Top 100 Angular Interview Questions & Answers (Frequently Asked)


  • 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.

  • 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
    This uses use 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
  • What is TypeScript?

    TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. 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.

  • Write a pictorial diagram of Angular architecture?

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

  • 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 represent 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.
  • 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) {
  = 'yellow';

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

    <p myHighlight>Highlight me!</p>
  • 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>
          <div>Learn Angular6 with examples</div>
       </div> `,
    export class AppComponent {
       title: string = 'Welcome to Angular world';
  • 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
  • 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>Learn Angular</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';
  • 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 ]
    export class AppModule { }

    The NgModule decorator has three 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
  • 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, ScreenShot
    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.
  • What is a data binding?

    Data binding is a core concept in Angular and allows to define 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.

    1. From the Component to the DOM: Interpolation: {{ value }}: Adds the value of a property from the component
    <li>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]="">
    1. 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>
    1. 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)]="">
  • 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';
      selector: 'my-component',
      template: '<div>Class decorator</div>',
    export class MyComponent {
      constructor() {
        console.log('Hey I am a component!');
      imports: [],
      declarations: [],
    export class MyModule {
      constructor() {
        console.log('Hey I am a module!');
    1. Property decorators Used for properties inside classes, e.g. @Input and @Output
    import { Component, Input } from '@angular/core';
        selector: 'my-component',
        template: '<div>Property decorator</div>'
    export class MyComponent {
        title: string;
    1. Method decorators Used for methods inside classes, e.g. @HostListener
    import { Component, HostListener } from '@angular/core';
        selector: 'my-component',
        template: '<div>Method decorator</div>'
    export class MyComponent {
        @HostListener('click', ['$event'])
        onHostClick(event: Event) {
            // clicked, `event` available
    1. Parameter decorators Used for parameters inside class constructors, e.g. @Inject
    import { Component, Inject } from '@angular/core';
    import { MyService } from './my-service';
        selector: 'my-component',
        template: '<div>Parameter decorator</div>'
    export class MyComponent {
        constructor(@Inject(MyService) myService) {
            console.log(myService); // MyService
  • 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
    1. Running the Project: ng serve
  • 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{
         //called first time before the ngOnInit()
         //called after the constructor and called  after the first ngOnChanges()
  • 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
    export class RepoService{
      constructor(private http: Http){
        return this.http.get('').map(res => res.json());

    The above service uses Http service as a dependency.

  • 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.

  • 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.

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

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

  • 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.

  • 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>';
  • 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,

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

    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.

  • What are template expressions?

    A template expression produces a value similar to any Javascript expression. Angular executes the expression and assigns it to a property of a binding target; the target might be an HTML element, a component, or a directive. In the property binding, a template expression appears in quotes to the right of the = symbol as in [property]=”expression”. In interpolation syntax, the template expression is surrounded by double curly braces. For example, in the below interpolation, the template expression is {{username}},

    <h3>{{username}}, welcome to Angular</h3>

    The below javascript expressions are prohibited in template expression

    1. assignments (=, +=, -=, …)
    2. new
    3. chaining expressions with ; or ,
    4. increment and decrement operators (++ and –)

  • What are template statements?

    A template statement responds to an event raised by a binding target such as an element, component, or directive. The template statements appear in quotes to the right of the = symbol like (event)=”statement”. Let’s take an example of button click event’s statement

    <button (click)="editProfile()">Edit Profile</button>

    In the above expression, editProfile is a template statement. The below JavaScript syntax expressions are not allowed.

    1. new
    2. increment and decrement operators, ++ and —
    3. operator assignment, such as += and -=
    4. the bitwise operators | and &
    5. the template expression operators

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

  • 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';
              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);
  • What is a custom pipe?

    Apart from built-inn pipes, you can write your own custom pipe with the below key characteristics,

    1. A pipe is a class decorated with pipe metadata @Pipe decorator, which you import from the core Angular library For example,
        @Pipe({name: 'myCustomPipe'})
    1. The pipe class implements the PipeTransform interface’s transform method that accepts an input value followed by optional parameters and returns the transformed value. The structure of pipeTransform would be as below,
    interface PipeTransform {
      transform(value: any, ...args: any[]): any
    1. The @Pipe decorator allows you to define the pipe name that you’ll use within template expressions. It must be a valid JavaScript identifier.
    template: `{{someInputValue | myCustomPipe: someOtherValue}}`
  • 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>
  • 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.

