1. What is Angular Framework?

   Angular is a TypeScript-based open-source front-end platform that makes it easy to build applications with in web/mobile/desktop. The major features of this framework such as declarative templates, dependency injection, end to end tooling, and many more other features are used to ease the development.

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2. What is the difference between AngularJS and Angular?

   Angular is a completely revived component-based framework in which an application is a tree of individual components.

   Some of the major difference in tabular form

   AngularJS	Angular
   It is based on MVC architecture	This is based on Service/Controller
   It uses JavaScript to build the application	Introduced the TypeScript to write the application
   Based on controllers concept	This is a component based UI approach
   Not a mobile friendly framework	Developed considering mobile platform
   Difficulty in SEO friendly application development	Ease to create SEO friendly applications

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3. What is TypeScript?

   TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, and many other features, and compiles to plain JavaScript. It helps catch errors during development Angular built entirely in TypeScript and used as a primary language. You can install it globally as

   npm install -g typescript

   Let's see a simple example of TypeScript usage,

   function greeter(person: string) {
       return "Hello, " + person;
   }
   
   let user = "Sudheer";
   
   document.body.innerHTML = greeter(user);

   The greeter method allows only string type as argument.

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4. Write a pictorial diagram of Angular architecture?

   The main building blocks of an Angular application is shown in the below diagram

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5. What are the key components of Angular?

   Angular has the below key components,
   1. Component: These are the basic building blocks of angular application to control HTML views.
   2. Modules: An angular module is set of angular basic building blocks like component, directives, services etc. An application is divided into logical pieces and each piece of code is called as "module" which perform a single task.
   3. Templates: This represents the views of an Angular application.
   4. Services: It is used to create components which can be shared across the entire application.
   5. Metadata: This can be used to add more data to an Angular class.

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6. What are directives?

   Directives add behaviour to an existing DOM element or an existing component instance.

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

   Now this directive extends HTML element behavior with a yellow background as below

   <p myHighlight>Highlight me!</p>

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7. What are components?

   Components are the most basic UI building block of an Angular app which formed a tree of Angular components. These components are subset of directives. Unlike directives, components always have a template and only one component can be instantiated per an element in a template. Let's see a simple example of Angular component

   import { Component } from '@angular/core';
   
   @Component ({
      selector: 'my-app',
      template: ` <div>
         <h1>{{title}}</h1>
         <div>Learn Angular6 with examples</div>
      </div> `,
   })
   
   export class AppComponent {
      title: string = 'Welcome to Angular world';
   }

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8. What are the differences between Component and Directive?

   In a short note, A component(@component) is a directive-with-a-template.

   Some of the major differences are mentioned in a tabular form

   Component	Directive
   To register a component we use @Component meta-data annotation	To register directives we use @Directive meta-data annotation
   Components are typically used to create UI widgets	Directive is used to add behavior to an existing DOM element
   Component is used to break up the application into smaller components	Directive is use to design re-usable components
   Only one component can be present per DOM element	Many directives can be used per DOM element
   @View decorator or templateurl/template are mandatory	Directive doesn't use View

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9. What is a template?

   A template is a HTML view where you can display data by binding controls to properties of an Angular component. You can store your component's template in one of two places. You can define it inline using the template property, or you can define the template in a separate HTML file and link to it in the component metadata using the @Component decorator's templateUrl property.

   Using inline template with template syntax,

   import { Component } from '@angular/core';
   
   @Component ({
      selector: 'my-app',
      template: '
         <div>
            <h1>{{title}}</h1>
            <div>Learn Angular</div>
         </div>
      '
   })
   
   export class AppComponent {
      title: string = 'Hello World';
   }

   Using separate template file such as app.component.html

   import { Component } from '@angular/core';
   
   @Component ({
      selector: 'my-app',
      templateUrl: 'app/app.component.html'
   })
   
   export class AppComponent {
      title: string = 'Hello World';
   }

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10. What is a module?

    Modules are logical boundaries in your application and the application is divided into separate modules to separate the functionality of your application. Lets take an example of app.module.ts root module declared with @NgModule decorator as below,

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

    The NgModule decorator has five important(among all) options

    1. The imports option is used to import other dependent modules. The BrowserModule is required by default for any web based angular application
    2. The declarations option is used to define components in the respective module
    3. The bootstrap option tells Angular which Component to bootstrap in the application
    4. The providers option is used to configure set of injectable objects that are available in the injector of this module.
    5. The entryComponents option is a set of components dynamically loaded into the view.

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11. What are lifecycle hooks available?

    Angular application goes through an entire set of processes or has a lifecycle right from its initiation to the end of the application. The representation of lifecycle in pictorial representation as follows,

    

    The description of each lifecycle method is as below,

    1. ngOnChanges: When the value of a data bound property changes, then this method is called.
    2. ngOnInit: This is called whenever the initialization of the directive/component after Angular first displays the data-bound properties happens.
    3. ngDoCheck: This is for the detection and to act on changes that Angular can't or won't detect on its own.
    4. ngAfterContentInit: This is called in response after Angular projects external content into the component's view.
    5. ngAfterContentChecked: This is called in response after Angular checks the content projected into the component.
    6. ngAfterViewInit: This is called in response after Angular initializes the component's views and child views.
    7. ngAfterViewChecked: This is called in response after Angular checks the component's views and child views.
    8. ngOnDestroy: This is the cleanup phase just before Angular destroys the directive/component.

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12. What is metadata?

    Metadata is used to decorate a class so that it can configure the expected behavior of the class. The metadata is represented by decorators
    1. Class decorators, e.g. @Component and @NgModule

       import { NgModule, Component } from '@angular/core';
       
       @Component({
         selector: 'my-component',
         template: '<div>Class decorator</div>',
       })
       export class MyComponent {
         constructor() {
           console.log('Hey I am a component!');
         }
       }
       
       @NgModule({
         imports: [],
         declarations: [],
       })
       export class MyModule {
         constructor() {
           console.log('Hey I am a module!');
         }
       }

    2. Property decorators Used for properties inside classes, e.g. @Input and @Output

       import { Component, Input } from '@angular/core';
       
       @Component({
           selector: 'my-component',
           template: '<div>Property decorator</div>'
       })
       
       export class MyComponent {
           @Input()
           title: string;
       }

    3. Method decorators Used for methods inside classes, e.g. @HostListener

       import { Component, HostListener } from '@angular/core';
       
       @Component({
           selector: 'my-component',
           template: '<div>Method decorator</div>'
       })
       export class MyComponent {
           @HostListener('click', ['$event'])
           onHostClick(event: Event) {
               // clicked, `event` available
           }
       }

    4. Parameter decorators Used for parameters inside class constructors, e.g. @Inject, Optional

       import { Component, Inject } from '@angular/core';
       import { MyService } from './my-service';
       
       @Component({
           selector: 'my-component',
           template: '<div>Parameter decorator</div>'
       })
       export class MyComponent {
           constructor(@Inject(MyService) myService) {
               console.log(myService); // MyService
           }
       }

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13. What is angular CLI?

    Angular CLI(Command Line Interface) is a command line interface to scaffold and build angular apps using nodejs style (commonJs) modules. You need to install using below npm command,

    npm install @angular/cli@latest
    

    Below are the list of few commands, which will come handy while creating angular projects

    1. Creating New Project: ng new

    2. Generating Components, Directives & Services: ng generate/g The different types of commands would be,

       • ng generate class my-new-class: add a class to your application
       • ng generate component my-new-component: add a component to your application
       • ng generate directive my-new-directive: add a directive to your application
       • ng generate enum my-new-enum: add an enum to your application
       • ng generate module my-new-module: add a module to your application
       • ng generate pipe my-new-pipe: add a pipe to your application
       • ng generate service my-new-service: add a service to your application

    3. Running the Project: ng serve

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14. What is the difference between constructor and ngOnInit?

    TypeScript classes has a default method called constructor which is normally used for the initialization purpose. Whereas ngOnInit method is specific to Angular, especially used to define Angular bindings. Even though constructor getting called first, it is preferred to move all of your Angular bindings to ngOnInit method. In order to use ngOnInit, you need to implement OnInit interface as below,

    export class App implements OnInit{
      constructor(){
         //called first time before the ngOnInit()
      }
    
      ngOnInit(){
         //called after the constructor and called  after the first ngOnChanges()
      }
    }

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15. What is a service?

    A service is used when a common functionality needs to be provided to various modules. Services allow for greater separation of concerns for your application and better modularity by allowing you to extract common functionality out of components.

    Let's create a repoService which can be used across components,

    import { Injectable } from '@angular/core';
    import { Http } from '@angular/http';
    
    @Injectable({ // The Injectable decorator is required for dependency injection to work
      // providedIn option registers the service with a specific NgModule
      providedIn: 'root',  // This declares the service with the root app (AppModule)
    })
    export class RepoService{
      constructor(private http: Http){
      }
    
      fetchAll(){
        return this.http.get('https://api.github.com/repositories');
      }
    }

    The above service uses Http service as a dependency.

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16. What is dependency injection in Angular?

    Dependency injection (DI), is an important application design pattern in which a class asks for dependencies from external sources rather than creating them itself. Angular comes with its own dependency injection framework for resolving dependencies( services or objects that a class needs to perform its function).So you can have your services depend on other services throughout your application.

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19. What is the purpose of async pipe?

    The AsyncPipe subscribes to an observable or promise and returns the latest value it has emitted. When a new value is emitted, the pipe marks the component to be checked for changes.

    Let's take a time observable which continuously updates the view for every 2 seconds with the current time.

    @Component({
      selector: 'async-observable-pipe',
      template: `<div><code>observable|async</code>:
           Time: {{ time | async }}</div>`
    })
    export class AsyncObservablePipeComponent {
      time = new Observable(observer =>
        setInterval(() => observer.next(new Date().toString()), 2000)
      );
    }

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17. What is the template variable

    При этом данную переменную мы можем использовать только внутри шаблона.

Использование шаблонных переменных открывает нам дополнительный способ взаимодействия между родительским и дочерним компонентом.

