Attribute binding link

Attribute binding in Angular helps you set values for attributes directly. With attribute binding, you can improve accessibility, style your application dynamically, and manage multiple CSS classes or styles simultaneously.

Prerequisites link

• Property Binding

Syntax link

Attribute binding syntax resembles property binding, but instead of an element property between brackets, you precede the name of the attribute with the prefix attr , followed by a dot. Then, you set the attribute value with an expression that resolves to a string.

      
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      <p [attr.attribute-you-are-targeting]="expression"></p>
    

Binding ARIA attributes link

One of the primary use cases for attribute binding is to set ARIA attributes.

To bind to an ARIA attribute, type the following:

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

Binding to colspan link

Another common use case for attribute binding is with the colspan attribute in tables. Binding to the colspan attribute helps you to keep your tables programmatically dynamic. Depending on the amount of data that your application populates a table with, the number of columns that a row spans could change.

To use attribute binding with the <td> attribute colspan

1. Specify the colspan attribute by using the following syntax: [attr.colspan] .
2. Set [attr.colspan] equal to an expression.

In the following example, you bind the colspan attribute to the expression 1 + 1 .

      
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      <!--  expression calculates colspan=2 -->
<tr><td [attr.colspan]="1 + 1">One-Two</td></tr>
    

This binding causes the <tr> to span two columns.

What’s next link

• Class & Style Binding

Attribute directives link

Change the appearance or behavior of DOM elements and Angular components with attribute directives.

Building an attribute directive link

This section walks you through creating a highlight directive that sets the background color of the host element to yellow.

1. To create a directive, use the CLI command ng generate directive .

         
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         ng generate directive highlight
       

   The CLI creates src/app/highlight.directive.ts , a corresponding test file src/app/highlight.directive.spec.ts , and declares the directive class in the AppModule .

   The CLI generates the default src/app/highlight.directive.ts as follows:

   src/app/highlight.directive.ts

         
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         import { Directive } from '@angular/core';
   
   @Directive({
     selector: '[appHighlight]'
   })
   export class HighlightDirective {
   
   }
       

   The @Directive() decorator's configuration property specifies the directive's CSS attribute selector, [appHighlight] .

2. Import ElementRef from @angular/core . ElementRef grants direct access to the host DOM element through its nativeElement property.

3. Add ElementRef in the directive's constructor() to inject a reference to the host DOM element, the element to which you apply appHighlight .

4. Add logic to the HighlightDirective class that sets the background to yellow.

   src/app/highlight.directive.ts

         
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         import { Directive, ElementRef } from '@angular/core';
   
   @Directive({
     selector: '[appHighlight]'
   })
   export class HighlightDirective {
       constructor(private el: ElementRef) {
          this.el.nativeElement.style.backgroundColor = 'yellow';
       }
   }
       

      
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      <p app:Highlight>This is invalid</p>
    

Applying an attribute directive link

1. To use the HighlightDirective , add a <p> element to the HTML template with the directive as an attribute.

   src/app/app.component.html

         
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         <p appHighlight>Highlight me!</p>
       

Angular creates an instance of the HighlightDirective class and injects a reference to the <p> element into the directive's constructor, which sets the <p> element's background style to yellow.

Handling user events link

This section shows you how to detect when a user mouses into or out of the element and to respond by setting or clearing the highlight color.

1. Import HostListener from '@angular/core'.

   src/app/highlight.directive.ts (imports)

         
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         import { Directive, ElementRef, HostListener } from '@angular/core';
       

2. Add two event handlers that respond when the mouse enters or leaves, each with the @HostListener() decorator.

   src/app/highlight.directive.ts (mouse-methods)

         
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         @HostListener('mouseenter') onMouseEnter() {
     this.highlight('yellow');
   }
   
   @HostListener('mouseleave') onMouseLeave() {
     this.highlight('');
   }
   
   private highlight(color: string) {
     this.el.nativeElement.style.backgroundColor = color;
   }
       

Subscribe to events of the DOM element that hosts an attribute directive, the <p> in this case, with the @HostListener() decorator.

