Introduction to components and templates link

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.

      
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      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 link

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 .

      
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      @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:

Templates and views link

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 link

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 .

      
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      <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 link

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.

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

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.

      
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      <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 link

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 ( | ).

      
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      {{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.

      
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      <!-- 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 link

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 link

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.

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

Attribute directives link

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.

      
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      <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.

Introduction to modules link

Angular applications are modular and Angular has its own modularity system called NgModules . NgModules are containers for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. They can contain components, service providers, and other code files whose scope is defined by the containing NgModule. They can import functionality that is exported from other NgModules, and export selected functionality for use by other NgModules.

Every Angular application has at least one NgModule class, the root module , which is conventionally named AppModule and resides in a file named app.module.ts . You launch your application by bootstrapping the root NgModule.

While a small application might have only one NgModule, most applications have many more feature modules . The root NgModule for an application is so named because it can include child NgModules in a hierarchy of any depth.

NgModule metadata link

An NgModule is defined by a class decorated with @NgModule() . The @NgModule() decorator is a function that takes a single metadata object, whose properties describe the module. The most important properties are as follows.

Here's a simple root NgModule definition.

      
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      import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
@NgModule({
  imports:      [ BrowserModule ],
  providers:    [ Logger ],
  declarations: [ AppComponent ],
  exports:      [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }
    

NgModules and components link

NgModules provide a compilation context for their components. A root NgModule always has a root component that is created during bootstrap but any NgModule can include any number of additional components, which can be loaded through the router or created through the template. The components that belong to an NgModule share a compilation context.

A component and its template together define a view . A component can contain a view hierarchy , which allows you to define arbitrarily complex areas of the screen that can be created, modified, and destroyed as a unit. A view hierarchy can mix views defined in components that belong to different NgModules. This is often the case, especially for UI libraries.

When you create a component, it's associated directly with a single view, called the host view . The host view can be the root of a view hierarchy, which can contain embedded views , which are in turn the host views of other components. Those components can be in the same NgModule, or can be imported from other NgModules. Views in the tree can be nested to any depth.

NgModules and JavaScript modules link

The NgModule system is different from, and unrelated to, the JavaScript (ES2015) module system for managing collections of JavaScript objects. These are complementary module systems that you can use together to write your applications.

In JavaScript each file is a module and all objects defined in the file belong to that module. The module declares some objects to be public by marking them with the export key word. Other JavaScript modules use import statements to access public objects from other modules.

      
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      import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
    

      
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      export class AppModule { }
    

Angular libraries link

Angular loads as a collection of JavaScript modules. You can think of them as library modules. Each Angular library name begins with the @angular prefix. Install them with the node package manager npm and import parts of them with JavaScript import statements.

For example, import Angular's Component decorator from the @angular/core library like this.

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

You also import NgModules from Angular libraries using JavaScript import statements. For example, the following code imports the BrowserModule NgModule from the platform-browser library.

      
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      import { BrowserModule } from '@angular/platform-browser';
    

In the example of the simple root module above, the application module needs material from within BrowserModule . To access that material, add it to the @NgModule metadata imports like this.

      
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      imports:      [ BrowserModule ],
    

In this way, you're using the Angular and JavaScript module systems together . Although it's easy to confuse the two systems, which share the common vocabulary of "imports" and "exports", you will become familiar with the different contexts in which they are used.

Next steps: tools and techniques link

After you understand the basic Angular building blocks, you can learn more about the features and tools that can help you develop and deliver Angular applications.

• Work through the Tour of Heroes tutorial to get a feel for how to fit the basic building blocks together to create a well-designed application.
• Check out the Glossary to understand Angular-specific terms and usage.
• Use the documentation to learn about key features in more depth, according to your stage of development and areas of interest.

Application architecture link

• The Main Concepts section located in the table of contents contains several topics that explain how to connect the application data in your components to your page-display templates, to create a complete interactive application.
• The NgModules guide provides in-depth information on the modular structure of an Angular application.
• The Routing and navigation guide provides in-depth information on how to construct applications that allow a user to navigate to different views within your single-page application.
• The Dependency injection guide provides in-depth information on how to construct an application such that each component class can acquire the services and objects it needs to perform its function.