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

@Component({
    selector: 'my-app',
    template: `<child-comp #counter></child-comp>
		<button (click)="counter.increment()">+</button>
		<button (click)="counter.decrement()">-</button>`
})
export class AppComponent { }
```

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18. What is the purpose of ngFor directive?

    We use Angular ngFor directive in the template to display each item in the list. For example, here we iterate over list of users,

    <li *ngFor="let user of users">
      {{ user }}
    </li>

    The user variable in the ngFor double-quoted instruction is a template input variable

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19. What is the purpose of ngIf directive?

    Sometimes an app needs to display a view or a portion of a view only under specific circumstances. The Angular ngIf directive inserts or removes an element based on a truthy/falsy condition. Let's take an example to display a message if the user age is more than 18,

    <p *ngIf="user.age > 18">You are not eligible for student pass!</p>

    Note: Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in the larger projects with many data bindings.

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20. What happens if you use script tag inside template?

    Angular recognizes the value as unsafe and automatically sanitizes it, which removes the script tag but keeps safe content such as the text content of the script tag. This way it eliminates the risk of script injection attacks. If you still use it then it will be ignored and a warning appears in the browser console.

    Let's take an example of innerHtml property binding which causes XSS vulnerability,

    export class InnerHtmlBindingComponent {
      // For example, a user/attacker-controlled value from a URL.
      htmlSnippet = 'Template <script>alert("0wned")</script> <b>Syntax</b>';
    }

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21. What is interpolation?

    Interpolation is a special syntax that Angular converts into property binding. It’s a convenient alternative to property binding. It is represented by double curly braces({{}}). The text between the braces is often the name of a component property. Angular replaces that name with the string value of the corresponding component property.

    Let's take an example,

    <h3>
      {{title}}
      <img src="{{url}}" style="height:30px">
    </h3>

    In the example above, Angular evaluates the title and url properties and fills in the blanks, first displaying a bold application title and then a URL.

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22. How do you categorize data binding types?

    Binding types can be grouped into three categories distinguished by the direction of data flow. They are listed as below,

    1. From the source-to-view
    2. From view-to-source
    3. View-to-source-to-view

    The possible binding syntax can be tabularized as below,

    Data direction	Syntax	Type
    From the source-to-view(One-way)	1. {{expression}} 2. [target]="expression" 3. bind-target="expression"	Interpolation, Property, Attribute, Class, Style
    From view-to-source(One-way)	1. (target)="statement" 2. on-target="statement"	Event
    View-to-source-to-view(Two-way)	1. [(target)]="expression" 2. bindon-target="expression"	Two-way

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23. What are pipes?

    A pipe takes in data as input and transforms it to a desired output. For example, let us take a pipe to transform a component's birthday property into a human-friendly date using date pipe.

    import { Component } from '@angular/core';
    
    @Component({
      selector: 'app-birthday',
      template: `<p>Birthday is {{ birthday | date }}</p>`
    })
    export class BirthdayComponent {
      birthday = new Date(1987, 6, 18); // June 18, 1987
    }

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24. What is a parameterized pipe?

    A pipe can accept any number of optional parameters to fine-tune its output. The parameterized pipe can be created by declaring the pipe name with a colon ( : ) and then the parameter value. If the pipe accepts multiple parameters, separate the values with colons. Let's take a birthday example with a particular format(dd/MM/yyyy):

    import { Component } from '@angular/core';
    
        @Component({
          selector: 'app-birthday',
          template: `<p>Birthday is {{ birthday | date:'dd/MM/yyyy'}}</p>` // 18/06/1987
        })
        export class BirthdayComponent {
          birthday = new Date(1987, 6, 18);
        }

    Note: The parameter value can be any valid template expression, such as a string literal or a component property.

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25. How do you chain pipes?

    You can chain pipes together in potentially useful combinations as per the needs. Let's take a birthday property which uses date pipe(along with parameter) and uppercase pipes as below

    import { Component } from '@angular/core';
    
            @Component({
              selector: 'app-birthday',
              template: `<p>Birthday is {{  birthday | date:'fullDate' | uppercase}} </p>` // THURSDAY, JUNE 18, 1987
            })
            export class BirthdayComponent {
              birthday = new Date(1987, 6, 18);
            }

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26. Give an example of custom pipe?

    You can create custom reusable pipes for the transformation of existing value. For example, let us create a custom pipe for finding file size based on an extension,

      import { Pipe, PipeTransform } from '@angular/core';
    
      @Pipe({name: 'customFileSizePipe'})
      export class FileSizePipe implements PipeTransform {
        transform(size: number, extension: string = 'MB'): string {
          return (size / (1024 * 1024)).toFixed(2) + extension;
        }
      }

    Now you can use the above pipe in template expression as below,

       template: `
          <h2>Find the size of a file</h2>
          <p>Size: {{288966 | customFileSizePipe: 'GB'}}</p>
        `

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27. What is the difference between pure and impure pipe?

    A pure pipe is only called when Angular detects a change in the value or the parameters passed to a pipe. For example, any changes to a primitive input value (String, Number, Boolean, Symbol) or a changed object reference (Date, Array, Function, Object). An impure pipe is called for every change detection cycle no matter whether the value or parameters changes. i.e, An impure pipe is called often, as often as every keystroke or mouse-move.

for Example (Pure pipe) @Pipe({ name: 'filterPipe', pure: false/true })

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28. What is a bootstrapping module?

    Every application has at least one Angular module, the root module that you bootstrap to launch the application is called as bootstrapping module. It is commonly known as AppModule. The default structure of AppModule generated by AngularCLI would be as follows,

    ```javascript
    /* JavaScript imports */
    import { BrowserModule } from '@angular/platform-browser';
    import { NgModule } from '@angular/core';
    import { FormsModule } from '@angular/forms';
    import { HttpClientModule } from '@angular/common/http';
    
    import { AppComponent } from './app.component';
    
    /* the AppModule class with the @NgModule decorator */
    @NgModule({
      declarations: [
        AppComponent
      ],
      imports: [
        BrowserModule,
        FormsModule,
        HttpClientModule
      ],
      providers: [],
      bootstrap: [AppComponent]
    })
    export class AppModule { }
    ```
    

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29. What is HttpClient and its benefits?

    Most of the Front-end applications communicate with backend services over HTTP protocol using either XMLHttpRequest interface or the fetch() API. Angular provides a simplified client HTTP API known as HttpClient which is based on top of XMLHttpRequest interface. This client is avaialble from @angular/common/http package. You can import in your root module as below,

    import { HttpClientModule } from '@angular/common/http';

    The major advantages of HttpClient can be listed as below,

    1. Contains testability features
    2. Provides typed request and response objects
    3. Intercept request and response
    4. Supports Observalbe APIs
    5. Supports streamlined error handling

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30. How can you read full response?

    The response body doesn't may not return full response data because sometimes servers also return special headers or status code which which are important for the application workflow. Inorder to get full response, you should use observe option from HttpClient,

    getUserResponse(): Observable<HttpResponse<User>> {
      return this.http.get<User>(
        this.userUrl, { observe: 'response' });
    }

    Now HttpClient.get() method returns an Observable of typed HttpResponse rather than just the JSON data.

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31. How do you perform Error handling?

    If the request fails on the server or failed to reach the server due to network issues then HttpClient will return an error object instead of a successful reponse. In this case, you need to handle in the component by passing error object as a second callback to subscribe() method.

    Let's see how it can be handled in the component with an example,

    fetchUser() {
      this.userService.getProfile()
        .subscribe(
          (data: User) => this.userProfile = { ...data }, // success path
          error => this.error = error // error path
        );
    }

    It is always a good idea to give the user some meaningful feedback instead of displaying the raw error object returned from HttpClient.

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32. What is RxJS?

    RxJS is a library for composing asynchronous and callback-based code in a functional, reactive style using Observables. Many APIs such as HttpClient produce and consume RxJS Observables and also uses operators for processing observables.

    For example, you can import observables and operators for using HttpClient as below,

    import { Observable, throwError } from 'rxjs';
    import { catchError, retry } from 'rxjs/operators';

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33. What is subscribing?

    An Observable instance begins publishing values only when someone subscribes to it. So you need to subscribe by calling the subscribe() method of the instance, passing an observer object to receive the notifications.

    Let's take an example of creating and subscribing to a simple observable, with an observer that logs the received message to the console.

    Creates an observable sequence of 5 integers, starting from 1
    const source = range(1, 5);
    
    // Create observer object
    const myObserver = {
      next: x => console.log('Observer got a next value: ' + x),
      error: err => console.error('Observer got an error: ' + err),
      complete: () => console.log('Observer got a complete notification'),
    };
    
    // Execute with the observer object and Prints out each item
    source.subscribe(myObserver);
    // => Observer got a next value: 1
    // => Observer got a next value: 2
    // => Observer got a next value: 3
    // => Observer got a next value: 4
    // => Observer got a next value: 5
    // => Observer got a complete notification

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34. What is an observable?

    An Observable is a unique Object similar to a Promise that can help manage async code. Observables are not part of the JavaScript language so we need to rely on a popular Observable library called RxJS. The observables are created using new keyword.

Observables are declarative which provide support for passing messages between publishers and subscribers in your application. They are mainly used for event handling, asynchronous programming, and handling multiple values. In this case, you define a function for publishing values, but it is not executed until a consumer subscribes to it. The subscribed consumer then receives notifications until the function completes, or until they unsubscribe.

Let see the simple example of observable,
```javascript
import { Observable } from 'rxjs';