  • 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 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 */
      declarations: [
      imports: [
      providers: [],
      bootstrap: [AppComponent]
    export class AppModule { }
  • What are observables?

    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.

  • 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
  • Explain on how to use HttpClient with an example?

    Below are the steps need to be followed for the usage of HttpClient.

    1. Import HttpClient into root module:
    import { HttpClientModule } from '@angular/common/http';
      imports: [
        // import HttpClientModule after BrowserModule.
     export class AppModule {}
    1. Inject the HttpClient into the application: Let’s create a userProfileService(userprofile.service.ts) as an example. It also defines get method of HttpClient
    import { Injectable } from '@angular/core';
    import { HttpClient } from '@angular/common/http';
    const userProfileUrl: string = 'assets/data/profile.json';
    export class UserProfileService {
      constructor(private http: HttpClient) { }
    getUserProfile() {
      return this.http.get(this.userProfileUrl);
    1. Create a component for subscribing service: Let’s create a component called UserProfileComponent(userprofile.component.ts) which inject UserProfileService and invokes the service method,
    fetchUserProfile() {
        .subscribe((data: User) => this.user = {
            id: data['userId'],
            name: data['firstName'],
            city:  data['city']

    Since the above service method returns an Observable which needs to be subscribed in the component.

  • 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.

  • 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() {
          (data: User) => this.userProfile = { }, // 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.

  • 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';
  • 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
    // => 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
  • 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. Let see the simple example of observable,

    import { Observable } from 'rxjs';
    const observable = new Observable(observer => {
      setTimeout(() => {'Hello from a Observable!');
      }, 2000);
  • 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,


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


  • What are the router imports?

    The Angular Router which represents a particular component view for a given URL is not part of Angular Core. It is available in library named @angular/router to import required router components. For example, we import them in app module as below,

    import { RouterModule, Routes } from '@angular/router';
  • What is router outlet?

    The RouterOutlet is a directive from the router library and it acts as a placeholder that marks the spot in the template where the router should display the components for that outlet. Router outlet is used like a component,

    <!-- Routed components go here -->
  • 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>
      <a routerLink="/todosList" >List of todos</a>
      <a routerLink="/completed" >Completed todos</a>
  • 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>
     <a routerLink="/todosList" routerLinkActive="active">List of todos</a>
     <a routerLink="/completed" routerLinkActive="active">Completed todos</a>
  • 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.

    class MyComponent {
      constructor(router: Router) {
        const state: RouterState = router.routerState;
        const root: ActivatedRoute = state.root;
        const child = root.firstChild;
        const id: Observable<string> = =>;
  • What are router events?

    During each navigation, the Router emits navigation events through the 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
  • 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,

    class MyComponent {
      constructor(route: ActivatedRoute) {
        const id: Observable<string> = route.params.pipe(map(p =>;
        const url: Observable<string> = route.url.pipe(map(segments => segments.join('')));
        // includes both `data` and `resolve`
        const user = => d.user));
  • 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 }
     imports: [
         { enableTracing: true } // <-- debugging purposes only
       // other imports here
    export class AppModule { }
  • 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 }
  • 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.

  • 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.

  • 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)
  • 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
  • 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.

  • 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.

  • 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.
  • What are the ways to control AOT compilation?

    You can control your app compilation in two ways

    1. By providing template compiler options in the tsconfig.json file
    2. By configuring Angular metadata with decorators
  • What are the restrictions of metadata?

    In Angular, You must write metadata with the following general constraints,

    1. Write expression syntax with in the supported range of javascript features
    2. The compiler can only reference symbols which are exported
    3. Only call the functions supported by the compiler
    4. Decorated and data-bound class members must be public.
  • What are the two phases of AOT?

    The AOT compiler works in three phases,

    1. Code Analysis: The compiler records a representation of the source
    2. Code generation: It handles the interpretation as well as places restrictions on what it interprets.
    3. Validation: In this phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates.
  • Can I use arrow functions in AOT?

    No, Arrow functions or lambda functions can’t be used to assign values to the decorator properties. For example, the following snippet is invalid:

      providers: [{
        provide: MyService, useFactory: () => getService()

    To fix this, it has to be changed as following exported function:

    function getService(){
      return new MyService();
      providers: [{
        provide: MyService, useFactory: getService

    If you still use arrow function, it generates an error node in place of the function. When the compiler later interprets this node, it reports an error to turn the arrow function into an exported function. Note: From Angular5 onwards, the compiler automatically performs this rewriting while emitting the .js file.

