child component in that they allow the child to communicate with the parent. Think of @Input() and @Output() like ports or doorways—@Input() is the doorway into the component allowing data to flow in while @Output() is the doorway out of the component, allowing the child component to send data out.

This section about @Input() and @Output() has its own . The following subsections highlight key points in the sample app.

@Input() and @Output() are independent

Though @Input() and @Output() often appear together in apps, you can use them separately. If the nested component is such that it only needs to send data to its parent, you wouldn't need an @Input(), only an @Output(). The reverse is also true in that if the child only needs to receive data from the parent, you'd only need @Input().

How to use @Input()

Use the @Input() decorator in a child component or directive to let Angular know that a property in that component can receive its value from its parent component. It helps to remember that the data flow is from the perspective of the child component. So an @Input() allows data to be input into the child component from the parent component.

To illustrate the use of @Input(), edit these parts of your app:

  • The child component class and template
  • The parent component class and template

In the child

To use the @Input() decorator in a child component class, first import Input and then decorate the property with @Input():

import { Component, Input } from '@angular/core'; // First, import Input
export class ItemDetailComponent {
  @Input() item: string; // decorate the property with @Input()
}

In this case, @Input() decorates the property item, which has a type of string, however, @Input() properties can have any type, such as number, string, boolean, or object. The value for item will come from the parent component, which the next section covers.

Next, in the child component template, add the following:

<p>
  Today's item: {{item}}
</p>

In the parent

The next step is to bind the property in the parent component's template. In this example, the parent component template is app.component.html.

First, use the child's selector, here <app-item-detail>, as a directive within the parent component template. Then, use property binding to bind the property in the child to the property of the parent.

<app-item-detail [item]="currentItem"></app-item-detail>

Next, in the parent component class, app.component.ts, designate a value for currentItem:

export class AppComponent {
  currentItem = 'Television';
}

With @Input(), Angular passes the value for currentItem to the child so that item renders as Television.

The following diagram shows this structure:

The target in the square brackets, [], is the property you decorate with @Input() in the child component. The binding source, the part to the right of the equal sign, is the data that the parent component passes to the nested component.

The key takeaway is that when binding to a child component's property in a parent component—that is, what's in square brackets—you must decorate the property with @Input() in the child component.

OnChanges and @Input()

To watch for changes on an @Input() property, use OnChanges, one of Angular's lifecycle hooks. OnChanges is specifically designed to work with properties that have the @Input() decorator. See the OnChanges section of the Lifecycle Hooks guide for more details and examples.

How to use @Output()

Use the @Output() decorator in the child component or directive to allow data to flow from the child out to the parent.

An @Output() property should normally be initialized to an Angular EventEmitter with values flowing out of the component as events.

Just like with @Input(), you can use @Output() on a property of the child component but its type should be EventEmitter.

@Output() marks a property in a child component as a doorway through which data can travel from the child to the parent. The child component then has to raise an event so the parent knows something has changed. To raise an event, @Output() works hand in hand with EventEmitter, which is a class in @angular/core that you use to emit custom events.

When you use @Output(), edit these parts of your app:

  • The child component class and template
  • The parent component class and template

The following example shows how to set up an @Output() in a child component that pushes data you enter in an HTML <input> to an array in the parent component.

The HTML element <input> and the Angular decorator @Input() are different. This documentation is about component communication in Angular as it pertains to @Input() and @Output(). For more information on the HTML element <input>, see the W3C Recommendation.

In the child

This example features an <input> where a user can enter a value and click a <button> that raises an event. The EventEmitter then relays the data to the parent component.

First, be sure to import Output and EventEmitter in the child component class:

import { Output, EventEmitter } from '@angular/core';

Next, still in the child, decorate a property with @Output() in the component class. The following example @Output() is called newItemEvent and its type is EventEmitter, which means it's an event.

@Output() newItemEvent = new EventEmitter<string>();

The different parts of the above declaration are as follows:

  • @Output()—a decorator function marking the property as a way for data to go from the child to the parent
  • newItemEvent—the name of the @Output()
  • EventEmitter<string>—the @Output()'s type
  • new EventEmitter<string>()—tells Angular to create a new event emitter and that the data it emits is of type string. The type could be any type, such as number, boolean, and so on. For more information on EventEmitter, see the EventEmitter API documentation.

Next, create an addNewItem() method in the same component class:

export class ItemOutputComponent {

  @Output() newItemEvent = new EventEmitter<string>();

  addNewItem(value: string) {
    this.newItemEvent.emit(value);
  }
}

The addNewItem() function uses the @Output(), newItemEvent, to raise an event in which it emits the value the user types into the <input>. In other words, when the user clicks the add button in the UI, the child lets the parent know about the event and gives that data to the parent.

In the child's template

The child's template has two controls. The first is an HTML <input> with a template reference variable , #newItem, where the user types in an item name. Whatever the user types into the <input> gets stored in the #newItem variable.