const observable = new Observable(observer => {
  setTimeout(() => {
    observer.next('Hello from a Observable!');
  }, 2000);
});
```

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35. What is an observer?

    Observer is an interface for a consumer of push-based notifications delivered by an Observable. It has below structure,

    interface Observer<T> {
      closed?: boolean;
      next: (value: T) => void;
      error: (err: any) => void;
      complete: () => void;
    }

    A handler that implements the Observer interface for receiving observable notifications will be passed as a parameter for observable as below,

    myObservable.subscribe(myObserver);

    Note: If you don't supply a handler for a notification type, the observer ignores notifications of that type.

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36. What is the difference between promise and observable?

    Below are the list of differences between promise and observable,

    Observable	Promise
    Declarative: Computation does not start until subscription so that they can be run whenever you need the result	Execute immediately on creation
    Provide multiple values over time	Provide only one
    Subscribe method is used for error handling which makes centralized and predictable error handling	Push errors to the child promises
    Provides chaining and subscription to handle complex applications	Uses only .then() clause

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37. What is multicasting?

    Multi-casting is the practice of broadcasting to a list of multiple subscribers in a single execution.

    Let's demonstrate the multi-casting feature,

    var source = Rx.Observable.from([1, 2, 3]);
    var subject = new Rx.Subject();
    var multicasted = source.multicast(subject);
    
    // These are, under the hood, `subject.subscribe({...})`:
    multicasted.subscribe({
      next: (v) => console.log('observerA: ' + v)
    });
    multicasted.subscribe({
      next: (v) => console.log('observerB: ' + v)
    });
    
    // This is, under the hood, `s

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38. How do you perform error handling in observables?

    You can handle errors by specifying an error callback on the observer instead of relying on try/catch which are ineffective in asynchronous environment.

    For example, you can define error callback as below,

    myObservable.subscribe({
      next(num) { console.log('Next num: ' + num)},
      error(err) { console.log('Received an errror: ' + err)}
    });

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39. What is the short hand notation for subscribe method?

    The subscribe() method can accept callback function definitions in line, for next, error, and complete handlers is known as short hand notation or Subscribe method with positional arguments.

    For example, you can define subscribe method as below,

    myObservable.subscribe(
      x => console.log('Observer got a next value: ' + x),
      err => console.error('Observer got an error: ' + err),
      () => console.log('Observer got a complete notification')
    );

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40. What are the utility functions provided by RxJS?

    The RxJS library also provides below utility functions for creating and working with observables.

    1. Converting existing code for async operations into observables
    2. Iterating through the values in a stream
    3. Mapping values to different types
    4. Filtering streams
    5. Composing multiple streams

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41. What are observable creation functions?

    RxJS provides creation functions for the process of creating observables from things such as promises, events, timers and Ajax requests. Let us explain each of them with an example,
    1. Create an observable from a promise

       import { from } from 'rxjs'; // from function
       const data = from(fetch('/api/endpoint')); //Created from Promise
       data.subscribe({
        next(response) { console.log(response); },
        error(err) { console.error('Error: ' + err); },
        complete() { console.log('Completed'); }
       });

    2. Create an observable that creates an AJAX request

       import { ajax } from 'rxjs/ajax'; // ajax function
       const apiData = ajax('/api/data'); // Created from AJAX request
       // Subscribe to create the request
       apiData.subscribe(res => console.log(res.status, res.response));

    3. Create an observable from a counter

       import { interval } from 'rxjs'; // interval function
       const secondsCounter = interval(1000); // Created from Counter value
       secondsCounter.subscribe(n =>
         console.log(`Counter value: ${n}`));

    4. Create an observable from an event

       import { fromEvent } from 'rxjs';
       const el = document.getElementById('custom-element');
       const mouseMoves = fromEvent(el, 'mousemove');
       const subscription = mouseMoves.subscribe((e: MouseEvent) => {
         console.log(`Coordnitaes of mouse pointer: ${e.clientX} * ${e.clientY}`);
         });

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42. What will happen if you do not supply handler for observer?

    Normally an observer object can define any combination of next, error and complete notification type handlers. If you don't supply a handler for a notification type, the observer just ignores notifications of that type.

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43. What are angular elements?

    Angular elements are Angular components packaged as custom elements(a web standard for defining new HTML elements in a framework-agnostic way). Angular Elements hosts an Angular component, providing a bridge between the data and logic defined in the component and standard DOM APIs, thus, providing a way to use Angular components in non-Angular environments.

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44. What are custom elements?

    Custom elements (or Web Components) are a Web Platform feature which extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code. The browser maintains a CustomElementRegistry of defined custom elements, which maps an instantiable JavaScript class to an HTML tag. Currently this feature is supported by Chrome, Firefox, Opera, and Safari, and available in other browsers through polyfills.

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45. Do I need to bootstrap custom elements?

    No, custom elements bootstrap (or start) automatically when they are added to the DOM, and are automatically destroyed when removed from the DOM. Once a custom element is added to the DOM for any page, it looks and behaves like any other HTML element, and does not require any special knowledge of Angular.

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46. Explain how custom elements works internally?

    Below are the steps in an order about custom elements functionality,

    1. App registers custom element with browser: Use the createCustomElement() function to convert a component into a class that can be registered with the browser as a custom element.
    2. App adds custom element to DOM: Add custom element just like a built-in HTML element directly into the DOM.
    3. Browser instantiate component based class: Browser creates an instance of the registered class and adds it to the DOM.
    4. Instance provides content with data binding and change detection: The content with in template is rendered using the component and DOM data. The flow chart of the custom elements functionality would be as follows,

    

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47. How to transfer components to custom elements?

    Transforming components to custom elements involves two major steps,

    1. Build custom element class: Angular provides the createCustomElement() function for converting an Angular component (along with its dependencies) to a custom element. The conversion process implements NgElementConstructor interface, and creates a constructor class which is used to produce a self-bootstrapping instance of Angular component.
    2. Register element class with browser: It uses customElements.define() JS function, to register the configured constructor and its associated custom-element tag with the browser's CustomElementRegistry. When the browser encounters the tag for the registered element, it uses the constructor to create a custom-element instance.

    The detailed structure would be as follows,

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48. What are the mapping rules between Angular component and custom element?

    The Component properties and logic maps directly into HTML attributes and the browser's event system. Let us describe them in two steps,
    1. The createCustomElement() API parses the component input properties with corresponding attributes for the custom element. For example, component @Input('myInputProp') converted as custom element attribute my-input-prop.
    2. The Component outputs are dispatched as HTML Custom Events, with the name of the custom event matching the output name. For example, component @Output() valueChanged = new EventEmitter() converted as custom element with dispatch event as "valueChanged".

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49. How do you define typings for custom elements?

    You can use the NgElement and WithProperties types exported from @angular/elements.

    Let's see how it can be applied by comparing with Angular component.

    1. The simple container with input property would be as below,

       @Component(...)
       class MyContainer {
         @Input() message: string;
       }

    2. After applying types typescript validates input value and their types,

       const container = document.createElement('my-container') as NgElement & WithProperties<{message: string}>;
       container.message = 'Welcome to Angular elements!';
       container.message = true;  // <-- ERROR: TypeScript knows this should be a string.
       container.greet = 'News';  // <-- ERROR: TypeScript knows there is no `greet` property on `container`.
       

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50. What are dynamic components?

    Dynamic components are the components in which components location in the application is not defined at build time.i.e, They are not used in any angular template. But the component is instantiated and placed in the application at runtime.

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51. What are the various kinds of directives?

    There are mainly three kinds of directives,
    1. Components — These are directives with a template.
    2. Structural directives — These directives change the DOM layout by adding and removing DOM elements.
    3. Attribute directives — These directives change the appearance or behavior of an element, component, or another directive.

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52. Give an example for attribute directives?

    Let's take simple highlighter behavior as a example directive for DOM element. You can create and apply the attribute directive using below steps,

    1. Create HighlightDirective class with the file name src/app/highlight.directive.ts. In this file, we need to import Directive from core library to apply the metadata and ElementRef in the directive's constructor to inject a reference to the host DOM element ,

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

    2. Apply the attribute directive as an attribute to the host element(for example,

       )

       <p appHighlight>Highlight me!</p>

    3. Run the application to see the highlight behavior on paragraph element

       ng serve

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53. What is Angular Router?

    Angular Router is a mechanism in which navigation happens from one view to the next as users perform application tasks. It borrows the concepts or model of browser's application navigation.

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54. What is the purpose of base href tag?

    The routing application should add element to the index.html as the first child in the tag in order to indicate how to compose navigation URLs. If app folder is the application root then you can set the href value as below

    <base href="/">

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55. What are active router links?

    RouterLinkActive is a directive that toggles css classes for active RouterLink bindings based on the current RouterState. i.e, the Router will add CSS classes when this link is active and and remove when the link is inactive. For example, you can add them to RouterLinks as below

    <h1>Angular Router</h1>
    <nav>
      <a routerLink="/todosList" routerLinkActive="active">List of todos</a>
      <a routerLink="/completed" routerLinkActive="active">Completed todos</a>
    </nav>
    <router-outlet></router-outlet>

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56. What is HostListener?

Декоратор @HostListener позволяет связать события DOM и методы директивы. В частности, в декоратор передается название события, по которому будет вызываться метод. В данном случае мы привязываем события mouseenter (наведения указателя мыши на элемент) и mouseleave (уведение указателя мыши с элемента) к методу setFontWeight(), который устанавливает стилевое свойство font-weight у элемента. Если мы наводим на элемент, то устанавливается выделение жирным. При отводе мыши выделение сбрасывается.

```typescript

@Directive({ selector: '[bold]' }) export class BoldDirective{

constructor(private element: ElementRef, private renderer: Renderer2){
     
    this.renderer.setStyle(this.element.nativeElement, "cursor", "pointer");
}
 
@HostListener("mouseenter") onMouseEnter() {
    this.setFontWeight("bold");
}

@HostListener("mouseleave") onMouseLeave() {
    this.setFontWeight("normal");
}

private setFontWeight(val: string) {
    this.renderer.setStyle(this.element.nativeElement, "font-weight", val);
}

} ```

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57. What is HostBinding?

