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Angular - Define animations and attach to an HTML template

Define animations and attach to an HTML template

Contents

Define the animation
Attach an animation to an HTML template

Animations are defined in the metadata of the component that controls the HTML element to be animated.

Define the animation

Put the code that defines your animations under the animations: property within the @Component() decorator.

src/app/open-close.component.ts

      
      @Component({
  selector: 'app-open-close',
  animations: [
    trigger('openClose', [
      // ...
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.8,
        backgroundColor: 'blue'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'open-close.component.html',
  styleUrls: ['open-close.component.css']
})
export class OpenCloseComponent {
  isOpen = true;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}

When an animation trigger for a component is defined, attach it to an element in the template. Wrap the trigger name in brackets and precede it with an @ symbol. Bind the trigger to a template expression using standard Angular property binding syntax. The triggerName is the name of the trigger, and expression evaluates to a defined animation state.

      
      <div [@triggerName]="expression">…</div>;

The animation is executed or triggered when the expression value changes to a new state.

The following code snippet binds the trigger to the value of the isOpen property.

src/app/open-close.component.html

      
      <nav>
  <button type="button" (click)="toggle()">Toggle Open/Close</button>
</nav>

<div [@openClose]="isOpen ? 'open' : 'closed'" class="open-close-container">
  <p>The box is now {{ isOpen ? 'Open' : 'Closed' }}!</p>
</div>

In this example, when the isOpen expression evaluates to a defined state of open or closed , it notifies the trigger openClose of a state change. Then it's up to the openClose code to handle the state change and kick off a state change animation.

For elements entering or leaving a page (inserted or removed from the DOM), you can make the animations conditional. For example, use *ngIf with the animation trigger in the HTML template.

NOTE :
In the component file, set the trigger that defines the animations as the value of the animations: property in the @Component() decorator.

Attach an animation to an HTML template

In the HTML template file, use the trigger name to attach the defined animations to the HTML element to be animated.

Last reviewed on Fri Oct 28 2022

Angular - Introduction to Angular animations

Introduction to Angular animations

About this guide
Prerequisites
Getting started
Step 1: Enabling the animations module
Step 2: Importing animation functions into component files
Step 3: Adding the animation metadata property
Animating a transition
Animation state and styles
Transitions and timing
Triggering the animation
Defining animations and attaching them to the HTML template
Code review
Summary
Animations API summary
More on Angular animations

Animation provides the illusion of motion: HTML elements change styling over time. Well-designed animations can make your application more fun and straightforward to use, but they aren't just cosmetic. Animations can improve your application and user experience in a number of ways:

Without animations, web page transitions can seem abrupt and jarring
Motion greatly enhances the user experience, so animations give users a chance to detect the application's response to their actions
Good animations intuitively call the user's attention to where it is needed

Typically, animations involve multiple style transformations over time. An HTML element can move, change color, grow or shrink, fade, or slide off the page. These changes can occur simultaneously or sequentially. You can control the timing of each transformation.

Angular's animation system is built on CSS functionality, which means you can animate any property that the browser considers animatable. This includes positions, sizes, transforms, colors, borders, and more. The W3C maintains a list of animatable properties on its CSS Transitions page.

About this guide

This guide covers the basic Angular animation features to get you started on adding Angular animations to your project.

The features described in this guide —and the more advanced features described in the related Angular animations guides— are demonstrated in an example application available as a live example / download example .

Prerequisites

The guide assumes that you're familiar with building basic Angular apps, as described in the following sections:

Tutorial
Architecture Overview

Getting started

The main Angular modules for animations are @angular/animations and @angular/platform-browser . When you create a new project using the CLI, these dependencies are automatically added to your project.

To get started with adding Angular animations to your project, import the animation-specific modules along with standard Angular functionality.

Step 1: Enabling the animations module

Import BrowserAnimationsModule , which introduces the animation capabilities into your Angular root application module.

src/app/app.module.ts

      
      import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

@NgModule({
  imports: [
    BrowserModule,
    BrowserAnimationsModule
  ],
  declarations: [ ],
  bootstrap: [ ]
})
export class AppModule { }

NOTE :
When you use the CLI to create your app, the root application module app.module.ts is placed in the src/app folder.

Step 2: Importing animation functions into component files

If you plan to use specific animation functions in component files, import those functions from @angular/animations .

src/app/app.component.ts

      
      import { Component, HostBinding } from '@angular/core';
import {
  trigger,
  state,
  style,
  animate,
  transition,
  // ...
} from '@angular/animations';

NOTE :
See a summary of available animation functions at the end of this guide.

Step 3: Adding the animation metadata property

In the component file, add a metadata property called animations: within the @Component() decorator. You put the trigger that defines an animation within the animations metadata property.

src/app/app.component.ts

      
      @Component({
  selector: 'app-root',
  templateUrl: 'app.component.html',
  styleUrls: ['app.component.css'],
  animations: [
    // animation triggers go here
  ]
})

Animating a transition

Let's animate a transition that changes a single HTML element from one state to another. For example, you can specify that a button displays either Open or Closed based on the user's last action. When the button is in the open state, it's visible and yellow. When it's the closed state, it's translucent and blue.

In HTML, these attributes are set using ordinary CSS styles such as color and opacity. In Angular, use the style() function to specify a set of CSS styles for use with animations. Collect a set of styles in an animation state, and give the state a name, such as open or closed .

Let's create a new open-close component to animate with simple transitions.

Run the following command in terminal to generate the component:

      
      ng g component open-close

This will create the component at src/app/open-close.component.ts .

Animation state and styles

Use Angular's state() function to define different states to call at the end of each transition. This function takes two arguments: A unique name like open or closed and a style() function.

Use the style() function to define a set of styles to associate with a given state name. You must use camelCase for style attributes that contain dashes, such as backgroundColor or wrap them in quotes, such as 'background-color' .

Let's see how Angular's state() function works with the style⁣(⁠) function to set CSS style attributes. In this code snippet, multiple style attributes are set at the same time for the state. In the open state, the button has a height of 200 pixels, an opacity of 1, and a yellow background color.

src/app/open-close.component.ts

      
      // ...
state('open', style({
  height: '200px',
  opacity: 1,
  backgroundColor: 'yellow'
})),

In the following closed state, the button has a height of 100 pixels, an opacity of 0.8, and a background color of blue.

src/app/open-close.component.ts

      
      state('closed', style({
  height: '100px',
  opacity: 0.8,
  backgroundColor: 'blue'
})),

Transitions and timing

In Angular, you can set multiple styles without any animation. However, without further refinement, the button instantly transforms with no fade, no shrinkage, or other visible indicator that a change is occurring.

To make the change less abrupt, you need to define an animation transition to specify the changes that occur between one state and another over a period of time. The transition() function accepts two arguments: The first argument accepts an expression that defines the direction between two transition states, and the second argument accepts one or a series of animate() steps.