  • What is the purpose of metadata json files?

    The metadata.json file can be treated as a diagram of the overall structure of a decorator’s metadata, represented as an abstract syntax tree(AST). During the analysis phase, the AOT collector scan the metadata recorded in the Angular decorators and outputs metadata information in .metadata.json files, one per .d.ts file.

  • Can I use any javascript feature for expression syntax in AOT?

    No, the AOT collector understands a subset of (or limited) JavaScript features. If an expression uses unsupported syntax, the collector writes an error node to the .metadata.json file. Later point of time, the compiler reports an error if it needs that piece of metadata to generate the application code.

  • What is folding?

    The compiler can only resolve references to exported symbols in the metadata. Where as some of the non-exported members are folded while generating the code. i.e Folding is a process in which the collector evaluate an expression during collection and record the result in the .metadata.json instead of the original expression. For example, the compiler couldn’t refer selector reference because it is not exported

    let selector = 'app-root';
      selector: selector

    Will be folded into inline selector

          selector: 'app-root'

    Remember that the compiler can’t fold everything. For example, spread operator on arrays, objects created using new keywords and function calls.

  • What are macros?

    The AOT compiler supports macros in the form of functions or static methods that return an expression in a single return expression. For example, let us take a below macro function,

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

    You can use it inside metadata as an expression,

      declarations: wrapInArray(TypicalComponent)
    export class TypicalModule {}

    The compiler treats the macro expression as it written directly

      declarations: [TypicalComponent]
    export class TypicalModule {}
  • Give an example of few metadata errors?

    Below are some of the errors encountered in metadata,

    1. Expression form not supported: Some of the language features outside of the compiler’s restricted expression syntax used in angular metadata can produce this error. Let’s see some of these examples,
      1. export class User { ... }
         const prop = typeof User; // typeof is not valid in metadata
      2. { provide: 'token', useValue: { [prop]: 'value' } }; // bracket notation is not valid in metadata
    2. ** Reference to a local (non-exported) symbol:** The compiler encountered a referenced to a locally defined symbol that either wasn’t exported or wasn’t initialized. Let’s take example of this error,
      // ERROR
      let username: string; // neither exported nor initialized
        selector: 'my-component',
        template: ... ,
        providers: [
          { provide: User, useValue: username }
      export class MyComponent {}

      You can fix this by either exporting or initializing the value,

      export let username: string; // exported
      let username = 'John'; // initialized
    3. Function calls are not supported: The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider’s useFactory to an anonymous function or arrow function as below.
       providers: [
          { provide: MyStrategy, useFactory: function() { ... } },
          { provide: OtherStrategy, useFactory: () => { ... } }

      You can fix this with exported function

      export function myStrategy() { ... }
      export function otherStrategy() { ... }
      ... // metadata
      providers: [
          { provide: MyStrategy, useFactory: myStrategy },
          { provide: OtherStrategy, useFactory: otherStrategy },
    4. Destructured variable or constant not supported: The compiler does not support references to variables assigned by destructuring. For example, you cannot write something like this:
      import { user } from './user';
      // destructured assignment to name and age
      const {name, age} = user;
      ... //metadata
      providers: [
          {provide: Name, useValue: name},
          {provide: Age, useValue: age},

      You can fix this by non-destructured values

      import { user } from './user';
      ... //metadata
      providers: [
          {provide: Name, useValue:},
          {provide: Age, useValue: user.age},
  • What is metadata rewriting?

    Metadata rewriting is the process in which the compiler converts the expression initializing the fields such as useClass, useValue, useFactory, and data into an exported variable, which replaces the expression. Remember that the compiler does this rewriting during the emit of the .js file but not in definition files( .d.ts file).

  • How do you provide configuration inheritance?

    Angular Compiler supports configuration inheritance through extends in the tsconfig.json on angularCompilerOptions. i.e, The configuration from the base file(for example, tsconfig.base.json) are loaded first, then overridden by those in the inheriting config file.

      "extends": "../tsconfig.base.json",
      "compilerOptions": {
        "experimentalDecorators": true,
      "angularCompilerOptions": {
        "fullTemplateTypeCheck": true,
        "preserveWhitespaces": true,
  • How do you specify angular template compiler options?

    The angular template compiler options are specified as members of the angularCompilerOptions object in the tsconfig.json file. These options will be specified adjecent to typescript compiler options.

      "compilerOptions": {
        "experimentalDecorators": true,
      "angularCompilerOptions": {
        "fullTemplateTypeCheck": true,
        "preserveWhitespaces": true,
  • How do you enable binding expression validation?