    HostBinding позволяет связать обычное свойство класса со свойством элемента, к которому применяется директива

@Directive({ selector: '[bold]' }) export class BoldDirective{

private fontWeight = "normal";
 
@HostBinding("style.fontWeight") get getFontWeight(){
     
    return this.fontWeight;
}
 
@HostBinding("style.cursor") get getCursor(){
    return "pointer";
}
 
@HostListener("mouseenter") onMouseEnter() {
    this.fontWeight ="bold";
}

@HostListener("mouseleave") onMouseLeave() {
    this.fontWeight = "normal";
}
```
Инструкция @HostBinding("style.fontWeight") get getFontWeight() связывает со свойством "style.fontWeight" значение, которое возвращается этим геттером getFontWeight. А он возвращает значение свойства fontWeight, которое также меняется при наведении указателя мыши.

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58. What are router links?

    The RouterLink is a directive on the anchor tags give the router control over those elements. Since the navigation paths are fixed, you can assign string values to router-link directive as below,

    <h1>Angular Router</h1>
    <nav>
      <a routerLink="/todosList" >List of todos</a>
      <a routerLink="/completed" >Completed todos</a>
    </nav>
    <router-outlet></router-outlet>

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59. What are active router links?

    RouterLinkActive is a directive that toggles css classes for active RouterLink bindings based on the current RouterState. i.e, the Router will add CSS classes when this link is active and and remove when the link is inactive. For example, you can add them to RouterLinks as below

    <h1>Angular Router</h1>
    <nav>
      <a routerLink="/todosList" routerLinkActive="active">List of todos</a>
      <a routerLink="/completed" routerLinkActive="active">Completed todos</a>
    </nav>
    <router-outlet></router-outlet>

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60. What is router state?

    RouterState is a tree of activated routes. Every node in this tree knows about the "consumed" URL segments, the extracted parameters, and the resolved data. You can access the current RouterState from anywhere in the application using the Router service and the routerState property.

    @Component({templateUrl:'template.html'})
    class MyComponent {
      constructor(router: Router) {
        const state: RouterState = router.routerState;
        const root: ActivatedRoute = state.root;
        const child = root.firstChild;
        const id: Observable<string> = child.params.map(p => p.id);
        //...
      }
    }

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61. What are router events?

    During each navigation, the Router emits navigation events through the Router.events property allowing you to track the lifecycle of the route.

    The sequence of router events is as below,

    1. NavigationStart,
    2. RouteConfigLoadStart,
    3. RouteConfigLoadEnd,
    4. RoutesRecognized,
    5. GuardsCheckStart,
    6. ChildActivationStart,
    7. ActivationStart,
    8. GuardsCheckEnd,
    9. ResolveStart,
    10. ResolveEnd,
    11. ActivationEnd
    12. ChildActivationEnd
    13. NavigationEnd,
    14. NavigationCancel,
    15. NavigationError
    16. Scroll

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62. What is activated route?

    ActivatedRoute contains the information about a route associated with a component loaded in an outlet. It can also be used to traverse the router state tree. The ActivatedRoute will be injected as a router service to access the information. In the below example, you can access route path and parameters,

    @Component({...})
    class MyComponent {
      constructor(route: ActivatedRoute) {
        const id: Observable<string> = route.params.pipe(map(p => p.id));
        const url: Observable<string> = route.url.pipe(map(segments => segments.join('')));
        // route.data includes both `data` and `resolve`
        const user = route.data.pipe(map(d => d.user));
      }
    }

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63. How do you define routes?

    A router must be configured with a list of route definitions. You configures the router with routes via the RouterModule.forRoot() method, and adds the result to the AppModule's imports array.

     const appRoutes: Routes = [
      { path: 'todo/:id',      component: TodoDetailComponent },
      {
        path: 'todos',
        component: TodosListComponent,
        data: { title: 'Todos List' }
      },
      { path: '',
        redirectTo: '/todos',
        pathMatch: 'full'
      },
      { path: '**', component: PageNotFoundComponent }
    ];
    
    @NgModule({
      imports: [
        RouterModule.forRoot(
          appRoutes,
          { enableTracing: true } // <-- debugging purposes only
        )
        // other imports here
      ],
      ...
    })
    export class AppModule { }

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64. What is the purpose of Wildcard route?

    If the URL doesn't match any predefined routes then it causes the router to throw an error and crash the app. In this case, you can use wildcard route. A wildcard route has a path consisting of two asterisks to match every URL.

    For example, you can define PageNotFoundComponent for wildcard route as below

    { path: '**', component: PageNotFoundComponent }

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65. Do I need a Routing Module always?

    No, the Routing Module is a design choice. You can skip routing Module (for example, AppRoutingModule) when the configuration is simple and merge the routing configuration directly into the companion module (for example, AppModule). But it is recommended when the configuration is complex and includes specialized guard and resolver services.

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66. What is Angular Universal?

    Angular Universal is a server-side rendering module for Angular applications in various scenarios. This is a community driven project and available under @angular/platform-server package. Recently Angular Universal is integrated with Angular CLI.

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67. What are different types of compilation in Angular?

    Angular offers two ways to compile your application,
    1. Just-in-Time (JIT)
    2. Ahead-of-Time (AOT)

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68. What is JIT?

    Just-in-Time (JIT) is a type of compilation that compiles your app in the browser at runtime. JIT compilation is the default when you run the ng build (build only) or ng serve (build and serve locally) CLI commands. i.e, the below commands used for JIT compilation,

    ng build
    ng serve

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69. What is AOT?

    Ahead-of-Time (AOT) is a type of compilation that compiles your app at build time. For AOT compilation, include the --aot option with the ng build or ng serve command as below,

    ng build --aot
    ng serve --aot

    Note: The ng build command with the --prod meta-flag (ng build --prod) compiles with AOT by default.

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70. Why do we need compilation process?

    The Angular components and templates cannot be understood by the browser directly. Due to that Angular applications require a compilation process before they can run in a browser. For example, In AOT compilation, both Angular HTML and TypeScript code converted into efficient JavaScript code during the build phase before browser runs it.

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71. What are the advantages with AOT?

    Below are the list of AOT benefits,

    1. Faster rendering: The browser downloads a pre-compiled version of the application. So it can render the application immediately without compiling the app.
    2. Fewer asynchronous requests: It inlines external HTML templates and CSS style sheets within the application javascript which eliminates separate ajax requests.
    3. Smaller Angular framework download size: Doesn't require downloading the Angular compiler. Hence it dramatically reduces the application payload.
    4. Detect template errors earlier: Detects and reports template binding errors during the build step itself
    5. Better security: It compiles HTML templates and components into JavaScript. So there won't be any injection attacks.

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72. What is Non null type assertion operator?

    You can use the non-null type assertion operator to suppress the Object is possibly 'undefined' error. In the following example, the user and contact properties are always set together, implying that contact is always non-null if user is non-null. The error is suppressed in the example by using contact!.email.

    @Component({
      selector: 'my-component',
      template: '<span *ngIf="user"> {{user.name}} contacted through {{contact!.email}} </span>'
    })
    class MyComponent {
      user?: User;
      contact?: Contact;
    
      setData(user: User, contact: Contact) {
        this.user = user;
        this.contact = contact;
      }
    }

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73. How do you describe various dependencies in angular application?

    The dependencies section of package.json with in an angular application can be divided as follow,

    1. Angular packages: Angular core and optional modules; their package names begin @angular/.
    2. Support packages: Third-party libraries that must be present for Angular apps to run.
    3. Polyfill packages: Polyfills plug gaps in a browser's JavaScript implementation.

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74. What is zone?

    A Zone is an execution context that persists across async tasks. Angular relies on zone.js to run Angular's change detection processes when native JavaScript operations raise events

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75. What is the purpose of common module?

    The commonly-needed services, pipes, and directives provided by @angular/common module. Apart from these HttpClientModule is available under @angular/common/http.

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76. What is codelyzer?

    Codelyzer provides set of tslint rules for static code analysis of Angular TypeScript projects. ou can run the static code analyzer over web apps, NativeScript, Ionic etc. Angular CLI has support for this and it can be use as below,

    ng new codelyzer
    ng lint

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77. How to inject the dynamic script in angular?

    Using DomSanitizer we can inject the dynamic Html,Style,Script,Url.

    import { Component, OnInit } from '@angular/core';
    import { DomSanitizer } from '@angular/platform-browser';
    @Component({
       selector: 'my-app',
       template: `
           <div [innerHtml]="htmlSnippet"></div>
       `,
    })
    export class App {
           constructor(protected sanitizer: DomSanitizer) {}
           htmlSnippet: string = this.sanitizer.bypassSecurityTrustScript("<script>safeCode()</script>");
       }
    

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78. What is a service worker and its role in Angular?

    A service worker is a script that runs in the web browser and manages caching for an application. Starting from 5.0.0 version, Angular ships with a service worker implementation. Angular service worker is designed to optimize the end user experience of using an application over a slow or unreliable network connection, while also minimizing the risks of serving outdated content.

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79. What are the design goals of service workers?

    Below are the list of design goals of Angular's service workers,

    1. It caches an application just like installing a native application
    2. A running application continues to run with the same version of all files without any incompatible files
    3. When you refresh the application, it loads the latest fully cached version
    4. When changes are published then it immediately updates in the background
    5. Service workers saves the bandwidth by downloading the resources only when they changed.

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80. What are the differences between AngularJS and Angular with respect to dependency injection?

    Dependency injection is a common component in both AngularJS and Angular, but there are some key differences between the two frameworks in how it actually works.

    AngularJS	Angular
    Dependency injection tokens are always strings	Tokens can have different types. They are often classes and sometimes can be strings.
    There is exactly one injector even though it is a multi-module applications	There is a tree hierarchy of injectors, with a root injector and an additional injector for each component.

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81. What is Angular Ivy?

    Angular Ivy is a new rendering engine for Angular. You can choose to opt in a preview version of Ivy from Angular version 8.

    1. You can enable ivy in a new project by using the --enable-ivy flag with the ng new command

       ng new ivy-demo-app --enable-ivy

    2. You can add it to an existing project by adding enableIvy option in the angularCompilerOptions in your project's tsconfig.app.json.

       {
         "compilerOptions": { ... },
         "angularCompilerOptions": {
           "enableIvy": true
         }
       }

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82. What are the features included in ivy preview?

    You can expect below features with Ivy preview,

    1. Generated code that is easier to read and debug at runtime
    2. Faster re-build time
    3. Improved payload size
    4. Improved template type checking