Use the animate() function to define the length, delay, and easing of a transition, and to designate the style function for defining styles while transitions are taking place. Use the animate() function to define the keyframes() function for multi-step animations. These definitions are placed in the second argument of the animate() function.

Animation metadata: duration, delay, and easing

The animate() function (second argument of the transition function) accepts the timings and styles input parameters.

The timings parameter takes either a number or a string defined in three parts.

      
      animate (duration)

      
      animate ('duration delay easing')

The first part, duration , is required. The duration can be expressed in milliseconds as a number without quotes, or in seconds with quotes and a time specifier. For example, a duration of a tenth of a second can be expressed as follows:

As a plain number, in milliseconds: 100
In a string, as milliseconds: '100ms'
In a string, as seconds: '0.1s'

The second argument, delay , has the same syntax as duration . For example:

Wait for 100ms and then run for 200ms: '0.2s 100ms'

The third argument, easing , controls how the animation accelerates and decelerates during its runtime. For example, ease-in causes the animation to begin slowly, and to pick up speed as it progresses.

Wait for 100ms, run for 200ms. Use a deceleration curve to start out fast and slowly decelerate to a resting point: '0.2s 100ms ease-out'
Run for 200ms, with no delay. Use a standard curve to start slow, accelerate in the middle, and then decelerate slowly at the end: '0.2s ease-in-out'
Start immediately, run for 200ms. Use an acceleration curve to start slow and end at full velocity: '0.2s ease-in'

NOTE :
See the Material Design website's topic on Natural easing curves for general information on easing curves.

This example provides a state transition from open to closed with a 1-second transition between states.

src/app/open-close.component.ts

      
      transition('open => closed', [
  animate('1s')
]),

In the preceding code snippet, the => operator indicates unidirectional transitions, and <=> is bidirectional. Within the transition, animate() specifies how long the transition takes. In this case, the state change from open to closed takes 1 second, expressed here as 1s .

This example adds a state transition from the closed state to the open state with a 0.5-second transition animation arc.

src/app/open-close.component.ts

      
      transition('closed => open', [
  animate('0.5s')
]),

NOTE :
Some additional notes on using styles within state and transition functions.

Use state() to define styles that are applied at the end of each transition, they persist after the animation completes
Use transition() to define intermediate styles, which create the illusion of motion during the animation
When animations are disabled, transition() styles can be skipped, but state() styles can't

Include multiple state pairs within the same transition() argument:

      
      transition( 'on => off, off => void' )

Triggering the animation

An animation requires a trigger , so that it knows when to start. The trigger() function collects the states and transitions, and gives the animation a name, so that you can attach it to the triggering element in the HTML template.

The trigger() function describes the property name to watch for changes. When a change occurs, the trigger initiates the actions included in its definition. These actions can be transitions or other functions, as we'll see later on.

In this example, we'll name the trigger openClose , and attach it to the button element. The trigger describes the open and closed states, and the timings for the two transitions.

NOTE :
Within each trigger() function call, an element can only be in one state at any given time. However, it's possible for multiple triggers to be active at once.

Defining animations and attaching them to the HTML template

Animations are defined in the metadata of the component that controls the HTML element to be animated. Put the code that defines your animations under the animations: property within the @Component() decorator.

src/app/open-close.component.ts

      
      @Component({
  selector: 'app-open-close',
  animations: [
    trigger('openClose', [
      // ...
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.8,
        backgroundColor: 'blue'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'open-close.component.html',
  styleUrls: ['open-close.component.css']
})
export class OpenCloseComponent {
  isOpen = true;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}

When you've defined an animation trigger for a component, attach it to an element in that component's template by wrapping the trigger name in brackets and preceding it with an @ symbol. Then, you can bind the trigger to a template expression using standard Angular property binding syntax as shown below, where triggerName is the name of the trigger, and expression evaluates to a defined animation state.

      
      <div [@triggerName]="expression">…</div>;

The animation is executed or triggered when the expression value changes to a new state.

The following code snippet binds the trigger to the value of the isOpen property.

src/app/open-close.component.html

      
      <nav>
  <button type="button" (click)="toggle()">Toggle Open/Close</button>
</nav>

<div [@openClose]="isOpen ? 'open' : 'closed'" class="open-close-container">
  <p>The box is now {{ isOpen ? 'Open' : 'Closed' }}!</p>
</div>

For elements entering or leaving a page (inserted or removed from the DOM), you can make the animations conditional. For example, use *ngIf with the animation trigger in the HTML template.

NOTE :
In the component file, set the trigger that defines the animations as the value of the animations: property in the @Component() decorator.

In the HTML template file, use the trigger name to attach the defined animations to the HTML element to be animated.

Code review

Here are the code files discussed in the transition example.

src/app/open-close.component.ts

src/app/open-close.component.html

src/app/open-close.component.css

      
      @Component({
  selector: 'app-open-close',
  animations: [
    trigger('openClose', [
      // ...
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.8,
        backgroundColor: 'blue'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'open-close.component.html',
  styleUrls: ['open-close.component.css']
})
export class OpenCloseComponent {
  isOpen = true;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}

Summary

You learned to add animation to a transition between two states, using style() and state() along with animate() for the timing.

Learn about more advanced features in Angular animations under the Animation section, beginning with advanced techniques in transition and triggers.

Animations API summary

The functional API provided by the @angular/animations module provides a domain-specific language (DSL) for creating and controlling animations in Angular applications. See the API reference for a complete listing and syntax details of the core functions and related data structures.

Function name	What it does
`trigger()`	Kicks off the animation and serves as a container for all other animation function calls. HTML template binds to `triggerName` . Use the first argument to declare a unique trigger name. Uses array syntax.
`style()`	Defines one or more CSS styles to use in animations. Controls the visual appearance of HTML elements during animations. Uses object syntax.
`state()`	Creates a named set of CSS styles that should be applied on successful transition to a given state. The state can then be referenced by name within other animation functions.
`animate()`	Specifies the timing information for a transition. Optional values for `delay` and `easing` . Can contain `style()` calls within.
`transition()`	Defines the animation sequence between two named states. Uses array syntax.
`keyframes()`	Allows a sequential change between styles within a specified time interval. Use within `animate()` . Can include multiple `style()` calls within each `keyframe()` . Uses array syntax.
`group()`	Specifies a group of animation steps ( inner animations ) to be run in parallel. Animation continues only after all inner animation steps have completed. Used within `sequence()` or `transition()` .
`query()`	Finds one or more inner HTML elements within the current element.
`sequence()`	Specifies a list of animation steps that are run sequentially, one by one.
`stagger()`	Staggers the starting time for animations for multiple elements.
`animation()`	Produces a reusable animation that can be invoked from elsewhere. Used together with `useAnimation()` .
`useAnimation()`	Activates a reusable animation. Used with `animation()` .
`animateChild()`	Allows animations on child components to be run within the same timeframe as the parent.