<label>Add an item: <input #newItem></label>
<button (click)="addNewItem(newItem.value)">Add to parent's list</button>

The second element is a <button> with an event binding. You know it's an event binding because the part to the left of the equal sign is in parentheses, (click).

The (click) event is bound to the addNewItem() method in the child component class which takes as its argument whatever the value of #newItem is.

Now the child component has an @Output() for sending data to the parent and a method for raising an event. The next step is in the parent.

In the parent

In this example, the parent component is AppComponent, but you could use any component in which you could nest the child.

The AppComponent in this example features a list of items in an array and a method for adding more items to the array.

export class AppComponent {
  items = ['item1', 'item2', 'item3', 'item4'];

  addItem(newItem: string) {
    this.items.push(newItem);
  }
}

The addItem() method takes an argument in the form of a string and then pushes, or adds, that string to the items array.

In the parent's template

Next, in the parent's template, bind the parent's method to the child's event. Put the child selector, here <app-item-output>, within the parent component's template, app.component.html.

<app-item-output (newItemEvent)="addItem($event)"></app-item-output>

The event binding, (newItemEvent)='addItem($event)', tells Angular to connect the event in the child, newItemEvent, to the method in the parent, addItem(), and that the event that the child is notifying the parent about is to be the argument of addItem(). In other words, this is where the actual hand off of data takes place. The $event contains the data that the user types into the <input> in the child template UI.

Now, in order to see the @Output() working, add the following to the parent's template:

<ul>
    <li *ngFor="let item of items">{{item}}</li>
  </ul>

The *ngFor iterates over the items in the items array. When you enter a value in the child's <input> and click the button, the child emits the event and the parent's addItem() method pushes the value to the items array and it renders in the list.

@Input() and @Output() together

You can use @Input() and @Output() on the same child component as in the following:

<app-input-output [item]="currentItem" (deleteRequest)="crossOffItem($event)"></app-input-output>

The target, item, which is an @Input() property in the child component class, receives its value from the parent's property, currentItem. When you click delete, the child component raises an event, deleteRequest, which is the argument for the parent's crossOffItem() method.

The following diagram is of an @Input() and an @Output() on the same child component and shows the different parts of each:

As the diagram shows, use inputs and outputs together in the same manner as using them separately. Here, the child selector is <app-input-output> with item and deleteRequest being @Input() and @Output() properties in the child component class. The property currentItem and the method crossOffItem() are both in the parent component class.

To combine property and event bindings using the banana-in-a-box syntax, [()], see Two-way Binding.

For more detail on how these work, see the previous sections on Input and Output. To see it in action, see the Inputs and Outputs Example.

@Input() and @Output() declarations

Instead of using the @Input() and @Output() decorators to declare inputs and outputs, you can identify members in the inputs and outputs arrays of the directive metadata, as in this example:

// tslint:disable: no-inputs-metadata-property no-outputs-metadata-property
inputs: ['clearanceItem'],
outputs: ['buyEvent']
// tslint:enable: no-inputs-metadata-property no-outputs-metadata-property

While declaring inputs and outputs in the @Directive and @Component metadata is possible, it is a better practice to use the @Input() and @Output() class decorators instead, as follows:

@Input() item: string;
@Output() deleteRequest = new EventEmitter<string>();

See the Decorate input and output properties section of the Style Guide for details.

If you get a template parse error when trying to use inputs or outputs, but you know that the properties do indeed exist, double check that your properties are annotated with @Input() / @Output() or that you've declared them in an inputs/outputs array:

Uncaught Error: Template parse errors:
Can't bind to 'item' since it isn't a known property of 'app-item-detail'

Aliasing inputs and outputs

Sometimes the public name of an input/output property should be different from the internal name. While it is a best practice to avoid this situation, Angular does offer a solution.

Aliasing in the metadata

Alias inputs and outputs in the metadata using a colon-delimited (:) string with the directive property name on the left and the public alias on the right:

// tslint:disable: no-inputs-metadata-property no-outputs-metadata-property
inputs: ['input1: saveForLaterItem'], // propertyName:alias
outputs: ['outputEvent1: saveForLaterEvent']
// tslint:disable: no-inputs-metadata-property no-outputs-metadata-property

Aliasing with the @Input()/@Output() decorator

You can specify the alias for the property name by passing the alias name to the @Input()/@Output() decorator. The internal name remains as usual.

@Input('wishListItem') input2: string; //  @Input(alias)
@Output('wishEvent') outputEvent2 = new EventEmitter<string>(); //  @Output(alias) propertyName = ...

Template expression operators

The Angular template expression language employs a subset of JavaScript syntax supplemented with a few special operators for specific scenarios. The next sections cover three of these operators:

The pipe operator (|)

The result of an expression might require some transformation before you're ready to use it in a binding. For example, you might display a number as a currency, change text to uppercase, or filter a list and sort it.

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 (|):

<p>Title through uppercase pipe: {{title | uppercase}}</p>

The pipe operator passes the result of an expression on the left to a pipe function on the right.