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83. What are the class decorators in Angular?

    A class decorator is a decorator that appears immediately before a class definition, which declares the class to be of the given type, and provides metadata suitable to the type

    The following list of decorators comes under class decorators,

    1. @Component()
    2. @Directive()
    3. @Pipe()
    4. @Injectable()
    5. @NgModule()

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84. What are class field decorators?

    The class field decorators are the statements declared immediately before a field in a class definition that defines the type of that field. Some of the examples are: @input and @output,

    @Input() myProperty;
    @Output() myEvent = new EventEmitter();

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85. What is declarable in Angular?

    Declarable is a class type that you can add to the declarations list of an NgModule. The class types such as components, directives, and pipes comes can be declared in the module. The structure of declarations would be,

    declarations: [
      YourComponent,
      YourPipe,
      YourDirective
    ],

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86. What are the restrictions on declarable classes?

    Below classes shouldn't be declared,

    1. A class that's already declared in another NgModule
    2. Ngmodule classes
    3. Service classes
    4. Helper classes

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87. What is a DI token?

    A DI token is a lookup token associated with a dependency provider in dependency injection system. The injector maintains an internal token-provider map that it references when asked for a dependency and the DI token is the key to the map. Let's take example of DI Token usage,

    const BASE_URL = new InjectionToken<string>('BaseUrl');
    const injector =
       Injector.create({providers: [{provide: BASE_URL, useValue: 'http://some-domain.com'}]});
    const url = injector.get(BASE_URL);

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88. what is an rxjs subject in Angular

    An RxJS Subject is a special type of Observable that allows values to be multicasted to many Observers. While plain Observables are unicast (each subscribed Observer owns an independent execution of the Observable), Subjects are multicast.

    A Subject is like an Observable, but can multicast to many Observers. Subjects are like EventEmitters: they maintain a registry of many listeners.

     import { Subject } from 'rxjs';
    
       const subject = new Subject<number>();
    
       subject.subscribe({
         next: (v) => console.log(`observerA: ${v}`)
       });
       subject.subscribe({
         next: (v) => console.log(`observerB: ${v}`)
       });
    
       subject.next(1);
       subject.next(2);

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89. What is Bazel tool?

    Bazel is a powerful build tool developed and massively used by Google and it can keep track of the dependencies between different packages and build targets. In Angular8, you can build your CLI application with Bazel. Note: The Angular framework itself is built with Bazel.

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90. What are the advantages of Bazel tool?

    Below are the list of key advantages of Bazel tool,

    1. It creates the possibility of building your back-ends and front-ends with the same tool
    2. The incremental build and tests
    3. It creates the possibility to have remote builds and cache on a build farm.

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91. What is platform in Angular?

    A platform is the context in which an Angular application runs. The most common platform for Angular applications is a web browser, but it can also be an operating system for a mobile device, or a web server. The runtime-platform is provided by the @angular/platform-* packages and these packages allow applications that make use of @angular/core and @angular/common to execute in different environments. i.e, Angular can be used as platform-independent framework in different environments, For example,

    1. While running in the browser, it uses platform-browser package.
    2. When SSR(server-side rendering ) is used, it uses platform-server package for providing web server implementation.

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92. What happens if I import the same module twice?

    If multiple modules imports the same module then angular evaluates it only once (When it encounters the module first time). It follows this condition even the module appears at any level in a hierarchy of imported NgModules.

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93. How do you select an element with in a component template?

    You can use @ViewChild directive to access elements in the view directly. Let's take input element with a reference,

    <input #uname>

    and define view child directive and access it in ngAfterViewInit lifecycle hook

    @ViewChild('uname') input;
    
    ngAfterViewInit() {
      console.log(this.input.nativeElement.value);
    }

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94. How do you detect route change in Angular?

    In Angular7, you can subscribe to router to detect the changes. The subscription for router events would be as below,

    this.router.events.subscribe((event: Event) => {})

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95. How do you pass headers for HTTP client?

    You can directly pass object map for http client or create HttpHeaders class to supply the headers.

    constructor(private _http: HttpClient) {}
    this._http.get('someUrl',{
       headers: {'header1':'value1','header2':'value2'}
    });
    
    (or)
    let headers = new HttpHeaders().set('header1', headerValue1); // create header object
    headers = headers.append('header2', headerValue2); // add a new header, creating a new object
    headers = headers.append('header3', headerValue3); // add another header
    
    let params = new HttpParams().set('param1', value1); // create params object
    params = params.append('param2', value2); // add a new param, creating a new object
    params = params.append('param3', value3); // add another param
    
    return this._http.get<any[]>('someUrl', { headers: headers, params: params })

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96. Is Angular supports dynamic imports?

    Yes, Angular 8 supports dynamic imports in router configuration. i.e, You can use the import statement for lazy loading the module using loadChildren method and it will be understood by the IDEs(VSCode and WebStorm), webpack, etc. Previously, you have been written as below to lazily load the feature module. By mistake, if you have typo in the module name it still accepts the string and throws an error during build time.

    {path: ‘user’, loadChildren: ‘./users/user.module#UserModulee’},

    This problem is resolved by using dynamic imports and IDEs are able to find it during compile time itself.

    {path: ‘user’, loadChildren: () => import(‘./users/user.module’).then(m => m.UserModule)};

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97. What are the ways to trigger change detection in Angular?

    You can inject either ApplicationRef or NgZone, or ChangeDetectorRef into your component and apply below specific methods to trigger change detection in Angular. i.e, There are 3 possible ways,

    1. ApplicationRef.tick(): Invoke this method to explicitly process change detection and its side-effects. It check the full component tree.
    2. NgZone.run(callback): It evaluate the callback function inside the Angular zone.
    3. ChangeDetectorRef.detectChanges(): It detects only the components and it's children.

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98. What are the security principles in angular?

    Below are the list of security principles in angular,

    1.	You should avoid direct use of the DOM APIs.
    2.  You should enable Content Security Policy (CSP) and configure your web server to return appropriate CSP HTTP headers.
    3.  You should Use the offline template compiler.
    4.  You should Use Server Side XSS protection.
    5.  You should Use DOM Sanitizer.
    6.  You should Preventing CSRF or XSRF attacks.
    

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99. What is schematic?

    It's a scaffolding library that defines how to generate or transform a programming project by creating, modifying, refactoring, or moving files and code. It defines rules that operate on a virtual file system called a tree.

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100. What are the best practices for security in angular?

     Below are the best practices of security in angular,

     1. Use the latest Angular library releases
     2. Don't modify your copy of Angular
     3. Avoid Angular APIs marked in the documentation as “Security Risk.”

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101. What is Angular security model for preventing XSS attacks?

     Angular treats all values as untrusted by default. i.e, Angular sanitizes and escapes untrusted values When a value is inserted into the DOM from a template, via property, attribute, style, class binding, or interpolation.

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102. What is the role of template compiler for prevention of XSS attacks?

     The offline template compiler prevents vulnerabilities caused by template injection, and greatly improves application performance. So it is recommended to use offline template compiler in production deployments without dynamically generating any template.

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103. What are the various security contexts in Angular?

     Angular defines the following security contexts for sanitization,

     1. HTML: It is used when interpreting a value as HTML such as binding to innerHtml.
     2. Style: It is used when binding CSS into the style property.
     3. URL: It is used for URL properties such as <a href>.
     4. Resource URL: It is a URL that will be loaded and executed as code such as <script src>.

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104. What is Sanitization? Is angular supports it?

     Sanitization is the inspection of an untrusted value, turning it into a value that's safe to insert into the DOM. Yes, Angular suppports sanitization. It sanitizes untrusted values for HTML, styles, and URLs but sanitizing resource URLs isn't possible because they contain arbitrary code.

     ⬆ Back to Top

105. What is the purpose of innerHTML?

     The innerHtml is a property of HTML-Elements, which allows you to set it's html-content programmatically. Let's display the below html code snippet in a <div> tag as below using innerHTML binding,

     <div [innerHTML]="htmlSnippet"></div>

     and define the htmlSnippet property from any component

     export class myComponent {
       htmlSnippet: string = '<b>Hello World</b>, Angular';
     }

     Unfortunately this property could cause Cross Site Scripting (XSS) security bugs when improperly handled.

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106. What is the difference between interpolated content and innerHTML?

     The main difference between interpolated and innerHTML code is the behavior of code interpreted. Interpolated content is always escaped i.e, HTML isn't interpreted and the browser displays angle brackets in the element's text content. Where as in innerHTML binding, the content is interpreted i.e, the browser will convert < and > characters as HTMLEntities. For example, the usage in template would be as below,

     <p>Interpolated value:</p>
     <div >{{htmlSnippet}}</div>
     <p>Binding of innerHTML:</p>
     <div [innerHTML]="htmlSnippet"></div>

     and the property defined in a component.

     export class InnerHtmlBindingComponent {
       htmlSnippet = 'Template <script>alert("XSS Attack")</script> <b>Code attached</b>';
     }

     Even though innerHTML binding create a chance of XSS attack, Angular recognizes the value as unsafe and automatically sanitizes it.

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107. How do you prevent automatic sanitization?

     Sometimes the applications genuinely need to include executable code such as displaying <iframe> from an URL. In this case, you need to prevent automatic sanitization in Angular by saying that you inspected a value, checked how it was generated, and made sure it will always be secure. Basically it involves 2 steps,

     1. Inject DomSanitizer: You can inject DomSanitizer in component as parameter in constructor

     2. Mark the trusted value by calling some of the below methods

        1. bypassSecurityTrustHtml
        2. bypassSecurityTrustScript
        3. bypassSecurityTrustStyle
        4. bypassSecurityTrustUrl
        5. bypassSecurityTrustResourceUrl

     For example,The usage of dangerous url to trusted url would be as below,

     constructor(private sanitizer: DomSanitizer) {
       this.dangerousUrl = 'javascript:alert("XSS attack")';
       this.trustedUrl = sanitizer.bypassSecurityTrustUrl(this.dangerousUrl);

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108. Is safe to use direct DOM API methods in terms of security?

     No,the built-in browser DOM APIs or methods don't automatically protect you from security vulnerabilities. In this case it is recommended to use Angular templates instead of directly interacting with DOM. If it is unavoidable then use the built-in Angular sanitization functions.

     ⬆ Back to Top

109. What is DOM sanitizer?

     DomSanitizer is used to help preventing Cross Site Scripting Security bugs (XSS) by sanitizing values to be safe to use in the different DOM contexts.