Ahead-of-time (AOT) compilation

Choosing a compiler
How AOT works
Compilation phases
Metadata restrictions
Configuring AOT compilation
Phase 1: Code analysis
Expression syntax limitations
No arrow functions
Code folding
Phase 2: code generation
Public symbols
Supported classes and functions
Functions and static method calls
Metadata rewriting
Phase 3: Template type checking
Type narrowing
Non-null type assertion operator

An Angular application consists mainly of components and their HTML templates. Because the components and templates provided by Angular cannot be understood by the browser directly, Angular applications require a compilation process before they can run in a browser.

The Angular ahead-of-time (AOT) compiler converts your Angular HTML and TypeScript code into efficient JavaScript code during the build phase before the browser downloads and runs that code. Compiling your application during the build process provides a faster rendering in the browser.

This guide explains how to specify metadata and apply available compiler options to compile your applications efficiently using the AOT compiler.

Watch Alex Rickabaugh explain the Angular compiler at AngularConnect 2019.

Here are some reasons you might want to use AOT.

Reasons	Details
Faster rendering	With AOT, the browser downloads a pre-compiled version of the application. The browser loads executable code so it can render the application immediately, without waiting to compile the application first.
Fewer asynchronous requests	The compiler inlines external HTML templates and CSS style sheets within the application JavaScript, eliminating separate ajax requests for those source files.
Smaller Angular framework download size	There's no need to download the Angular compiler if the application is already compiled. The compiler is roughly half of Angular itself, so omitting it dramatically reduces the application payload.
Detect template errors earlier	The AOT compiler detects and reports template binding errors during the build step before users can see them.
Better security	AOT compiles HTML templates and components into JavaScript files long before they are served to the client. With no templates to read and no risky client-side HTML or JavaScript evaluation, there are fewer opportunities for injection attacks.

Choosing a compiler

Angular offers two ways to compile your application:

Angular compile	Details
Just-in-Time (JIT)	Compiles your application in the browser at runtime. This was the default until Angular 8.
Ahead-of-Time (AOT)	Compiles your application and libraries at build time. This is the default starting in Angular 9.

When you run the ng build (build only) or ng serve (build and serve locally) CLI commands, the type of compilation (JIT or AOT) depends on the value of the aot property in your build configuration specified in angular.json . By default, aot is set to true for new CLI applications.

See the CLI command reference and Building and serving Angular apps for more information.

How AOT works

The Angular AOT compiler extracts metadata to interpret the parts of the application that Angular is supposed to manage. You can specify the metadata explicitly in decorators such as @Component() and @Input() , or implicitly in the constructor declarations of the decorated classes. The metadata tells Angular how to construct instances of your application classes and interact with them at runtime.

In the following example, the @Component() metadata object and the class constructor tell Angular how to create and display an instance of TypicalComponent .

      
      @Component({
  selector: 'app-typical',
  template: '<div>A typical component for {{data.name}}</div>'
})
export class TypicalComponent {
  @Input() data: TypicalData;
  constructor(private someService: SomeService) { … }
}

The Angular compiler extracts the metadata once and generates a factory for TypicalComponent . When it needs to create a TypicalComponent instance, Angular calls the factory, which produces a new visual element, bound to a new instance of the component class with its injected dependency.

Compilation phases

There are three phases of AOT compilation.

	Phase	Details
1	code analysis	In this phase, the TypeScript compiler and AOT collector create a representation of the source. The collector does not attempt to interpret the metadata it collects. It represents the metadata as best it can and records errors when it detects a metadata syntax violation.
2	code generation	In this phase, the compiler's `StaticReflector` interprets the metadata collected in phase 1, performs additional validation of the metadata, and throws an error if it detects a metadata restriction violation.
3	template type checking	In this optional phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates. You can enable this phase explicitly by setting the `fullTemplateTypeCheck` configuration option; see Angular compiler options.

Metadata restrictions

You write metadata in a subset of TypeScript that must conform to the following general constraints:

Limit expression syntax to the supported subset of JavaScript
Only reference exported symbols after code folding
Only call functions supported by the compiler
Decorated and data-bound class members must be public

For additional guidelines and instructions on preparing an application for AOT compilation, see Angular: Writing AOT-friendly applications.

Errors in AOT compilation commonly occur because of metadata that does not conform to the compiler's requirements (as described more fully below). For help in understanding and resolving these problems, see AOT Metadata Errors.

Configuring AOT compilation

You can provide options in the TypeScript configuration file that controls the compilation process. See Angular compiler options for a complete list of available options.

Phase 1: Code analysis

The TypeScript compiler does some of the analytic work of the first phase. It emits the .d.ts type definition files with type information that the AOT compiler needs to generate application code. At the same time, the AOT collector analyzes the metadata recorded in the Angular decorators and outputs metadata information in .metadata.json files, one per .d.ts file.

You can think of .metadata.json as a diagram of the overall structure of a decorator's metadata, represented as an abstract syntax tree (AST).

Angular's schema.ts describes the JSON format as a collection of TypeScript interfaces.

Expression syntax limitations

The AOT collector only understands a subset of JavaScript. Define metadata objects with the following limited syntax:

Syntax	Example
Literal object	`{cherry: true, apple: true, mincemeat: false}`
Literal array	`['cherries', 'flour', 'sugar']`
Spread in literal array	`['apples', 'flour', ...]`
Calls	`bake(ingredients)`
New	`new Oven()`
Property access	`pie.slice`
Array index	`ingredients[0]`
Identity reference	`Component`
A template string	`pie is ${multiplier} times better than cake`
Literal string	`'pi'`
Literal number	`3.14153265`
Literal boolean	`true`
Literal null	`null`
Supported prefix operator	`!cake`
Supported binary operator	`a+b`
Conditional operator	`a ? b : c`
Parentheses	`(a+b)`

If an expression uses unsupported syntax, the collector writes an error node to the .metadata.json file. The compiler later reports the error if it needs that piece of metadata to generate the application code.

If you want ngc to report syntax errors immediately rather than produce a .metadata.json file with errors, set the strictMetadataEmit option in the TypeScript configuration file.

      
      "angularCompilerOptions": {
  …
  "strictMetadataEmit" : true
}

Angular libraries have this option to ensure that all Angular .metadata.json files are clean and it is a best practice to do the same when building your own libraries.

No arrow functions

The AOT compiler does not support function expressions and arrow functions, also called lambda functions.

Consider the following component decorator:

      
      @Component({
  …
  providers: [{provide: server, useFactory: () => new Server()}]
})

The AOT collector does not support the arrow function, () => new Server() , in a metadata expression. It generates an error node in place of the function. When the compiler later interprets this node, it reports an error that invites you to turn the arrow function into an exported function .