Responsive programming link

The template syntax and related topics contain details about how to display your component data when and where you want it within a view, and how to collect input from users that you can respond to.

Additional pages and sections describe some basic programming techniques for Angular applications.

• Lifecycle hooks: Tap into key moments in the lifetime of a component, from its creation to its destruction, by implementing the lifecycle hook interfaces.
• Observables and event processing: How to use observables with components and services to publish and subscribe to messages of any type, such as user-interaction events and asynchronous operation results.
• Angular elements: How to package components as custom elements using Web Components, a web standard for defining new HTML elements in a framework-agnostic way.
• Forms: Support complex data entry scenarios with HTML-based input validation.
• Animations: Use Angular's animation library to animate component behavior without deep knowledge of animation techniques or CSS.

Client-server interaction link

Angular provides a framework for single-page applications, where most of the logic and data resides on the client. Most applications still need to access a server using the HttpClient to access and save data. For some platforms and applications, you might also want to use the PWA (Progressive Web App) model to improve the user experience.

• HTTP: Communicate with a server to get data, save data, and invoke server-side actions with an HTTP client.
• Server-side rendering: Angular Universal generates static application pages on the server through server-side rendering (SSR). This allows you to run your Angular application on the server in order to improve performance and show the first page quickly on mobile and low-powered devices, and also facilitate web crawlers.
• Service workers and PWA: Use a service worker to reduce dependency on the network and significantly improve the user experience.
• Web workers: Learn how to run CPU-intensive computations in a background thread.

Support for the development cycle link

• CLI Command Reference: The Angular CLI is a command-line tool that you use to create projects, generate application and library code, and perform a variety of ongoing development tasks such as testing, bundling, and deployment.
• Compilation: Angular provides just-in-time (JIT) compilation for the development environment, and ahead-of-time (AOT) compilation for the production environment.
• Testing platform: Run unit tests on your application parts as they interact with the Angular framework.
• Deployment: Learn techniques for deploying your Angular application to a remote server.
• Security guidelines: Learn about Angular's built-in protections against common web-application vulnerabilities and attacks such as cross-site scripting attacks.
• Internationalization: Make your application available in multiple languages with Angular's internationalization (i18n) tools.
• Accessibility: Make your application accessible to all users.

File structure, configuration, and dependencies link

• Workspace and file structure: Understand the structure of Angular workspace and project folders.
• Building and serving: Learn to define different build and proxy server configurations for your project, such as development, staging, and production.
• npm packages: The Angular Framework, Angular CLI, and components used by Angular applications are packaged as npm packages and distributed using the npm registry. The Angular CLI creates a default package.json file, which specifies a starter set of packages that work well together and jointly support many common application scenarios.
• TypeScript configuration: TypeScript is the primary language for Angular application development.
• Browser support: Make your applications compatible across a wide range of browsers.

Extending Angular link

• Angular libraries: Learn about using and creating re-usable libraries.
• Schematics: Learn about customizing and extending the CLI's generation capabilities.
• CLI builders: Learn about customizing and extending the CLI's ability to apply tools to perform complex tasks, such as building and testing applications.

Introduction to services and dependency injection link

Service is a broad category encompassing any value, function, or feature that an application needs. A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.

Angular distinguishes components from services to increase modularity and reusability.

Ideally, a component's job is to enable only the user experience. A component should present properties and methods for data binding to mediate between the view and the application logic. The view is what the template renders and the application logic is what includes the notion of a model .

A component should use services for tasks that don't involve the view or application logic. Services are good for tasks such as fetching data from the server, validating user input, or logging directly to the console. By defining such processing tasks in an injectable service class , you make those tasks available to any component. You can also make your application more adaptable by injecting different providers of the same kind of service, as appropriate in different circumstances.

Angular doesn't enforce these principles. Instead, Angular helps you follow these principles by making it easy to factor your application logic into services. In Angular, dependency injection makes those services available to components.

Service examples link

Here's an example of a service class that logs to the browser console.

      
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      export class Logger {
  log(msg: any)   { console.log(msg); }
  error(msg: any) { console.error(msg); }
  warn(msg: any)  { console.warn(msg); }
}
    

Services can depend on other services. For example, here's a HeroService that depends on the Logger service, and also uses BackendService to get heroes. That service in turn might depend on the HttpClient service to fetch heroes asynchronously from a server.

