Angular is a framework that is used to develop a single page application. Angular is based on TypeScript.  Now read and learn carefully interview questions below:



Angular Interview questions (11 to 22)
11.    What are lifecycle hooks available?
The 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,
                          i.          ngOnChanges: When the value of data-bind property changes, then this method is called.
                         ii.          ngOnInit: This is called whenever the initialization of the directive/component after Angular first displays the data-bound properties happen.
                        iii.          ngDoCheck: This is for the detection and to act on changes that Angular can't or won't detect on its own.
                        iv.          ngAfterContentInit: This is called in response after Angular projects external content into the component's view.
                         v.          ngAfterContentChecked: This is called in response after Angular checks the content projected into the component.
                        vi.          ngAfterViewInit: This is called in response after Angular initializes the component's views and child views.
                      vii.          ngAfterViewChecked: This is called in response after Angular checks the component's views and child views.
                     viii.          ngOnDestroy: This is the cleanup phase just before Angular destroys the directive/component.
12.    What is data binding?
Data binding is a core concept in Angular and allows defining communication between a component and the DOM, making it very easy to define interactive applications without worrying about pushing and pulling data. There are four forms of data binding(divided as 3 categories) which differ in the way the data is flowing.
                          i.          From the Component to the DOM: Interpolation: {{ value }}: Adds the value of a property from the component
<li>Name: {{ user.name }}</li>
<li>Address: {{ user.address }}</li>
Property binding: [property]=”value”: The value is passed from the component to the specified property or simple HTML attribute
<input type="email" [value]="user.email">
                         ii.          From the DOM to the Component: Event binding: (event)=”function”: When a specific DOM event happens (eg.: click, change, keyup), call the specified method in the component
<button (click)="logout()"></button>
                        iii.          Two-way binding: Two-way data binding: [(ngModel)]=”value”: Two-way data binding allows to have the data flow both ways. For example, in the below code snippet, both the email DOM input and component email property are in sync
<input type="email" [(ngModel)]="user.email">
13.    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
                        iii.          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!');
  }
}
                         ii.          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;
}
                        iii.          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
    }
}
                        iv.          Parameter decorators Used for parameters inside class constructors, e.g. @Inject
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
    }
}
14.    What is angular CLI?
Angular CLI(Command Line Interface) is a command-line interface to scaffold and build angular apps using nodejs style (common) modules. You need to install using below npm command,
npm install @angular/cli@latest
Below is the list of few commands, which will come handy while creating angular projects
                        iv.          Creating a New Project: ng new
                         v.          Generating Components, Directives & Services: ng generate/g The different types of commands would be,
           o     ng generate class my-new-class: add a class to your application
           o     ng generate component my-new-component: add a component to your application
           o     ng generate directive my-new-directive: add a directive to your application
           o     ng generate enum my-new-enum: add an enum to your application
           o     ng generate module my-new-module: add a module to your application
           o     ng generate pipe my-new-pipe: add a pipe to your application
           o     ng generate service my-new-service: add a service to your application
    1. Running the Project: ng serve
  1. 15. 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()
  }
}
16.    What is the 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.
17.    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.
18.    How is Dependency Hierarchy formed?
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)
  );
}
20.    What is the option to choose between inline and external template file?
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. The choice between inline and separate HTML is a matter of taste, circumstances, and organization policy. But normally we use inline template for small portion of code and external template file for bigger views. By default, the Angular CLI generates components with a template file. But you can override that with the below command,
ng generate component hero -it
21.    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
22.    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.

Part-1 Most asked Interviews questions for Angular 2,4,6,7 and 8