You can fix the error by converting to this:

      
      export function serverFactory() {
  return new Server();
}

@Component({
  …
  providers: [{provide: server, useFactory: serverFactory}]
})

In version 5 and later, the compiler automatically performs this rewriting while emitting the .js file.

Code folding

The compiler can only resolve references to exported symbols. The collector, however, can evaluate an expression during collection and record the result in the .metadata.json , rather than the original expression. This allows you to make limited use of non-exported symbols within expressions.

For example, the collector can evaluate the expression 1 + 2 + 3 + 4 and replace it with the result, 10 . This process is called folding . An expression that can be reduced in this manner is foldable .

The collector can evaluate references to module-local const declarations and initialized var and let declarations, effectively removing them from the .metadata.json file.

Consider the following component definition:

      
      const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template
})
export class HeroComponent {
  @Input() hero: Hero;
}

The compiler could not refer to the template constant because it isn't exported. The collector, however, can fold the template constant into the metadata definition by in-lining its contents. The effect is the same as if you had written:

      
      @Component({
  selector: 'app-hero',
  template: '<div>{{hero.name}}</div>'
})
export class HeroComponent {
  @Input() hero: Hero;
}

There is no longer a reference to template and, therefore, nothing to trouble the compiler when it later interprets the collector's output in .metadata.json .

You can take this example a step further by including the template constant in another expression:

      
      const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template + '<div>{{hero.title}}</div>'
})
export class HeroComponent {
  @Input() hero: Hero;
}

The collector reduces this expression to its equivalent folded string:

      
      '<div>{{hero.name}}</div><div>{{hero.title}}</div>'

Foldable syntax

The following table describes which expressions the collector can and cannot fold:

Syntax	Foldable
Literal object	yes
Literal array	yes
Spread in literal array	no
Calls	no
New	no
Property access	yes, if target is foldable
Array index	yes, if target and index are foldable
Identity reference	yes, if it is a reference to a local
A template with no substitutions	yes
A template with substitutions	yes, if the substitutions are foldable
Literal string	yes
Literal number	yes
Literal boolean	yes
Literal null	yes
Supported prefix operator	yes, if operand is foldable
Supported binary operator	yes, if both left and right are foldable
Conditional operator	yes, if condition is foldable
Parentheses	yes, if the expression is foldable

If an expression is not foldable, the collector writes it to .metadata.json as an AST for the compiler to resolve.

Phase 2: code generation

The collector makes no attempt to understand the metadata that it collects and outputs to .metadata.json . It represents the metadata as best it can and records errors when it detects a metadata syntax violation. It's the compiler's job to interpret the .metadata.json in the code generation phase.

The compiler understands all syntax forms that the collector supports, but it may reject syntactically correct metadata if the semantics violate compiler rules.

Public symbols

The compiler can only reference exported symbols .

Decorated component class members must be public. You cannot make an @Input() property private or protected.
Data bound properties must also be public

Supported classes and functions

The collector can represent a function call or object creation with new as long as the syntax is valid. The compiler, however, can later refuse to generate a call to a particular function or creation of a particular object.

The compiler can only create instances of certain classes, supports only core decorators, and only supports calls to macros (functions or static methods) that return expressions.

Compiler action	Details
New instances	The compiler only allows metadata that create instances of the class `InjectionToken` from `@angular/core` .
Supported decorators	The compiler only supports metadata for the Angular decorators in the `@angular/core` module.
Function calls	Factory functions must be exported, named functions. The AOT compiler does not support lambda expressions ("arrow functions") for factory functions.

Functions and static method calls

The collector accepts any function or static method that contains a single return statement. The compiler, however, only supports macros in the form of functions or static methods that return an expression .

For example, consider the following function:

      
      export function wrapInArray<T>(value: T): T[] {
  return [value];
}

You can call the wrapInArray in a metadata definition because it returns the value of an expression that conforms to the compiler's restrictive JavaScript subset.

You might use wrapInArray() like this:

      
      @NgModule({
  declarations: wrapInArray(TypicalComponent)
})
export class TypicalModule {}

The compiler treats this usage as if you had written:

      
      @NgModule({
  declarations: [TypicalComponent]
})
export class TypicalModule {}

The Angular RouterModule exports two macro static methods, forRoot and forChild , to help declare root and child routes. Review the source code for these methods to see how macros can simplify configuration of complex NgModules.

Metadata rewriting

The compiler treats object literals containing the fields useClass , useValue , useFactory , and data specially, converting the expression initializing one of these fields into an exported variable that replaces the expression. This process of rewriting these expressions removes all the restrictions on what can be in them because the compiler doesn't need to know the expression's value —it just needs to be able to generate a reference to the value.

You might write something like:

      
      class TypicalServer {

}

@NgModule({
  providers: [{provide: SERVER, useFactory: () => TypicalServer}]
})
export class TypicalModule {}

Without rewriting, this would be invalid because lambdas are not supported and TypicalServer is not exported. To allow this, the compiler automatically rewrites this to something like:

      
      class TypicalServer {

}

export const θ0 = () => new TypicalServer();

@NgModule({
  providers: [{provide: SERVER, useFactory: θ0}]
})
export class TypicalModule {}

This allows the compiler to generate a reference to θ0 in the factory without having to know what the value of θ0 contains.

The compiler does the rewriting during the emit of the .js file. It does not, however, rewrite the .d.ts file, so TypeScript doesn't recognize it as being an export. And it does not interfere with the ES module's exported API.

Phase 3: Template type checking

One of the Angular compiler's most helpful features is the ability to type-check expressions within templates, and catch any errors before they cause crashes at runtime. In the template type-checking phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates.

Enable this phase explicitly by adding the compiler option "fullTemplateTypeCheck" in the "angularCompilerOptions" of the project's TypeScript configuration file (see Angular Compiler Options).

Template validation produces error messages when a type error is detected in a template binding expression, similar to how type errors are reported by the TypeScript compiler against code in a .ts file.

For example, consider the following component:

      
      @Component({
  selector: 'my-component',
  template: '{{person.addresss.street}}'
})
class MyComponent {
  person?: Person;
}

This produces the following error:

      
      my.component.ts.MyComponent.html(1,1): : Property 'addresss' does not exist on type 'Person'. Did you mean 'address'?

The file name reported in the error message, my.component.ts.MyComponent.html , is a synthetic file generated by the template compiler that holds contents of the MyComponent class template. The compiler never writes this file to disk. The line and column numbers are relative to the template string in the @Component annotation of the class, MyComponent in this case. If a component uses templateUrl instead of template , the errors are reported in the HTML file referenced by the templateUrl instead of a synthetic file.