      
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      export class HeroService {
  private heroes: Hero[] = [];

  constructor(
    private backend: BackendService,
    private logger: Logger) { }

  getHeroes() {
    this.backend.getAll(Hero).then( (heroes: Hero[]) => {
      this.logger.log(`Fetched ${heroes.length} heroes.`);
      this.heroes.push(...heroes); // fill cache
    });
    return this.heroes;
  }
}
    

Dependency injection (DI) link

Dependency injection (DI) is the part of the Angular framework that provides components with access to services and other resources. Angular provides the ability for you to inject a service into a component to give that component access to the service.

The @Injectable() decorator defines a class as a service in Angular and allows Angular to inject it into a component as a dependency . Likewise, the @Injectable() decorator indicates that a component, class, pipe, or NgModule has a dependency on a service.

• The injector is the main mechanism. Angular creates an application-wide injector for you during the bootstrap process, and additional injectors as needed. You don't have to create injectors.

• An injector creates dependencies and maintains a container of dependency instances that it reuses, if possible.

• A provider is an object that tells an injector how to obtain or create a dependency

For any dependency that you need in your app, you must register a provider with the application's injector, so that the injector can use the provider to create new instances. For a service, the provider is typically the service class itself.

When Angular creates a new instance of a component class, it determines which services or other dependencies that component needs by looking at the constructor parameter types. For example, the constructor of HeroListComponent needs HeroService .

      
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      constructor(private service: HeroService) { }
    

When Angular discovers that a component depends on a service, it first checks if the injector has any existing instances of that service. If a requested service instance doesn't yet exist, the injector makes one using the registered provider and adds it to the injector before returning the service to Angular.

When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments.

The process of HeroService injection looks something like this.

Providing services link

You must register at least one provider of any service you are going to use. The provider can be part of the service's own metadata, making that service available everywhere, or you can register providers with specific modules or components. You register providers in the metadata of the service (in the @Injectable() decorator), or in the @NgModule() or @Component() metadata

• By default, the Angular CLI command ng generate service registers a provider with the root injector for your service by including provider metadata in the @Injectable() decorator. The tutorial uses this method to register the provider of HeroService class definition.

        
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        @Injectable({
   providedIn: 'root',
  })
      

  When you provide the service at the root level, Angular creates a single, shared instance of HeroService and injects it into any class that asks for it. Registering the provider in the @Injectable() metadata also allows Angular to optimize an app by removing the service from the compiled application if it isn't used, a process known as tree-shaking .

• When you register a provider with a specific NgModule, the same instance of a service is available to all components in that NgModule. To register at this level, use the providers property of the @NgModule() decorator.

        
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        @NgModule({
    providers: [
    BackendService,
    Logger
   ],
   …
  })
      

• When you register a provider at the component level, you get a new instance of the service with each new instance of that component. At the component level, register a service provider in the providers property of the @Component() metadata.

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

        
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        @Component({
    selector:    'app-hero-list',
    templateUrl: './hero-list.component.html',
    providers:  [ HeroService ]
  })
      

For more detailed information, see the Dependency Injection section.

Introduction to Angular concepts link

Angular is a platform and framework for building single-page client applications using HTML and TypeScript. Angular is written in TypeScript. It implements core and optional functionality as a set of TypeScript libraries that you import into your applications.

The architecture of an Angular application relies on certain fundamental concepts. The basic building blocks of the Angular framework are Angular components that are organized into NgModules . NgModules collect related code into functional sets; an Angular application is defined by a set of NgModules. An application always has at least a root module that enables bootstrapping, and typically has many more feature modules .

• Components define views , which are sets of screen elements that Angular can choose among and modify according to your program logic and data
• Components use services , which provide specific functionality not directly related to views. Service providers can be injected into components as dependencies , making your code modular, reusable, and efficient.

Modules, components and services are classes that use decorators . These decorators mark their type and provide metadata that tells Angular how to use them.

• The metadata for a component class associates it with a template that defines a view. A template combines ordinary HTML with Angular directives and binding markup that allow Angular to modify the HTML before rendering it for display.