The complete directive is as follows:

      
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      @Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {

  constructor(private el: ElementRef) { }

  @HostListener('mouseenter') onMouseEnter() {
    this.highlight('yellow');
  }

  @HostListener('mouseleave') onMouseLeave() {
    this.highlight('');
  }

  private highlight(color: string) {
    this.el.nativeElement.style.backgroundColor = color;
  }

}
    

The background color appears when the pointer hovers over the paragraph element and disappears as the pointer moves out.

Passing values into an attribute directive link

This section walks you through setting the highlight color while applying the HighlightDirective .

1. In highlight.directive.ts , import Input from @angular/core .

   src/app/highlight.directive.ts (imports)

         
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         import { Directive, ElementRef, HostListener, Input } from '@angular/core';
       

2. Add an appHighlight @Input() property.

   src/app/highlight.directive.ts

         
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         @Input() appHighlight = '';
       

   The @Input() decorator adds metadata to the class that makes the directive's appHighlight property available for binding.

3. In app.component.ts , add a color property to the AppComponent .

   src/app/app.component.ts (class)

         
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         export class AppComponent {
     color = 'yellow';
   }
       

4. To simultaneously apply the directive and the color, use property binding with the appHighlight directive selector, setting it equal to color .

   src/app/app.component.html (color)

         
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         <p [appHighlight]="color">Highlight me!</p>
       

   The [appHighlight] attribute binding performs two tasks:

   • Applies the highlighting directive to the <p> element
   • Sets the directive's highlight color with a property binding

Setting the value with user input link

This section guides you through adding radio buttons to bind your color choice to the appHighlight directive.

1. Add markup to app.component.html for choosing a color as follows:

   src/app/app.component.html (v2)

         
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         <h1>My First Attribute Directive</h1>
   
   <h2>Pick a highlight color</h2>
   <div>
     <input type="radio" name="colors" (click)="color='lightgreen'">Green
     <input type="radio" name="colors" (click)="color='yellow'">Yellow
     <input type="radio" name="colors" (click)="color='cyan'">Cyan
   </div>
   <p [appHighlight]="color">Highlight me!</p>
       

2. Revise the AppComponent.color so that it has no initial value.

   src/app/app.component.ts (class)

         
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         export class AppComponent {
     color = '';
   }
       

3. In highlight.directive.ts , revise onMouseEnter method so that it first tries to highlight with appHighlight and falls back to red if appHighlight is undefined .

   src/app/highlight.directive.ts (mouse-enter)

         
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         @HostListener('mouseenter') onMouseEnter() {
     this.highlight(this.appHighlight || 'red');
   }
       

4. Serve your application to verify that the user can choose the color with the radio buttons.

   

Binding to a second property link

This section guides you through configuring your application so the developer can set the default color.

1. Add a second Input() property to HighlightDirective called defaultColor .

   src/app/highlight.directive.ts (defaultColor)

         
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         @Input() defaultColor = '';
       

2. Revise the directive's onMouseEnter so that it first tries to highlight with the appHighlight , then with the defaultColor , and falls back to red if both properties are undefined .

   src/app/highlight.directive.ts (mouse-enter)

         
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         @HostListener('mouseenter') onMouseEnter() {
     this.highlight(this.appHighlight || this.defaultColor || 'red');
   }
       

3. To bind to the AppComponent.color and fall back to "violet" as the default color, add the following HTML. In this case, the defaultColor binding doesn't use square brackets, [] , because it is static.

   src/app/app.component.html (defaultColor)

         
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         <p [appHighlight]="color" defaultColor="violet">
     Highlight me too!
   </p>
       

   As with components, you can add multiple directive property bindings to a host element.

The default color is red if there is no default color binding. When the user chooses a color the selected color becomes the active highlight color.