The error location is the beginning of the text node that contains the interpolation expression with the error. If the error is in an attribute binding such as [value]="person.address.street" , the error location is the location of the attribute that contains the error.

The validation uses the TypeScript type checker and the options supplied to the TypeScript compiler to control how detailed the type validation is. For example, if the strictTypeChecks is specified, the error

      
      my.component.ts.MyComponent.html(1,1): : Object is possibly 'undefined'

is reported as well as the above error message.

Type narrowing

The expression used in an ngIf directive is used to narrow type unions in the Angular template compiler, the same way the if expression does in TypeScript. For example, to avoid Object is possibly 'undefined' error in the template above, modify it to only emit the interpolation if the value of person is initialized as shown below:

      
      @Component({
  selector: 'my-component',
  template: ' {{person.address.street}} '
})
class MyComponent {
  person?: Person;
}

Using *ngIf allows the TypeScript compiler to infer that the person used in the binding expression will never be undefined .

For more information about input type narrowing, see Improving template type checking for custom directives.

Non-null type assertion operator

Use the non-null type assertion operator to suppress the Object is possibly 'undefined' error when it is inconvenient to use *ngIf or when some constraint in the component ensures that the expression is always non-null when the binding expression is interpolated.

In the following example, the person and address properties are always set together, implying that address is always non-null if person is non-null. There is no convenient way to describe this constraint to TypeScript and the template compiler, but the error is suppressed in the example by using address!.street .

      
      @Component({
  selector: 'my-component',
  template: '<span *ngIf="person"> {{person.name}} lives on {{address!.street}} </span>'
})
class MyComponent {
  person?: Person;
  address?: Address;

  setData(person: Person, address: Address) {
    this.person = person;
    this.address = address;
  }
}

The non-null assertion operator should be used sparingly as refactoring of the component might break this constraint.

In this example it is recommended to include the checking of address in the *ngIf as shown below:

      
      @Component({
  selector: 'my-component',
  template: '<span *ngIf="person && address"> {{person.name}} lives on {{address.street}} </span>'
})
class MyComponent {
  person?: Person;
  address?: Address;

  setData(person: Person, address: Address) {
    this.person = person;
    this.address = address;
  }
}

Last reviewed on Mon Feb 28 2022

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Angular - AOT metadata errors

AOT metadata errors

Expression form not supported
Reference to a local (non-exported) symbol
Only initialized variables and constants
Reference to a non-exported class
Reference to a non-exported function
Function calls are not supported
Destructured variable or constant not supported
Could not resolve type
Name expected
Unsupported enum member name
Tagged template expressions are not supported
Symbol reference expected

The following are metadata errors you may encounter, with explanations and suggested corrections.

Expression form not supported
Reference to a local (non-exported) symbol
Only initialized variables and constants
Reference to a non-exported class
Reference to a non-exported function
Function calls are not supported
Destructured variable or constant not supported
Could not resolve type
Name expected
Unsupported enum member name
Tagged template expressions are not supported
Symbol reference expected

Expression form not supported

The compiler encountered an expression it didn't understand while evaluating Angular metadata.

Language features outside of the compiler's restricted expression syntax can produce this error, as seen in the following example:

      
      // ERROR
export class Fooish { … }
…
const prop = typeof Fooish; // typeof is not valid in metadata
  …
  // bracket notation is not valid in metadata
  { provide: 'token', useValue: { [prop]: 'value' } };
  …

You can use typeof and bracket notation in normal application code. You just can't use those features within expressions that define Angular metadata.

Avoid this error by sticking to the compiler's restricted expression syntax when writing Angular metadata and be wary of new or unusual TypeScript features.

Reference to a local (non-exported) symbol

Reference to a local (non-exported) symbol 'symbol name'. Consider exporting the symbol.

The compiler encountered a referenced to a locally defined symbol that either wasn't exported or wasn't initialized.

Here's a provider example of the problem.

      
      // ERROR
let foo: number; // neither exported nor initialized

@Component({
  selector: 'my-component',
  template: … ,
  providers: [
    { provide: Foo, useValue: foo }
  ]
})
export class MyComponent {}

The compiler generates the component factory, which includes the useValue provider code, in a separate module. That factory module can't reach back to this source module to access the local (non-exported) foo variable.

You could fix the problem by initializing foo .

      
      let foo = 42; // initialized

The compiler will fold the expression into the provider as if you had written this.

      
      providers: [
  { provide: Foo, useValue: 42 }
]

Alternatively, you can fix it by exporting foo with the expectation that foo will be assigned at runtime when you actually know its value.

      
      // CORRECTED
export let foo: number; // exported

@Component({
  selector: 'my-component',
  template: … ,
  providers: [
    { provide: Foo, useValue: foo }
  ]
})
export class MyComponent {}

Adding export often works for variables referenced in metadata such as providers and animations because the compiler can generate references to the exported variables in these expressions. It doesn't need the values of those variables.

Adding export doesn't work when the compiler needs the actual value in order to generate code. For example, it doesn't work for the template property.

      
      // ERROR
export let someTemplate: string; // exported but not initialized

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

The compiler needs the value of the template property right now to generate the component factory. The variable reference alone is insufficient. Prefixing the declaration with export merely produces a new error, " Only initialized variables and constants can be referenced ".

Only initialized variables and constants

Only initialized variables and constants can be referenced because the value of this variable is needed by the template compiler.

The compiler found a reference to an exported variable or static field that wasn't initialized. It needs the value of that variable to generate code.

The following example tries to set the component's template property to the value of the exported someTemplate variable which is declared but unassigned .

      
      // ERROR
export let someTemplate: string;

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

You'd also get this error if you imported someTemplate from some other module and neglected to initialize it there.

      
      // ERROR - not initialized there either
import { someTemplate } from './config';

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

The compiler cannot wait until runtime to get the template information. It must statically derive the value of the someTemplate variable from the source code so that it can generate the component factory, which includes instructions for building the element based on the template.

To correct this error, provide the initial value of the variable in an initializer clause on the same line .

      
      // CORRECTED
export let someTemplate = '<h1>Greetings from Angular</h1>';

@Component({
  selector: 'my-component',
  template: someTemplate
})
export class MyComponent {}

Reference to a non-exported class

Reference to a non-exported class <class name> . Consider exporting the class.

Metadata referenced a class that wasn't exported.

For example, you may have defined a class and used it as an injection token in a providers array but neglected to export that class.

      
      // ERROR
abstract class MyStrategy { }

  …
  providers: [
    { provide: MyStrategy, useValue: … }
  ]
  …

Angular generates a class factory in a separate module and that factory can only access exported classes. To correct this error, export the referenced class.

      
      // CORRECTED
export abstract class MyStrategy { }

  …
  providers: [
    { provide: MyStrategy, useValue: … }
  ]
  …

Reference to a non-exported function

Metadata referenced a function that wasn't exported.