You can chain expressions through multiple pipes:

<!-- convert title to uppercase, then to lowercase -->
<p>Title through a pipe chain: {{title | uppercase | lowercase}}</p>

And you can also apply parameters to a pipe:

<!-- pipe with configuration argument => "February 25, 1980" -->
<p>Manufacture date with date format pipe: {{item.manufactureDate | date:'longDate'}}</p>

The json pipe is particularly helpful for debugging bindings:

<p>Item json pipe: {{item | json}}</p>

The generated output would look something like this:

{ "name": "Telephone",
    "manufactureDate": "1980-02-25T05:00:00.000Z",
    "price": 98 }

The pipe operator has a higher precedence than the ternary operator (?:), which means a ? b : c | x is parsed as a ? b : (c | x). Nevertheless, for a number of reasons, the pipe operator cannot be used without parentheses in the first and second operands of ?:. A good practice is to use parentheses in the third operand too.

The safe navigation operator ( ? ) and null property paths

The Angular safe navigation operator, ?, guards against null and undefined values in property paths. Here, it protects against a view render failure if item is null.

<p>The item name is: {{item?.name}}</p>

If item is null, the view still renders but the displayed value is blank; you see only "The item name is:" with nothing after it.

Consider the next example, with a nullItem.

The null item name is {{nullItem.name}}

Since there is no safe navigation operator and nullItem is null, JavaScript and Angular would throw a null reference error and break the rendering process of Angular:

TypeError: Cannot read property 'name' of null.

Sometimes however, null values in the property path may be OK under certain circumstances, especially when the value starts out null but the data arrives eventually.

With the safe navigation operator, ?, Angular stops evaluating the expression when it hits the first null value and renders the view without errors.

It works perfectly with long property paths such as a?.b?.c?.d.

The non-null assertion operator ( ! )

As of Typescript 2.0, you can enforce strict null checking with the --strictNullChecks flag. TypeScript then ensures that no variable is unintentionally null or undefined.

In this mode, typed variables disallow null and undefined by default. The type checker throws an error if you leave a variable unassigned or try to assign null or undefined to a variable whose type disallows null and undefined.

The type checker also throws an error if it can't determine whether a variable will be null or undefined at runtime. You tell the type checker not to throw an error by applying the postfix non-null assertion operator, !.

The Angular non-null assertion operator, !, serves the same purpose in an Angular template. For example, after you use *ngIf to check that item is defined, you can assert that item properties are also defined.

<!--No color, no error -->
<p *ngIf="item">The item's color is: {{item!.color}}</p>

When the Angular compiler turns your template into TypeScript code, it prevents TypeScript from reporting that item might be null or undefined.

Unlike the safe navigation operator, the non-null assertion operator does not guard against null or undefined. Rather, it tells the TypeScript type checker to suspend strict null checks for a specific property expression.

The non-null assertion operator, !, is optional with the exception that you must use it when you turn on strict null checks.

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Built-in template functions

The $any() type cast function

Sometimes a binding expression triggers a type error during AOT compilation and it is not possible or difficult to fully specify the type. To silence the error, you can use the $any() cast function to cast the expression to the any type as in the following example:

<p>The item's undeclared best by date is: {{$any(item).bestByDate}}</p>

When the Angular compiler turns this template into TypeScript code, it prevents TypeScript from reporting that bestByDate is not a member of the item object when it runs type checking on the template.

The $any() cast function also works with this to allow access to undeclared members of the component.

<p>The item's undeclared best by date is: {{$any(this).bestByDate}}</p>

The $any() cast function works anywhere in a binding expression where a method call is valid.

SVG in templates

It is possible to use SVG as valid templates in Angular. All of the template syntax below is applicable to both SVG and HTML. Learn more in the SVG 1.1 and 2.0 specifications.

Why would you use SVG as template, instead of simply adding it as image to your application?

When you use an SVG as the template, you are able to use directives and bindings just like with HTML templates. This means that you will be able to dynamically generate interactive graphics.

Refer to the sample code snippet below for a syntax example:

import { Component } from '@angular/core';

@Component({
  selector: 'app-svg',
  templateUrl: './svg.component.svg',
  styleUrls: ['./svg.component.css']
})
export class SvgComponent {
  fillColor = 'rgb(255, 0, 0)';

  changeColor() {
    const r = Math.floor(Math.random() * 256);
    const g = Math.floor(Math.random() * 256);
    const b = Math.floor(Math.random() * 256);
    this.fillColor = `rgb(${r}, ${g}, ${b})`;
  }
}

Add the following code to your svg.component.svg file:

<svg>
  <g>
    <rect x="0" y="0" width="100" height="100" [attr.fill]="fillColor" (click)="changeColor()" />
    <text x="120" y="50">click the rectangle to change the fill color</text>
  </g>
</svg>

Here you can see the use of a click() event binding and the property binding syntax ([attr.fill]="fillColor").

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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/guide/template-syntax