Deactivating Angular processing with NgNonBindable link

To prevent expression evaluation in the browser, add ngNonBindable to the host element. ngNonBindable deactivates interpolation, directives, and binding in templates.

In the following example, the expression {{ 1 + 1 }} renders just as it does in your code editor, and does not display 2 .

      
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      <p>Use ngNonBindable to stop evaluation.</p>
<p ngNonBindable>This should not evaluate: {{ 1 + 1 }}</p>
    

Applying ngNonBindable to an element stops binding for that element's child elements. However, ngNonBindable still lets directives work on the element where you apply ngNonBindable . In the following example, the appHighlight directive is still active but Angular does not evaluate the expression {{ 1 + 1 }} .

      
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      <h3>ngNonBindable with a directive</h3>
<div ngNonBindable [appHighlight]="'yellow'">This should not evaluate: {{ 1 +1 }}, but will highlight yellow.
</div>
    

If you apply ngNonBindable to a parent element, Angular disables interpolation and binding of any sort, such as property binding or event binding, for the element's children.

Understanding binding link

In an Angular template, a binding creates a live connection between a part of the UI created from a template (a DOM element, directive, or component) and the model (the component instance to which the template belongs). This connection can be used to synchronize the view with the model, to notify the model when an event or user action takes place in the view, or both. Angular's Change Detection algorithm is responsible for keeping the view and the model in sync.

Examples of binding include:

• text interpolations
• property binding
• event binding
• two-way binding

Bindings always have two parts: a target which will receive the bound value, and a template expression which produces a value from the model.

Syntax link

Template expressions are similar to JavaScript expressions. Many JavaScript expressions are legal template expressions, with the following exceptions.

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

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

Other notable differences from JavaScript syntax include:

• No support for the bitwise operators such as | and &
• New template expression operators, such as |

Expression context link

Interpolated expressions have a context—a particular part of the application to which the expression belongs. Typically, this context is the component instance.

In the following snippet, the expression recommended and the expression itemImageUrl2 refer to properties of the AppComponent .

      
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      <h4>{{recommended}}</h4>
<img alt="item 2" [src]="itemImageUrl2">
    

An expression can also refer to properties of the template's context such as a template input variable or a template reference variable.

The following example uses a template input variable of customer .

      
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      <ul>
  <li *ngFor="let customer of customers">{{customer.name}}</li>
</ul>
    

This next example features a template reference variable, #customerInput .

      
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      <label>Type something:
  <input #customerInput>{{customerInput.value}}
</label>
    

Preventing name collisions link

The context against which an expression evaluates is the union 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, Angular applies the following precedence logic to determine the context:

1. The template variable name.
2. A name in the directive's context.
3. The component's member names.

To avoid variables shadowing variables in another context, keep variable names unique. In the following example, the AppComponent template greets the customer , Padma.

An ngFor then lists each customer in the customers array.

      
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      @Component({
  template: `
    <div>
      <!-- Hello, Padma -->
      <h1>Hello, {{customer}}</h1>
      <ul>
        <!-- Ebony and Chiho in a list-->
        <li *ngFor="let customer of customers">{{ customer.value }}</li>
      </ul>
    </div>
  `
})
class AppComponent {
  customers = [{value: 'Ebony'}, {value: 'Chiho'}];
  customer = 'Padma';
}
    

The customer within the ngFor is in the context of an <ng-template> and so refers to the customer in the customers array, in this case Ebony and Chiho. This list does not feature Padma because customer outside of the ngFor is in a different context. Conversely, customer in the <h1> doesn't include Ebony or Chiho because the context for this customer is the class and the class value for customer is Padma.

Expression best practices link

When using a template expression, follow these best practices:

• Use short expressions

Use property names or method calls whenever possible. Keep application and business logic in the component, where it is accessible to develop and test.