For example, you may have set a providers useFactory property to a locally defined function that you neglected to export.

      
      // ERROR
function myStrategy() { … }

  …
  providers: [
    { provide: MyStrategy, useFactory: myStrategy }
  ]
  …

Angular generates a class factory in a separate module and that factory can only access exported functions. To correct this error, export the function.

      
      // CORRECTED
export function myStrategy() { … }

  …
  providers: [
    { provide: MyStrategy, useFactory: myStrategy }
  ]
  …

Function calls are not supported

Function calls are not supported. Consider replacing the function or lambda with a reference to an exported function.

The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider's useFactory to an anonymous function or arrow function like this.

      
      // ERROR
  …
  providers: [
    { provide: MyStrategy, useFactory: function() { … } },
    { provide: OtherStrategy, useFactory: () => { … } }
  ]
  …

You also get this error if you call a function or method in a provider's useValue .

      
      // ERROR
import { calculateValue } from './utilities';

  …
  providers: [
    { provide: SomeValue, useValue: calculateValue() }
  ]
  …

To correct this error, export a function from the module and refer to the function in a useFactory provider instead.

      
      // CORRECTED
import { calculateValue } from './utilities';

export function myStrategy() { … }
export function otherStrategy() { … }
export function someValueFactory() {
  return calculateValue();
}
  …
  providers: [
    { provide: MyStrategy, useFactory: myStrategy },
    { provide: OtherStrategy, useFactory: otherStrategy },
    { provide: SomeValue, useFactory: someValueFactory }
  ]
  …

Destructured variable or constant not supported

Referencing an exported destructured variable or constant is not supported by the template compiler. Consider simplifying this to avoid destructuring.

The compiler does not support references to variables assigned by destructuring.

For example, you cannot write something like this:

      
      // ERROR
import { configuration } from './configuration';

// destructured assignment to foo and bar
const {foo, bar} = configuration;
  …
  providers: [
    {provide: Foo, useValue: foo},
    {provide: Bar, useValue: bar},
  ]
  …

To correct this error, refer to non-destructured values.

      
      // CORRECTED
import { configuration } from './configuration';
  …
  providers: [
    {provide: Foo, useValue: configuration.foo},
    {provide: Bar, useValue: configuration.bar},
  ]
  …

Could not resolve type

The compiler encountered a type and can't determine which module exports that type.

This can happen if you refer to an ambient type. For example, the Window type is an ambient type declared in the global .d.ts file.

You'll get an error if you reference it in the component constructor, which the compiler must statically analyze.

      
      // ERROR
@Component({ })
export class MyComponent {
  constructor (private win: Window) { … }
}

TypeScript understands ambient types so you don't import them. The Angular compiler does not understand a type that you neglect to export or import.

In this case, the compiler doesn't understand how to inject something with the Window token.

Do not refer to ambient types in metadata expressions.

If you must inject an instance of an ambient type, you can finesse the problem in four steps:

Create an injection token for an instance of the ambient type.
Create a factory function that returns that instance.
Add a useFactory provider with that factory function.
Use @Inject to inject the instance.

Here's an illustrative example.

      
      // CORRECTED
import { Inject } from '@angular/core';

export const WINDOW = new InjectionToken('Window');
export function _window() { return window; }

@Component({
  …
  providers: [
    { provide: WINDOW, useFactory: _window }
  ]
})
export class MyComponent {
  constructor (@Inject(WINDOW) private win: Window) { … }
}

The Window type in the constructor is no longer a problem for the compiler because it uses the @Inject(WINDOW) to generate the injection code.

Angular does something similar with the DOCUMENT token so you can inject the browser's document object (or an abstraction of it, depending upon the platform in which the application runs).

      
      import { Inject }   from '@angular/core';
import { DOCUMENT } from '@angular/common';

@Component({ … })
export class MyComponent {
  constructor (@Inject(DOCUMENT) private doc: Document) { … }
}

Name expected

The compiler expected a name in an expression it was evaluating.

This can happen if you use a number as a property name as in the following example.

      
      // ERROR
provider: [{ provide: Foo, useValue: { 0: 'test' } }]

Change the name of the property to something non-numeric.

      
      // CORRECTED
provider: [{ provide: Foo, useValue: { '0': 'test' } }]

Unsupported enum member name

Angular couldn't determine the value of the enum member that you referenced in metadata.

The compiler can understand simple enum values but not complex values such as those derived from computed properties.

      
      // ERROR
enum Colors {
  Red = 1,
  White,
  Blue = "Blue".length // computed
}

  …
  providers: [
    { provide: BaseColor,   useValue: Colors.White } // ok
    { provide: DangerColor, useValue: Colors.Red }   // ok
    { provide: StrongColor, useValue: Colors.Blue }  // bad
  ]
  …

Avoid referring to enums with complicated initializers or computed properties.

Tagged template expressions are not supported

Tagged template expressions are not supported in metadata.

The compiler encountered a JavaScript ES2015 tagged template expression such as the following.

      
      // ERROR
const expression = 'funky';
const raw = String.raw`A tagged template ${expression} string`;
 …
 template: '<div>' + raw + '</div>'
 …

String.raw() is a tag function native to JavaScript ES2015.

The AOT compiler does not support tagged template expressions; avoid them in metadata expressions.

Symbol reference expected

The compiler expected a reference to a symbol at the location specified in the error message.

This error can occur if you use an expression in the extends clause of a class.

Last reviewed on Mon Feb 28 2022

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Angular - App shell

App shell

Contents

Step 1: Prepare the application
Step 2: Create the application shell
Step 3: Verify the application is built with the shell content

Application shell is a way to render a portion of your application using a route at build time. It can improve the user experience by quickly launching a static rendered page (a skeleton common to all pages) while the browser downloads the full client version and switches to it automatically after the code loads.

This gives users a meaningful first paint of your application that appears quickly because the browser can render the HTML and CSS without the need to initialize any JavaScript.

Learn more in The App Shell Model.

Step 1: Prepare the application

Do this with the following Angular CLI command:

      
      ng new my-app --routing

For an existing application, you have to manually add the RouterModule and defining a <router-outlet> within your application.

Step 2: Create the application shell

Use the Angular CLI to automatically create the application shell.

      
      ng generate app-shell

For more information about this command, see App shell command.

After running this command you can see that the angular.json configuration file has been updated to add two new targets, with a few other changes.