• The metadata for a service class provides the information Angular needs to make it available to components through dependency injection (DI)

An application's components typically define many views, arranged hierarchically. Angular provides the Router service to help you define navigation paths among views. The router provides sophisticated in-browser navigational capabilities.

Modules link

Angular NgModules differ from and complement JavaScript (ES2015) modules. An NgModule declares a compilation context for a set of components that is dedicated to an application domain, a workflow, or a closely related set of capabilities. An NgModule can associate its components with related code, such as services, to form functional units.

Every Angular application has a root module , conventionally named AppModule , which provides the bootstrap mechanism that launches the application. An application typically contains many functional modules.

Like JavaScript modules, NgModules can import functionality from other NgModules, and allow their own functionality to be exported and used by other NgModules. For example, to use the router service in your app, you import the Router NgModule.

Organizing your code into distinct functional modules helps in managing development of complex applications, and in designing for reusability. In addition, this technique lets you take advantage of lazy-loading —that is, loading modules on demand— to minimize the amount of code that needs to be loaded at startup.

Components link

Every Angular application has at least one component, the root component that connects a component hierarchy with the page document object model (DOM). Each component defines a class that contains application data and logic, and is associated with an HTML template that defines a view to be displayed in a target environment.

The @Component() decorator identifies the class immediately below it as a component, and provides the template and related component-specific metadata.

Templates, directives, and data binding link

A template combines HTML with Angular markup that can modify HTML elements before they are displayed. Template directives provide program logic, and binding markup connects your application data and the DOM. There are two types of data binding:

Before a view is displayed, Angular evaluates the directives and resolves the binding syntax in the template to modify the HTML elements and the DOM, according to your program data and logic. Angular supports two-way data binding , meaning that changes in the DOM, such as user choices, are also reflected in your program data.

Your templates can use pipes to improve the user experience by transforming values for display. For example, use pipes to display dates and currency values that are appropriate for a user's locale. Angular provides predefined pipes for common transformations, and you can also define your own pipes.

Services and dependency injection link

For data or logic that isn't associated with a specific view, and that you want to share across components, you create a service class. A service class definition is immediately preceded by the @Injectable() decorator. The decorator provides the metadata that allows other providers to be injected as dependencies into your class.

Dependency injection (DI) lets you keep your component classes lean and efficient. They don't fetch data from the server, validate user input, or log directly to the console; they delegate such tasks to services.

Routing link

The Angular Router NgModule provides a service that lets you define a navigation path among the different application states and view hierarchies in your application. It is modeled on the familiar browser navigation conventions:

• Enter a URL in the address bar and the browser navigates to a corresponding page
• Click links on the page and the browser navigates to a new page
• Click the browser's back and forward buttons and the browser navigates backward and forward through the history of pages you've seen

The router maps URL-like paths to views instead of pages. When a user performs an action, such as clicking a link, that would load a new page in the browser, the router intercepts the browser's behavior, and shows or hides view hierarchies.

If the router determines that the current application state requires particular functionality, and the module that defines it hasn't been loaded, the router can lazy-load the module on demand.

The router interprets a link URL according to your application's view navigation rules and data state. You can navigate to new views when the user clicks a button or selects from a drop box, or in response to some other stimulus from any source. The router logs activity in the browser's history, so the back and forward buttons work as well.

To define navigation rules, you associate navigation paths with your components. A path uses a URL-like syntax that integrates your program data, in much the same way that template syntax integrates your views with your program data. You can then apply program logic to choose which views to show or to hide, in response to user input and your own access rules.

What's next link

You've learned the basics about the main building blocks of an Angular application. The following diagram shows how these basic pieces are related.

• Together, a component and template define an Angular view
  • A decorator on a component class adds the metadata, including a pointer to the associated template
  • Directives and binding markup in a component's template modify views based on program data and logic
• The dependency injector provides services to a component, such as the router service that lets you define navigation among views

Each of these subjects is introduced in more detail in the following pages.

• Introduction to Modules
• Introduction to Components
  • Templates and views
  • Component metadata
  • Data binding
  • Directives
  • Pipes
• Introduction to services and dependency injection

When you're familiar with these fundamental building blocks, you can explore them in more detail in the documentation. To learn about more tools and techniques that are available to help you build and deploy Angular applications, see Next steps: tools and techniques.