• Quick execution

Angular executes a template expression after every change detection cycle. Many asynchronous activities trigger change detection cycles, such as promise resolutions, HTTP results, timer events, key presses, and mouse moves.

An expression should finish quickly to keep the user experience as efficient as possible, especially on slower devices. Consider caching values when their computation requires greater resources.

No visible side effects link

According to Angular's unidirectional data flow model, a template expression should not change any application state other than the value of the target property. Reading a component value should not change some other displayed value. The view should be stable throughout a single rendering pass.

What's next link

• Property binding
• Event binding

Binding syntax link

Data binding automatically keeps your page up-to-date based on your application's state. You use data binding to specify things such as the source of an image, the state of a button, or data for a particular user.

Data binding and HTML link

Developers can customize HTML by specifying attributes with string values. In the following example, class , src , and disabled modify the <div> , <img> , and <button> elements respectively.

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

Use data binding to control things like the state of a button:

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

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

HTML attributes and DOM properties link

Angular binding distinguishes between HTML attributes and DOM properties.

Attributes initialize DOM properties and you can configure them to modify an element's behavior. Properties are features of DOM 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
      

In Angular, the only role of HTML attributes is to initialize element and directive state.

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

Example 1: an <input> link

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

      
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      <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 live example / download example especially for binding syntax.

Example 2: a disabled button link

A button's disabled property is false by default so the button is enabled.

When you add the disabled attribute, you are initializing the button's disabled property to true which disables the button.

      
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      <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 instead.

Property and attribute comparison link

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

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

The first line, which uses the disabled property, uses a boolean value. The second line, which uses the disabled attribute checks for null .

Generally, use property binding over attribute binding as a boolean value is easy to read, the syntax is shorter, and a property is more performant.

To see the disabled button example in a functioning application, see the live example / download example . This example shows you how to toggle the disabled property from the component.

Types of data binding link

Angular provides three categories of data binding according to the direction of data flow:

• From source to view
• From view to source
• In a two-way sequence of view to source to view

      
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      {{expression}} 
[target]="expression"
    

      
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      (target)="statement"
    

      
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      [(target)]="expression"
    

Binding types other than interpolation have a target name to the left of the equal sign. The target of a binding is a property or event, which you surround with square bracket ( [ ] ) characters, parenthesis ( ( ) ) characters, or both ( [( )] ) characters.

The binding punctuation of [] , () , [()] , and the prefix specify the direction of data flow.

• Use [] to bind from source to view
• Use () to bind from view to source
• Use [()] to bind in a two-way sequence of view to source to view

Place the expression or statement to the right of the equal sign within double quote ( "" ) characters. For more information see Interpolation and Template statements.

Binding types and targets link

The target of a data binding can be a property, an event, or an attribute name. Every public member of a source directive is automatically available for binding in a template expression or statement. The following table summarizes the targets for the different binding types.

      
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      <img [alt]="hero.name" [src]="heroImageUrl">
<app-hero-detail [hero]="currentHero"></app-hero-detail>
<div [ngClass]="{'special': isSpecial}"></div>
    

      
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      <button type="button" (click)="onSave()">Save</button>
<app-hero-detail (deleteRequest)="deleteHero()"></app-hero-detail>
<div (myClick)="clicked=$event" clickable>click me</div>
    

      
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      <input [(ngModel)]="name">
    

      
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      <button type="button" [attr.aria-label]="help">help</button>
    

      
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      <div [class.special]="isSpecial">Special</div>
    

      
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      <button type="button" [style.color]="isSpecial ? 'red' : 'green'">
    

Launching your app with a root module link

Prerequisites link

A basic understanding of the following:

• JavaScript Modules vs. NgModules

An NgModule describes how the application parts fit together. Every application has at least one Angular module, the root module, which must be present for bootstrapping the application on launch. By convention and by default, this NgModule is named AppModule .