    You can enable binding expression validation explicitly by adding the compiler option fullTemplateTypeCheck in the “angularCompilerOptions” of the project’s tsconfig.json. It produces error messages when a type error is detected in a template binding expression. For example, consider the following component:

      selector: 'my-component',
      template: '{{}}'
    class MyComponent {
      user?: User;

    This will produce the following error:

    my.component.ts.MyComponent.html(1,1): : Property 'contacts' does not exist on type 'User'. Did you mean 'contact'?
  • What is the purpose of any type cast function?

    You can disable binding expression type checking using $any() type cast function(by surrounding the expression). In the following example, the error Property contacts does not exist is suppressed by casting user to the any type.

      template: '{{$any(user)}}'

    The $any() cast function also works with this to allow access to undeclared members of the component.

       template: '{{$any(this)}}'
  • 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.

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

    The expression used in an ngIf directive is used to narrow type unions in the Angular template compiler similar to if expression in typescript. So *ngIf allows the typeScript compiler to infer that the data used in the binding expression will never be undefined.

      selector: 'my-component',
      template: '<span *ngIf="user"> {{}} </span>'
    class MyComponent {
      user?: User;
  • 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.
  • 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

  • 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.

  • 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


  • What is angular animation?

    Angular’s animation system is built on CSS functionality in order to animate any property that the browser considers animatable. These properties includes positions, sizes, transforms, colors, borders etc. The Angular modules for animations are @angular/animations and @angular/platform-browser and these dependencies are automatically added to your project when you create a project using Angular CLI.

  • What are the steps to use animation module?

    You need to follow below steps to implement animation in your angular project,

    1. Enabling the animations module: Import BrowserAnimationsModule to add animation capabilities into your Angular root application module(for example, src/app/app.module.ts).
    import { NgModule } from '@angular/core';
    import { BrowserModule } from '@angular/platform-browser';
    import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
      imports: [
      declarations: [ ],
      bootstrap: [ ]
    export class AppModule { }
    1. Importing animation functions into component files: Import required animation functions from @angular/animations in component files(for example, src/app/app.component.ts).
    import {
      // ...
    } from '@angular/animations';
    1. Adding the animation metadata property: add a metadata property called animations: within the @Component() decorator in component files(for example, src/app/app.component.ts)
      selector: 'app-root',
      templateUrl: 'app.component.html',
      styleUrls: ['app.component.css'],
      animations: [
        // animation triggers go here
  • What is State function?

    Angular’s state() function is used to define different states to call at the end of each transition. This function takes two arguments: a unique name like open or closed and a style() function. For example, you can write a open state function

    state('open', style({
      height: '300px',
      opacity: 0.5,
      backgroundColor: 'blue'
  • What is Style function?

    The style function is used to define a set of styles to associate with a given state name. You need to use it along with state() function to set CSS style attributes. For example, in the close state, the button has a height of 100 pixels, an opacity of 0.8, and a background color of green.

    state('close', style({
      height: '100px',
      opacity: 0.8,
      backgroundColor: 'green'

    Note: The style attributes must be in camelCase

  • What is the purpose of animate function?

    Angular Animations are a powerful way to implement sophisticated and compelling animations for your Angular single page web application.

    import { Component, OnInit, Input } from '@angular/core';
    import { trigger, state, style, animate, transition } from '@angular/animations';
    selector: 'app-animate',
    templateUrl: `<div [@changeState]="currentState" class="myblock mx-auto"></div>`,
    styleUrls: `.myblock {
        background-color: green;
        width: 300px;
        height: 250px;
        border-radius: 5px;
        margin: 5rem;
    animations: [
        trigger('changeState', [
        state('state1', style({
            backgroundColor: 'green',
            transform: 'scale(1)'
        state('state2', style({
            backgroundColor: 'red',
            transform: 'scale(1.5)'
        transition('*=>state1', animate('300ms')),
        transition('*=>state2', animate('2000ms'))
    export class AnimateComponent implements OnInit {
        @Input() currentState;
        constructor() { }
        ngOnInit() {
  • What is transition function?

    The animation transition function is used to specify the changes that occur between one state and another over a period of time. It accepts two arguments: the first argument accepts an expression that defines the direction between two transition states, and the second argument accepts an animate() function. Let’s take an example state transition from open to closed with an half second transition between states.

    transition('open => closed', [


  • 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';
   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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