      
      "server": {
  "builder": "@angular-devkit/build-angular:server",
  "defaultConfiguration": "production",
  "options": {
    "outputPath": "dist/my-app/server",
    "main": "src/main.server.ts",
    "tsConfig": "tsconfig.server.json"
  },
  "configurations": {
    "development": {
      "outputHashing": "none",
    },
    "production": {
      "outputHashing": "media",
      "fileReplacements": [
        {
          "replace": "src/environments/environment.ts",
          "with": "src/environments/environment.prod.ts"
        }
      ],
      "sourceMap": false,
      "optimization": true
    }
  }
},
"app-shell": {
  "builder": "@angular-devkit/build-angular:app-shell",
  "defaultConfiguration": "production",
  "options": {
    "route": "shell"
  },
  "configurations": {
    "development": {
      "browserTarget": "my-app:build:development",
      "serverTarget": "my-app:server:development",
    },
    "production": {
      "browserTarget": "my-app:build:production",
      "serverTarget": "my-app:server:production"
    }
  }
}

Step 3: Verify the application is built with the shell content

Use the Angular CLI to build the app-shell target.

      
      ng run my-app:app-shell:development

Or to use the production configuration.

      
      ng run my-app:app-shell:production

To verify the build output, open dist/my-app/browser/index.html . Look for default text app-shell works! to show that the application shell route was rendered as part of the output.

Last reviewed on Mon Feb 28 2022

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Angular - Introduction to components and templates

Introduction to components and templates

Component metadata
Templates and views
Template syntax
Data binding
Pipes
Directives

A component controls a patch of screen called a view . It consists of a TypeScript class, an HTML template, and a CSS style sheet. The TypeScript class defines the interaction of the HTML template and the rendered DOM structure, while the style sheet describes its appearance.

An Angular application uses individual components to define and control different aspects of the application. For example, an application could include components to describe:

The application root with the navigation links
The list of heroes
The hero editor

In the following example, the HeroListComponent class includes:

A heroes property that holds an array of heroes.
A selectedHero property that holds the last hero selected by the user.
A selectHero() method sets a selectedHero property when the user clicks to choose a hero from that list.

The component initializes the heroes property by using the HeroService service, which is a TypeScript parameter property on the constructor. Angular's dependency injection system provides the HeroService service to the component.

src/app/hero-list.component.ts (class)

      
      export class HeroListComponent implements OnInit {
  heroes: Hero[] = [];
  selectedHero: Hero | undefined;

  constructor(private service: HeroService) { }

  ngOnInit() {
    this.heroes = this.service.getHeroes();
  }

  selectHero(hero: Hero) { this.selectedHero = hero; }
}

Angular creates, updates, and destroys components as the user moves through the application. Your application can take action at each moment in this lifecycle through optional lifecycle hooks, like ngOnInit() .

Component metadata

The @Component decorator identifies the class immediately below it as a component class, and specifies its metadata. In the example code below, you can see that HeroListComponent is just a class, with no special Angular notation or syntax at all. It's not a component until you mark it as one with the @Component decorator.

The metadata for a component tells Angular where to get the major building blocks that it needs to create and present the component and its view. In particular, it associates a template with the component, either directly with inline code, or by reference. Together, the component and its template describe a view .

In addition to containing or pointing to the template, the @Component metadata configures, for example, how the component can be referenced in HTML and what services it requires.

Here's an example of basic metadata for HeroListComponent .

src/app/hero-list.component.ts (metadata)

      
      @Component({
  selector:    'app-hero-list',
  templateUrl: './hero-list.component.html',
  providers:  [ HeroService ]
})
export class HeroListComponent implements OnInit {
  /* . . . */
}

This example shows some of the most useful @Component configuration options:

Configuration options	Details
`selector`	A CSS selector that tells Angular to create and insert an instance of this component wherever it finds the corresponding tag in template HTML. For example, if an application's HTML contains `<app-hero-list></app-hero-list>` , then Angular inserts an instance of the `HeroListComponent` view between those tags.
`templateUrl`	The module-relative address of this component's HTML template. Alternatively, you can provide the HTML template inline, as the value of the `template` property. This template defines the component's host view .
`providers`	An array of providers for services that the component requires. In the example, this tells Angular how to provide the `HeroService` instance that the component's constructor uses to get the list of heroes to display.

Templates and views

You define a component's view with its companion template. A template is a form of HTML that tells Angular how to render the component.

Views are typically organized hierarchically, allowing you to modify or show and hide entire UI sections or pages as a unit. The template immediately associated with a component defines that component's host view . The component can also define a view hierarchy , which contains embedded views , hosted by other components.

A view hierarchy can include views from components in the same NgModule and from those in different NgModules.

Template syntax

A template looks like regular HTML, except that it also contains Angular template syntax, which alters the HTML based on your application's logic and the state of application and DOM data. Your template can use data binding to coordinate the application and DOM data, pipes to transform data before it is displayed, and directives to apply application logic to what gets displayed.

For example, here is a template for the Tutorial's HeroListComponent .

src/app/hero-list.component.html

      
      <h2>Hero List</h2>

<p><em>Select a hero from the list to see details.</em></p>
<ul>
  <li *ngFor="let hero of heroes">
    <button type="button" (click)="selectHero(hero)">
      {{hero.name}}
    </button>
  </li>
</ul>

<app-hero-detail *ngIf="selectedHero" [hero]="selectedHero"></app-hero-detail>

This template uses typical HTML elements like <h2> and <p> . It also includes Angular template-syntax elements, *ngFor , {{hero.name}} , (click) , [hero] , and <app-hero-detail> . The template-syntax elements tell Angular how to render the HTML to the screen, using program logic and data.

The *ngFor directive tells Angular to iterate over a list
{{hero.name}} , (click) , and [hero] bind program data to and from the DOM, responding to user input. See more about data binding below.
The <app-hero-detail> element tag in the example represents a new component, HeroDetailComponent . The HeroDetailComponent defines the hero-detail portion of the rendered DOM structure specified by the HeroListComponent component.

Notice how these custom components mix with native HTML.

Data binding

Without a framework, you would be responsible for pushing data values into the HTML controls and turning user responses into actions and value updates. Writing such push and pull logic by hand is tedious, error-prone, and a nightmare to read, as any experienced front-end JavaScript programmer can attest.

Angular supports two-way data binding , a mechanism for coordinating the parts of a template with the parts of a component. Add binding markup to the template HTML to tell Angular how to connect both sides.

The following diagram shows the four forms of data binding markup. Each form has a direction: to the DOM, from the DOM, or both.

This example from the HeroListComponent template uses three of these forms.

src/app/hero-list.component.html (binding)

      
      <app-hero-detail [hero]="selectedHero"></app-hero-detail>
<button type="button" (click)="selectHero(hero)">
  {{hero.name}}
</button>

Data bindings	Details
`[hero]` property binding	Passes the value of `selectedHero` from the parent `HeroListComponent` to the `hero` property of the child `HeroDetailComponent` .
`(click)` event binding	Calls the component's `selectHero` method when the user clicks a hero's name.
`{{hero.name}}` interpolation	Displays the component's `hero.name` property value within the `<button>` element.