When you use the Angular CLI command ng new to generate an app, the default AppModule looks like the following:

      
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      /* JavaScript imports */
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';

/* the AppModule class with the @NgModule decorator */
@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }
    

After the import statements is a class with the @NgModule decorator.

The @NgModule decorator identifies AppModule as an NgModule class. @NgModule takes a metadata object that tells Angular how to compile and launch the application.

The default application created by the Angular CLI only has one component, AppComponent , so it is in both the declarations and the bootstrap arrays.

The declarations array link

The module's declarations array tells Angular which components belong to that module. As you create more components, add them to declarations .

You must declare every component in exactly one NgModule class. If you use a component without declaring it, Angular returns an error message.

The declarations array only takes declarables. Declarables are components, directives, and pipes. All of a module's declarables must be in the declarations array. Declarables must belong to exactly one module. The compiler emits an error if you try to declare the same class in more than one module.

These declared classes are visible within the module but invisible to components in a different module, unless they are exported from this module and the other module imports this one.

An example of what goes into a declarations array follows:

      
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      declarations: [
  YourComponent,
  YourPipe,
  YourDirective
],
    

A declarable can only belong to one module, so only declare it in one @NgModule . When you need it elsewhere, import the module that contains the declarable you need.

Using directives with @NgModule link

Use the declarations array for directives. To use a directive, component, or pipe in a module, you must do a few things:

1. Export it from the file where you wrote it.
2. Import it into the appropriate module.
3. Declare it in the @NgModule declarations array.

Those three steps look like the following. In the file where you create your directive, export it. The following example, named ItemDirective is the default directive structure that the CLI generates in its own file, item.directive.ts :

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

@Directive({
  selector: '[appItem]'
})
export class ItemDirective {
// code goes here
  constructor() { }

}
    

The key point here is that you have to export it, so that you can import it elsewhere. Next, import it into the NgModule , in this example app.module.ts , with a JavaScript import statement:

      
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      import { ItemDirective } from './item.directive';
    

And in the same file, add it to the @NgModule declarations array:

      
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      declarations: [
  AppComponent,
  ItemDirective
],
    

Now you could use your ItemDirective in a component. This example uses AppModule , but you'd do it the same way for a feature module. For more about directives, see Attribute Directives and Structural Directives. You'd also use the same technique for pipes and components.

Remember, components, directives, and pipes belong to one module only. You only need to declare them once in your application because you share them by importing the necessary modules. This saves you time and helps keep your application lean.

The imports array link

The module's imports array appears exclusively in the @NgModule metadata object. It tells Angular about other NgModules that this particular module needs to function properly.

      
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      imports: [
  BrowserModule,
  FormsModule,
  HttpClientModule
],
    

This list of modules are those that export components, directives, or pipes that component templates in this module reference. In this case, the component is AppComponent , which references components, directives, or pipes in BrowserModule , FormsModule , or HttpClientModule . A component template can reference another component, directive, or pipe when the referenced class is declared in this module, or the class was imported from another module.

The providers array link

The providers array is where you list the services the application needs. When you list services here, they are available app-wide. You can scope them when using feature modules and lazy loading. For more information, see Providers.

The bootstrap array link

The application launches by bootstrapping the root AppModule , which is also referred to as an entryComponent . Among other things, the bootstrapping process creates the component(s) listed in the bootstrap array and inserts each one into the browser DOM.

Each bootstrapped component is the base of its own tree of components. Inserting a bootstrapped component usually triggers a cascade of component creations that fill out that tree.

While you can put more than one component tree on a host web page, most applications have only one component tree and bootstrap a single root component.

This one root component is usually called AppComponent and is in the root module's bootstrap array.

In a situation where you want to bootstrap a component based on an API response, or you want to mount the AppComponent in a different DOM node that doesn't match the component selector, please refer to ApplicationRef.bootstrap() documentation.

More about Angular Modules link

For more on NgModules you're likely to see frequently in applications, see Frequently Used Modules.