     ⬆ Back to Top

110. How do you support server side XSS protection in Angular application?

     The server-side XSS protection is supported in an angular application by using a templating language that automatically escapes values to prevent XSS vulnerabilities on the server. But don't use a templating language to generate Angular templates on the server side which creates a high risk of introducing template-injection vulnerabilities.

     ⬆ Back to Top

111. Is angular prevents http level vulnerabilities?

     Angular has built-in support for preventing http level vulnerabilities such as as cross-site request forgery (CSRF or XSRF) and cross-site script inclusion (XSSI). Even though these vulnerabilities need to be mitigated on server-side, Angular provides helpers to make the integration easier on the client side.

     1. HttpClient supports a token mechanism used to prevent XSRF attacks
     2. HttpClient library recognizes the convention of prefixed JSON responses(which non-executable js code with ")]}',\n" characters) and automatically strips the string ")]}',\n" from all responses before further parsing

     ⬆ Back to Top

112. What are Http Interceptors?

     Http Interceptors are part of @angular/common/http, which inspect and transform HTTP requests from your application to the server and vice-versa on HTTP responses. These interceptors can perform a variety of implicit tasks, from authentication to logging.

     The syntax of HttpInterceptor interface looks like as below,

     interface HttpInterceptor {
       intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>>
     }

     You can use interceptors by declaring a service class that implements the intercept() method of the HttpInterceptor interface.

     @Injectable()
     export class MyInterceptor implements HttpInterceptor {
         constructor() {}
         intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
             ...
         }
     }

     After that you can use it in your module,

     @NgModule({
         ...
         providers: [
             {
                 provide: HTTP_INTERCEPTORS,
                 useClass: MyInterceptor,
                 multi: true
             }
         ]
         ...
     })
     export class AppModule {}

     ⬆ Back to Top

113. What are the applications of HTTP interceptors?

     The HTTP Interceptors can be used for different variety of tasks,

     1. Authentication
     2. Logging
     3. Caching
     4. Fake backend
     5. URL transformation
     6. Modifying headers

     ⬆ Back to Top

114. What is TestBed?

     TestBed is an api for writing unit tests for Angular applications and it's libraries. Even though We still write our tests in Jasmine and run using Karma, this API provides an easier way to create components, handle injection, test asynchronous behaviour and interact with our application.

     ⬆ Back to Top

115. What is protractor?

     Protractor is an end-to-end test framework for Angular and AngularJS applications. It runs tests against your application running in a real browser, interacting with it as a user would.

     npm install -g protractor

     ⬆ Back to Top

116. What is router state?

     The RouteState is an interface which represents the state of the router as a tree of activated routes.

     interface RouterState extends Tree {
       snapshot: RouterStateSnapshot
       toString(): string
     }

     You can access the current RouterState from anywhere in the Angular app using the Router service and the routerState property.

     ⬆ Back to Top

117. What is the difference between ngIf and hidden property?

     The main difference is that *ngIf will remove the element from the DOM, while [hidden] actually plays with the CSS style by setting display:none. Generally it is expensive to add and remove stuff from the DOM for frequent actions.

     ⬆ Back to Top

118. What is slice pipe?

     The slice pipe is used to create a new Array or String containing a subset (slice) of the elements. The syntax looks like as below,

     {{ value_expression | slice : start [ : end ] }}

     For example, you can provide 'hello' list based on a greeting array,

     @Component({
       selector: 'list-pipe',
       template: `<ul>
         <li *ngFor="let i of greeting | slice:0:5">{{i}}</li>
       </ul>`
     })
     export class PipeListComponent {
       greeting: string[] = ['h', 'e', 'l', 'l', 'o', 'm','o', 'r', 'n', 'i', 'n', 'g'];
     }

     ⬆ Back to Top

119. What is the purpose of ngFor trackBy?

     The main purpose of using *ngFor with trackBy option is performance optimization. Normally if you use NgFor with large data sets, a small change to one item by removing or adding an item, can trigger a cascade of DOM manipulations. In this case, Angular sees only a fresh list of new object references and to replace the old DOM elements with all new DOM elements. You can help Angular to track which items added or removed by providing a trackBy function which takes the index and the current item as arguments and needs to return the unique identifier for this item.

     For example, lets set trackBy to the trackByTodos() method

     <div *ngFor="let todo of todos; trackBy: trackByTodos">
       ({{todo.id}}) {{todo.name}}
     </div>

     and define the trackByTodos method,

     trackByTodos(index: number, item: Todo): number { return todo.id; }

     ⬆ Back to Top

120. What is the purpose of ngSwitch directive?

     NgSwitch directive is similar to JavaScript switch statement which displays one element from among several possible elements, based on a switch condition. In this case only the selected element placed into the DOM. It has been used along with NgSwitch, NgSwitchCase and NgSwitchDefault directives.

     For example, let's display the browser details based on selected browser using ngSwitch directive.

     <div [ngSwitch]="currentBrowser.name">
       <chrome-browser    *ngSwitchCase="'chrome'"    [item]="currentBrowser"></chrome-browser>
       <firefox-browser   *ngSwitchCase="'firefox'"     [item]="currentBrowser"></firefox-browser>
       <opera-browser     *ngSwitchCase="'opera'"  [item]="currentBrowser"></opera-browser>
       <safari-browser     *ngSwitchCase="'safari'"   [item]="currentBrowser"></safari-browser>
       <ie-browser  *ngSwitchDefault           [item]="currentItem"></ie-browser>
     </div>

     ⬆ Back to Top

121. What is safe navigation operator?

     The safe navigation operator(?)(or known as Elvis Operator) is used to guard against null and undefined values in property paths when you are not aware whether a path exists or not. i.e. It returns value of the object path if it exists, else it returns the null value.

     For example, you can access nested properties of a user profile easily without null reference errors as below,

     <p>The user firstName is: {{user?.fullName.firstName}}</p>

     Using this safe navigation operator, Angular framework stops evaluating the expression when it hits the first null value and renders the view without any errors.

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122. What is an entry component?

     An entry component is any component that Angular loads imperatively(i.e, not referencing it in the template) by type. Due to this behavior, they can’t be found by the Angular compiler during compilation. These components created dynamically with ComponentFactoryResolver.

     Basically, there are two main kinds of entry components which are following -

     1. The bootstrapped root component
     2. A component you specify in a route

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123. What is a bootstrapped component?

     A bootstrapped component is an entry component that Angular loads into the DOM during the bootstrap process or application launch time. Generally, this bootstrapped or root component is named as AppComponent in your root module using bootstrap property as below.

     @NgModule({
       declarations: [
         AppComponent
       ],
       imports: [
         BrowserModule,
         FormsModule,
         HttpClientModule,
         AppRoutingModule
       ],
       providers: [],
       bootstrap: [AppComponent] // bootstrapped entry component need to be declared here
     })

     ⬆ Back to Top

124. Is it necessary for bootstrapped component to be entry component?

     Yes, the bootstrapped component needs to be an entry component. This is because the bootstrapping process is an imperative process.

     ⬆ Back to Top

125. What is a routed entry component?

     The components referenced in router configuration are called as routed entry components. This routed entry component defined in a route definition as below,

     const routes: Routes = [
       {
         path: '',
         component: TodoListComponent // router entry component
       }
     ];

     Since router definition requires you to add the component in two places (router and entryComponents), these components are always entry components.

     Note: The compilers are smart enough to recognize a router definition and automatically add the router component into entryComponents.

     ⬆ Back to Top

126. Is it all components generated in production build?

     No, only the entry components and template components appears in production builds. If a component isn't an entry component and isn't found in a template, the tree shaker will throw it away. Due to this reason, make sure to add only true entry components to reduce the bundle size.

     ⬆ Back to Top

127. Give few examples for NgModules?

     The Angular core libraries and third-party libraries are available as NgModules.

     1. Angular libraries such as FormsModule, HttpClientModule, and RouterModule are NgModules.
     2. Many third-party libraries such as Material Design, Ionic, and AngularFire2 are NgModules.

     ⬆ Back to Top

128. What are the steps to use declaration elements?

     Below are the steps to be followed to use declaration elements.

     1. Create the element(component, directive and pipes) and export it from the file where you wrote it
     2. Import it into the appropriate module.
     3. Declare it in the @NgModule declarations array.

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129. What happens if browserModule used in feature module?

     If you do import BrowserModule into a lazy loaded feature module, Angular returns an error telling you to use CommonModule instead. Because BrowserModule’s providers are for the entire app so it should only be in the root module, not in feature module. Whereas Feature modules only need the common directives in CommonModule.

     

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130. What are the types of feature modules?

     Below are the five categories of feature modules,

     1. Domain: Deliver a user experience dedicated to a particular application domain(For example, place an order, registration etc)
     2. Routed: These are domain feature modules whose top components are the targets of router navigation routes.
     3. Routing: It provides routing configuration for another module.
     4. Service: It provides utility services such as data access and messaging(For example, HttpClientModule)
     5. Widget: It makes components, directives, and pipes available to external modules(For example, third-party libraries such as Material UI)

     ⬆ Back to Top

131. What is a provider?

     A provider is an instruction to the Dependency Injection system on how to obtain a value for a dependency(aka services created). The service can be provided using Angular CLI as below,

     ng generate service my-service

     The created service by CLI would be as below,

     import { Injectable } from '@angular/core';
     
     @Injectable({
       providedIn: 'root', //Angular provide the service in root injector
     })
     export class MyService {
     }

     ⬆ Back to Top

132. What is the recommendation for provider scope?

     You should always provide your service in the root injector unless there is a case where you want the service to be available only if you import a particular @NgModule.

     ⬆ Back to Top

133. How do you restrict provider scope to a module?

     It is possible to restrict service provider scope to a specific module instead making available to entire application. There are two possible ways to do it.

     1. Using providedIn in service:

        import { Injectable } from '@angular/core';
        import { SomeModule } from './some.module';
        
        @Injectable({
          providedIn: SomeModule,
        })
        export class SomeService {
        }

     2. Declare provider for the service in module:

        import { NgModule } from '@angular/core';
        
        import { SomeService } from './some.service';
        
        @NgModule({
          providers: [SomeService],
        })
        export class SomeModule {
        }

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134. How do you provide a singleton service?

     There are two possible ways to provide a singleton service.

     1. Set the providedIn property of the @Injectable() to "root". This is the preferred way(starting from Angular 6.0) of creating a singleton service since it makes your services tree-shakable.

        import { Injectable } from '@angular/core';
        
        @Injectable({
          providedIn: 'root',
        })
        export class MyService {
        }

     2. Include the service in root module or in a module that is only imported by root module. It has been used to register services before Angular 6.0.

        @NgModule({
          ...
          providers: [MyService],
          ...
        })

     ⬆ Back to Top

135. What are the different ways to remove duplicate service registration?