Two-way data binding (used mainly in template-driven forms) combines property and event binding in a single notation. Here's an example from the HeroDetailComponent template that uses two-way data binding with the ngModel directive.

src/app/hero-detail.component.html (ngModel)

      
      <input type="text" id="hero-name" [(ngModel)]="hero.name">

In two-way binding, a data property value flows to the input box from the component as with property binding. The user's changes also flow back to the component, resetting the property to the latest value, as with event binding.

Angular processes all data bindings once for each JavaScript event cycle, from the root of the application component tree through all child components.

Data binding plays an important role in communication between a template and its component, and is also important for communication between parent and child components.

Pipes

Angular pipes let you declare display-value transformations in your template HTML. A class with the @Pipe decorator defines a function that transforms input values to output values for display in a view.

Angular defines various pipes, such as the date pipe and currency pipe. For a complete list, see the Pipes API list. You can also define new pipes.

To specify a value transformation in an HTML template, use the pipe operator ( | ).

      
      {{interpolated_value | pipe_name}}

You can chain pipes, sending the output of one pipe function to be transformed by another pipe function. A pipe can also take arguments that control how it performs its transformation. For example, you can pass the desired format to the date pipe.

      
      <!-- Default format: output 'Jun 15, 2015'-->
<p>Today is {{today | date}}</p>

<!-- fullDate format: output 'Monday, June 15, 2015'-->
<p>The date is {{today | date:'fullDate'}}</p>

<!-- shortTime format: output '9:43 AM'-->
<p>The time is {{today | date:'shortTime'}}</p>

Directives

Angular templates are dynamic . When Angular renders them, it transforms the DOM according to the instructions given by directives . A directive is a class with a @Directive() decorator.

A component is technically a directive. However, components are so distinctive and central to Angular applications that Angular defines the @Component() decorator, which extends the @Directive() decorator with template-oriented features.

In addition to components, there are two other kinds of directives: structural and attribute . Angular defines a number of directives of both kinds, and you can define your own using the @Directive() decorator.

Just as for components, the metadata for a directive associates the decorated class with a selector element that you use to insert it into HTML. In templates, directives typically appear within an element tag as attributes, either by name or as the target of an assignment or a binding.

Structural directives

Structural directives alter layout by adding, removing, and replacing elements in the DOM. The example template uses two built-in structural directives to add application logic to how the view is rendered.

src/app/hero-list.component.html (structural)

      
      <li *ngFor="let hero of heroes"></li>
<app-hero-detail *ngIf="selectedHero"></app-hero-detail>

Directives	Details
`*ngFor`	An iterative , which tells Angular to create one `<li>` per hero in the `heroes` list.
`*ngIf`	A conditional , which includes the `HeroDetail` component only if a selected hero exists.

Attribute directives

Attribute directives alter the appearance or behavior of an existing element. In templates they look like regular HTML attributes, hence the name.

The ngModel directive, which implements two-way data binding, is an example of an attribute directive. ngModel modifies the behavior of an existing element (typically <input> ) by setting its display value property and responding to change events.

src/app/hero-detail.component.html (ngModel)

      
      <input type="text" id="hero-name" [(ngModel)]="hero.name">

Angular includes pre-defined directives that change:

The layout structure, such as ngSwitch, and
Aspects of DOM elements and components, such as ngStyle and ngClass.

Learn more in the Attribute Directives and Structural Directives guides.

Last reviewed on Mon Feb 28 2022

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Angular - Define animations and attach to an HTML template

Define animations and attach to an HTML template link

Define the animation link

Attach an animation to an HTML template link

Angular - Introduction to Angular animations

Introduction to Angular animations link

About this guide link

Prerequisites link

Getting started link

Step 1: Enabling the animations module link

Step 2: Importing animation functions into component files link

Step 3: Adding the animation metadata property link

Animating a transition link

Animation state and styles link

Transitions and timing link

Animation metadata: duration, delay, and easing link

Triggering the animation link

Defining animations and attaching them to the HTML template link

Code review link

Summary link

Animations API summary link

More on Angular animations link

Angular - Ahead-of-time (AOT) compilation

Ahead-of-time (AOT) compilation link

Choosing a compiler link

How AOT works link

Compilation phases link

Metadata restrictions link

Configuring AOT compilation link

Phase 1: Code analysis link

Expression syntax limitations link

No arrow functions link

Code folding link

Foldable syntax link

Phase 2: code generation link

Public symbols link

Supported classes and functions link

Functions and static method calls link

Metadata rewriting link

Phase 3: Template type checking link

Type narrowing link

Non-null type assertion operator link

Angular - AOT metadata errors

AOT metadata errors link

Expression form not supported link

Reference to a local (non-exported) symbol link

Only initialized variables and constants link

Reference to a non-exported class link

Reference to a non-exported function link

Function calls are not supported link

Destructured variable or constant not supported link

Could not resolve type link

Name expected link

Unsupported enum member name link

Tagged template expressions are not supported link

Symbol reference expected link

Angular - App shell

App shell link

Step 1: Prepare the application link

Step 2: Create the application shell link

Step 3: Verify the application is built with the shell content link

Angular - Introduction to components and templates

Introduction to components and templates link

Component metadata link

Templates and views link

Template syntax link

Data binding link

Pipes link

Directives link

Structural directives link

Attribute directives link

Define animations and attach to an HTML template

Define the animation

Attach an animation to an HTML template

Introduction to Angular animations

About this guide

Prerequisites

Getting started

Step 1: Enabling the animations module

Step 2: Importing animation functions into component files

Step 3: Adding the animation metadata property

Animating a transition

Animation state and styles

Transitions and timing

Animation metadata: duration, delay, and easing

Triggering the animation

Defining animations and attaching them to the HTML template

Code review

Summary

Animations API summary

More on Angular animations

Ahead-of-time (AOT) compilation

Choosing a compiler

How AOT works

Compilation phases

Metadata restrictions

Configuring AOT compilation

Phase 1: Code analysis

Expression syntax limitations

No arrow functions

Code folding

Foldable syntax

Phase 2: code generation

Public symbols

Supported classes and functions

Functions and static method calls

Metadata rewriting

Phase 3: Template type checking

Type narrowing

Non-null type assertion operator

AOT metadata errors

Expression form not supported

Reference to a local (non-exported) symbol

Only initialized variables and constants

Reference to a non-exported class

Reference to a non-exported function

Function calls are not supported

Destructured variable or constant not supported

Could not resolve type

Name expected

Unsupported enum member name

Tagged template expressions are not supported

Symbol reference expected

App shell

Step 1: Prepare the application

Step 2: Create the application shell

Step 3: Verify the application is built with the shell content

Introduction to components and templates

Component metadata

Templates and views

Template syntax

Data binding

Pipes

Directives

Structural directives

Attribute directives