     If a module defines provides and declarations then loading the module in multiple feature modules will duplicate the registration of the service. Below are the different ways to prevent this duplicate behavior.

     1. Use the providedIn syntax instead of registering the service in the module.
     2. Separate your services into their own module.
     3. Define forRoot() and forChild() methods in the module.

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136. How does forRoot method helpful to avoid duplicate router instances?

     If the RouterModule module didn’t have forRoot() static method then each feature module would instantiate a new Router instance, which leads to broken application due to duplicate instances. After using forRoot() method, the root application module imports RouterModule.forRoot(...) and gets a Router, and all feature modules import RouterModule.forChild(...) which does not instantiate another Router.

     ⬆ Back to Top

137. What is a shared module?

     The Shared Module is the module in which you put commonly used directives, pipes, and components into one module that is shared(import it) throughout the application.

     For example, the below shared module imports CommonModule, FormsModule for common directives and components, pipes and directives based on the need,

     import { CommonModule } from '@angular/common';
     import { NgModule } from '@angular/core';
     import { FormsModule } from '@angular/forms';
     import { UserComponent } from './user.component';
     import { NewUserDirective } from './new-user.directive';
     import { OrdersPipe } from './orders.pipe';
     
     @NgModule({
      imports:      [ CommonModule ],
      declarations: [ UserComponent, NewUserDirective, OrdersPipe ],
      exports:      [ UserComponent, NewUserDirective, OrdersPipe,
                      CommonModule, FormsModule ]
     })
     export class SharedModule { }

     ⬆ Back to Top

138. Can I share services using modules?

     No, it is not recommended to share services by importing module. i.e Import modules when you want to use directives, pipes, and components only. The best approach to get a hold of shared services is through 'Angular dependency injection' because importing a module will result in a new service instance.

     ⬆ Back to Top

139. What is NgZone?

     Angular provides a service called NgZone which creates a zone named angular to automatically trigger change detection when the following conditions are satisfied.

     1. When a sync or async function is executed.
     2. When there is no microTask scheduled.

     ⬆ Back to Top

140. What is NoopZone?

     Zone is loaded/required by default in Angular applications and it helps Angular to know when to trigger the change detection. This way, it make sures developers focus on application development rather core part of Angular. You can also use Angular without Zone but the change detection need to be implemented on your own and noop zone need to be configured in bootstrap process. Let's follow the below two steps to remove zone.js,

     1. Remove the zone.js import from polyfills.ts.

        /***************************************************************************************************
         * Zone JS is required by default for Angular itself.
         */
        // import 'zone.js/dist/zone';  // Included with Angular CLI.

     2. Bootstrap Angular with noop zone in src/main.ts.

        platformBrowserDynamic().bootstrapModule(AppModule, {ngZone: 'noop'})
          .catch(err => console.error(err));

     ⬆ Back to Top

141. What are the possible data update scenarios for change detection?

     The change detection works in the following scenarios where the data changes needs to update the application HTML.

     1. Component initialization: While bootstrapping the Angular application, Angular triggers the ApplicationRef.tick() to call change detection and View Rendering.
     2. Event listener: The DOM event listener can update the data in an Angular component and trigger the change detection too.

        @Component({
          selector: 'app-event-listener',
          template: `
            <button (click)="onClick()">Click</button>
            {{message}}`
        })
        export class EventListenerComponent {
          message = '';
        
          onClick() {
            this.message = 'data updated';
          }
        }

     3. HTTP Data Request: You can get data from a server through an HTTP request

        data = 'default value';
        constructor(private httpClient: HttpClient) {}
        
          ngOnInit() {
            this.httpClient.get(this.serverUrl).subscribe(response => {
              this.data = response.data; // change detection will happen automatically
            });
          }

     4. Macro tasks setTimeout() or setInterval(): You can update the data in the callback function of setTimeout or setInterval

        data = 'default value';
        
          ngOnInit() {
            setTimeout(() => {
              this.data = 'data updated'; // Change detection will happen automatically
            });
          }

     5. Micro tasks Promises: You can update the data in the callback function of promise

        data = 'initial value';
        
          ngOnInit() {
            Promise.resolve(1).then(v => {
              this.data = v; // Change detection will happen automatically
            });
          }

     6. Async operations like Web sockets and Canvas: The data can be updated asynchronously using WebSocket.onmessage() and Canvas.toBlob().

     ⬆ Back to Top

142. What is a zone context?

     Execution Context is an abstract concept that holds information about the environment within the current code being executed. A zone provides an execution context that persists across asynchronous operations is called as zone context. For example, the zone context will be same in both outside and inside setTimeout callback function,

     zone.run(() => {
       // outside zone
       expect(zoneThis).toBe(zone);
       setTimeout(function() {
         // the same outside zone exist here
         expect(zoneThis).toBe(zone);
       });
     });

     The current zone is retrieved through Zone.current.

     ⬆ Back to Top

143. What are the methods of NgZone used to control change detection?

     NgZone service provides a run() method that allows you to execute a function inside the angular zone. This function is used to execute third party APIs which are not handled by Zone and trigger change detection automatically at the correct time.

     export class AppComponent implements OnInit {
       constructor(private ngZone: NgZone) {}
       ngOnInit() {
         // use ngZone.run() to make the asynchronous operation in the angular zone
         this.ngZone.run(() => {
           someNewAsyncAPI(() => {
             // update the data of the component
           });
         });
       }
     }

     Whereas runOutsideAngular() method is used when you don't want to trigger change detection.

     export class AppComponent implements OnInit {
       constructor(private ngZone: NgZone) {}
       ngOnInit() {
         // Use this method when you know no data will be updated
         this.ngZone.runOutsideAngular(() => {
           setTimeout(() => {
             // update component data and don't trigger change detection
           });
         });
       }
     }

     ⬆ Back to Top

144. How do you configure injectors with providers at different levels?

     You can configure injectors with providers at different levels of your application by setting a metadata value. The configuration can happen in one of three places,

     1. In the @Injectable() decorator for the service itself
     2. In the @NgModule() decorator for an NgModule
     3. In the @Component() decorator for a component

     ⬆ Back to Top

145. Is it mandatory to use injectable on every service class?

     No. The @Injectable() decorator is not strictly required if the class has other Angular decorators on it or does not have any dependencies. But the important thing here is any class that is going to be injected with Angular is decorated. i.e, If we add the decorator, the metadata design:paramtypes is added, and the dependency injection can do it's job. That is the exact reason to add the @Injectable() decorator on a service if this service has some dependencies itself. For example, Let's see the different variations of AppService in a root component,

     1. The below AppService can be injected in AppComponent without any problems. This is because there are no dependency services inside AppService.

        export class AppService {
          constructor() {
            console.log('A new app service');
          }
        }

     2. The below AppService with dummy decorator and httpService can be injected in AppComponent without any problems. This is because meta information is generated with dummy decorator.

        function SomeDummyDecorator() {
          return (constructor: Function) => console.log(constructor);
        }
        
        @SomeDummyDecorator()
        export class AppService {
          constructor(http: HttpService) {
            console.log(http);
          }
        }

     and the generated javascript code of above service has meta information about HttpService, js var AppService = (function () { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], AppService); return AppService; }()); exports.AppService = AppService; 3. The below AppService with @injectable decorator and httpService can be injected in AppComponent without any problems. This is because meta information is generated with Injectable decorator. js @Injectable({ providedIn: 'root', }) export class AppService { constructor(http: HttpService) { console.log(http); } } ⬆ Back to Top

146. What is an optional dependency?

     The optional dependency is a parameter decorator to be used on constructor parameters, which marks the parameter as being an optional dependency. Due to this, the DI framework provides null if the dependency is not found. For example, If you don't register a logger provider anywhere, the injector sets the value of logger(or logger service) to null in the below class.

     import { Optional } from '@angular/core';
     
     constructor(@Optional() private logger?: Logger) {
       if (this.logger) {
         this.logger.log('This is an optional dependency message');
       } else {
         console.log('The logger is not registered');
       }
     }

     ⬆ Back to Top

147. What are the types of injector hierarchies?

     There are two types of injector hierarchies in Angular

     1. ModuleInjector hierarchy: It configure on a module level using an @NgModule() or @Injectable() annotation.
     2. ElementInjector hierarchy: It created implicitly at each DOM element. Also it is empty by default unless you configure it in the providers property on @Directive() or @Component().

     ⬆ Back to Top

148. What are reactive forms?

     Reactive forms is a model-driven approach for creating forms in a reactive style(form inputs changes over time). These are built around observable streams, where form inputs and values are provided as streams of input values. Let's follow the below steps to create reactive forms,

     1. Register the reactive forms module which declares reactive-form directives in your app

        import { ReactiveFormsModule } from '@angular/forms';
        
        @NgModule({
          imports: [
            // other imports ...
            ReactiveFormsModule
          ],
        })
        export class AppModule { }

     2. Create a new FormControl instance and save it in the component.

        import { Component } from '@angular/core';
        import { FormControl } from '@angular/forms';
        
        @Component({
          selector: 'user-profile',
          styleUrls: ['./user-profile.component.css']
        })
        export class UserProfileComponent {
          userName = new FormControl('');
        }

     3. Register the FormControl in the template.

        <label>
          User name:
          <input type="text" [formControl]="userName">
        </label>

     Finally, the component with reactive form control appears as below, ```js import { Component } from '@angular/core'; import { FormControl } from '@angular/forms';

     @Component({
       selector: 'user-profile',
       styleUrls: ['./user-profile.component.css']
       template: `
          <label>
            User name:
            <input type="text" [formControl]="userName">
          </label>
       `
     })
     export class UserProfileComponent {
       userName = new FormControl('');
     }
     ```
     

     ⬆ Back to Top

149. What are dynamic forms?

     Dynamic forms is a pattern in which we build a form dynamically based on metadata that describes a business object model. You can create them based on reactive form API. ⬆ Back to Top

150. What are template driven forms?

     Template driven forms are model-driven forms where you write the logic, validations, controls etc, in the template part of the code using directives. They are suitable for simple scenarios and uses two-way binding with [(ngModel)] syntax. For example, you can create register form easily by following the below simple steps,

     1. Import the FormsModule into the Application module's imports array

           import { BrowserModule } from '@angular/platform-browser';
           import { NgModule } from '@angular/core';
           import {FormsModule} from '@angular/forms'
           import { RegisterComponent } from './app.component';
           @NgModule({
             declarations: [
               RegisterComponent,
             ],
             imports: [
               BrowserModule,
               FormsModule
             ],
             providers: [],
             bootstrap: [RegisterComponent]
           })
           export class AppModule { }

     2. Bind the form from template to the component using ngModel syntax

        <input type="text" class="form-control" id="name"
               required
               [(ngModel)]="model.name" name="name">

     3. Attach NgForm directive to the tag in order to create FormControl instances and register them

        <form #registerForm="ngForm">

     4. Apply the validation message for form controls

        <label for="name">Name</label>
        <input type="text" class="form-control" id="name"
               required
               [(ngModel)]="model.name" name="name"
               #name="ngModel">
        <div [hidden]="name.valid || name.pristine"
             class="alert alert-danger">
          Please enter your name
        </div>

     5. Let's submit the form with ngSubmit directive and add type="submit" button at the bottom of the form to trigger form submit.

        <form (ngSubmit)="onSubmit()" #heroForm="ngForm">
        // Form goes here
        <button type="submit" class="btn btn-success" [disabled]="!registerForm.form.valid">Submit</button>

     Finally, the completed template-driven registration form will be appeared as follow.

     ```html
     <div class="container">
         <h1>Registration Form</h1>
         <form (ngSubmit)="onSubmit()" #registerForm="ngForm">
           <div class="form-group">
             <label for="name">Name</label>
             <input type="text" class="form-control" id="name"
                    required
                    [(ngModel)]="model.name" name="name"
                    #name="ngModel">
             <div [hidden]="name.valid || name.pristine"
                  class="alert alert-danger">
               Please enter your name
             </div>
           </div>
                 <button type="submit" class="btn btn-success" [disabled]="!registerForm.form.valid">Submit</button>
         </form>
     </div>
     ```
     

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151. What are the differences between reactive forms and template driven forms?

     Below are the main differences between reactive forms and template driven forms

     Feature	Reactive	Template-Driven
     Form model setup	Created(FormControl instance) in component explicitly	Created by directives
     Data updates	Synchronous	Asynchronous
     Form custom validation	Defined as Functions	Defined as Directives
     Testing	No interaction with change detection cycle	Need knowledge of the change detection process
     Mutability	Immutable(by always returning new value for FormControl instance)	Mutable(Property always modified to new value)
     Scalability	More scalable using low-level APIs	Less scalable using due to abstraction on APIs

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152. What are the different ways to group form controls?

     Reactive forms provide two ways of grouping multiple related controls.

     1. FormGroup: It defines a form with a fixed set of controls those can be managed together in an one object. It has same properties and methods similar to a FormControl instance. This FormGroup can be nested to create complex forms as below.

        import { Component } from '@angular/core';
        import { FormGroup, FormControl } from '@angular/forms';
        
        @Component({
          selector: 'user-profile',
          templateUrl: './user-profile.component.html',
          styleUrls: ['./user-profile.component.css']
        })
        export class UserProfileComponent {
          userProfile = new FormGroup({
            firstName: new FormControl(''),
            lastName: new FormControl(''),
            address: new FormGroup({
                  street: new FormControl(''),
                  city: new FormControl(''),
                  state: new FormControl(''),
                  zip: new FormControl('')
                })
          });
        
          onSubmit() {
            // Store this.userProfile.value in DB
          }
        }

        <form [formGroup]="userProfile" (ngSubmit)="onSubmit()">
        
          <label>
            First Name:
            <input type="text" formControlName="firstName">
          </label>
        
          <label>
            Last Name:
            <input type="text" formControlName="lastName">
          </label>
        
          <div formGroupName="address">
            <h3>Address</h3>
        
            <label>
              Street:
              <input type="text" formControlName="street">
            </label>
        
            <label>
              City:
              <input type="text" formControlName="city">
            </label>
        
            <label>
              State:
              <input type="text" formControlName="state">
            </label>
        
            <label>
              Zip Code:
              <input type="text" formControlName="zip">
            </label>
           </div>
            <button type="submit" [disabled]="!userProfile.valid">Submit</button>
        
        </form>

     2. FormArray: It defines a dynamic form in an array format, where you can add and remove controls at run time. This is useful for dynamic forms when you don’t know how many controls will be present within the group.

         import { Component } from '@angular/core';
         import { FormArray, FormControl } from '@angular/forms';
        
         @Component({
           selector: 'order-form',
           templateUrl: './order-form.component.html',
           styleUrls: ['./order-form.component.css']
         })
         export class OrderFormComponent {
           constructor () {
             this.orderForm = new FormGroup({
               firstName: new FormControl('John', Validators.minLength(3)),
               lastName: new FormControl('Rodson'),
               items: new FormArray([
                 new FormControl(null)
               ])
             });
           }
        
           onSubmitForm () {
             // Save the items this.orderForm.value in DB
           }
        
           onAddItem () {
             this.orderForm.controls
             .items.push(new FormControl(null));
           }
        
           onRemoveItem (index) {
             this.orderForm.controls['items'].removeAt(index);
           }
         }

        <form [formControlName]="orderForm" (ngSubmit)="onSubmit()">
        
          <label>
            First Name:
            <input type="text" formControlName="firstName">
          </label>
        
          <label>
            Last Name:
            <input type="text" formControlName="lastName">
          </label>
        
          <div>
          <p>Add items</p>
          <ul formArrayName="items">
            <li *ngFor="let item of orderForm.controls.items.controls; let i = index">
              <input type="text" formControlName="{{i}}">
              <button type="button" title="Remove Item" (click)="onRemoveItem(i)">Remove</button>
            </li>
          </ul>
          <button type="button" (click)="onAddItem">
            Add an item
          </button>
         </div>

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153. How do you update specific properties of a form model?

     You can use patchValue() method to update specific properties defined in the form model. For example,you can update the name and street of certain profile on click of the update button as shown below.

     updateProfile() {
       this.userProfile.patchValue({
         firstName: 'John',
         address: {
           street: '98 Crescent Street'
         }
       });
     }

       <button (click)="updateProfile()">Update Profile</button>

     You can also use setValue method to update properties.

     Note: Remember to update the properties against the exact model structure.

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154. What is the purpose of FormBuilder?

     FormBuilder is used as syntactic sugar for easily creating instances of a FormControl, FormGroup, or FormArray. This is helpful to reduce the amount of boilerplate needed to build complex reactive forms. It is available as an injectable helper class of the @angular/forms package.

     For example, the user profile component creation becomes easier as shown here.

     export class UserProfileComponent {
       profileForm = this.formBuilder.group({
         firstName: [''],
         lastName: [''],
         address: this.formBuilder.group({
           street: [''],
           city: [''],
           state: [''],
           zip: ['']
         }),
       });
       constructor(private formBuilder: FormBuilder) { }
       }

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155. What are the state CSS classes provided by ngModel?

     The ngModel directive updates the form control with special Angular CSS classes to reflect it's state. Let's find the list of classes in a tabular format,

     Form control state	If true	If false
     Visited	ng-touched	ng-untouched
     Value has changed	ng-dirty	ng-pristine
     Value is valid	ng-valid	ng-invalid

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156. How do you reset the form?

     In a model-driven form, you can reset the form just by calling the function reset() on our form model. For example, you can reset the form model on submission as follows,

     onSubmit() {
       if (this.myform.valid) {
         console.log("Form is submitted");
         // Perform business logic here
         this.myform.reset();
       }
     }

     Now, your form model resets the form back to its original pristine state.

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157. What are the types of validator functions?

     In reactive forms, the validators can be either synchronous or asynchronous functions,

     1. Sync validators: These are the synchronous functions which take a control instance and immediately return either a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are simple checks like whether a field is empty, whether it exceeds a maximum length etc.
     2. Async validators: These are the asynchronous functions which take a control instance and return a Promise or Observable that later emits a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are complex validations like hitting a server to check the availability of a username or email.

     The representation of these validators looks like below

     this.myForm = formBuilder.group({
         firstName: ['value'],
         lastName: ['value', *Some Sync validation function*],
         email: ['value', *Some validation function*, *Some asynchronous validation function*]
     });

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158. Can you give an example of built-in validators?

     In reactive forms, you can use built-in validator like required and minlength on your input form controls. For example, the registration form can have these validators on name input field

     this.registrationForm = new FormGroup({
         'name': new FormControl(this.hero.name, [
           Validators.required,
           Validators.minLength(4),
         ])
       });

     Whereas in template-driven forms, both required and minlength validators available as attributes.

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159. How do you optimize the performance of async validators?

     Since all validators run after every form value change, it creates a major impact on performance with async validators by hitting the external API on each keystroke. This situation can be avoided by delaying the form validity by changing the updateOn property from change (default) to submit or blur. The usage would be different based on form types,

     1. Template-driven forms: Set the property on ngModelOptions directive

        <input [(ngModel)]="name" [ngModelOptions]="{updateOn: 'blur'}">

     2. Reactive-forms: Set the property on FormControl instance

        name = new FormControl('', {updateOn: 'blur'});

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160. What is host property in css?

     The :host pseudo-class selector is used to target styles in the element that hosts the component. Since the host element is in a parent component's template, you can't reach the host element from inside the component by other means. For example, you can create a border for parent element as below,

     //Other styles for app.component.css
     //...
     :host {
       display: block;
       border: 1px solid black;
       padding: 20px;
     }

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161. How do you get the current route?

     In Angular, there is an url property of router package to get the current route. You need to follow the below few steps,

     1. Import Router from @angular/router

       import { Router } from '@angular/router';

     2. Inject router inside constructor

     constructor(private router: Router ) {
     
     }

     3. Access url parameter

       console.log(this.router.url); //  /routename

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162. What is Component Test Harnesses?

     A component harness is a testing API around an Angular directive or component to make tests simpler by hiding implementation details from test suites. This can be shared between unit tests, integration tests, and end-to-end tests. The idea for component harnesses comes from the PageObject pattern commonly used for integration testing.

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163. What is the benefit of Automatic Inlining of Fonts?

     During compile time, Angular CLI will download and inline the fonts that your application is using. This performance update speed up the first contentful paint(FCP) and this feature is enabled by default in apps built with version 11.

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