Unlocking angular: Some angular interview questions

Prepare to master Angular interviews with this compilation of insightful Angular interview questions and detailed answers. Elevate your understanding of this popular web framework and boost your chances of acing your next interview.

1. what is Reactive programming.

Reactive programming is a programming paradigm that treats streams of data, called Observables, as its basic units of programming

2. what is rxjs. how will u create observable. what all methods it has.

RxJS library is being used to bring Reactive programming(adaptable to change) to our application. You remember previously Object.Observe was introduce in ECMA7 but later on becouse of browser's performance issue it got removed.

RxJS brings similar kind of functionality where we can create observable objects. Methods like map, filter,reduce,take, takeWhile exists on simple streams and in my codebase most of the time I had used Observables or subjects. Where we can subscribe them and in our subscription we get stream of data.

3. what is diffrence between observable and promise

Obseravable can be canceled in middle but promises can not. Promises are given natively in javascript but we need libraries for observable e.g. rxJS

4. what are changes done in angular 2,4,5,6, 10+

When discussing about chenges within angular version 2,4,5,6 are very minimal syntexwize like in angular 2 syntax for using route/http were diffrent or there source library was diffrent but now they had changed.

Animation part is well improved in later version of angular. Several bug fixes are done with performance improvement.

Most of the angular changes its version because of its dependency version changes.

5. how you will communicate between two components.

I can communicate among controller using following ways.

  • using window object(very bad way)
  • using @input, @output
  • using services and events(good way).

I personally like service way more because of its modular way and its simplicity.

6. what is typescript, what all advantage it has.

Initially javascript was developped just to bring dynamicity to web pages but later on people started using it for bigger application.

Since javascript is a loosly typed language it is not compactable for bigger application. because if Team A is using module developped by Team B it will be hard to know what their methods signature is. Typescript solves this issue for us by introducing type.


  • Working on bigger/complex application becomes easy.
  • performance of our application increases since memory consumption by type conversion get reduced at some extent.


  • it is not supported by browsers/javascript engines.
  • it includes some learning curve.

7. what is zoneJS, what all advantage it has.

A zone is an execution context that persists across async tasks, and allows the creator of the zone to observe and control execution of the code within the zone. Pros:

  • Resource utilisation goes high

8. what is AOT

AOT stands for Ahead-of-Time compilation in Angular. It is a compilation process that takes place before the application is executed in the browser. In the context of Angular, the traditional compilation process involves interpreting the application code and translating it into JavaScript at runtime, which is known as Just-In-Time (JIT) compilation.

On the other hand, AOT compilation occurs during the build phase before the application is deployed. It involves translating the Angular templates and components into highly optimized JavaScript code. This optimized code is then shipped to the browser, resulting in faster rendering and improved performance.

Key advantages of AOT compilation in Angular include:

  • Faster Load Times: AOT compilation reduces the size of the JavaScript code that needs to be downloaded by the browser, leading to quicker application load times.

  • Detecting Errors Early: AOT compilation catches template-related errors at build time, enabling developers to identify and resolve issues before deploying the application.

  • Better Security: Since templates are converted to JavaScript during the build process, there is no need to ship the Angular compiler to the client-side, enhancing the security of the application.

  • Optimized Performance: AOT-compiled code tends to have better runtime performance compared to JIT-compiled code.

To enable AOT compilation, you can use the Angular CLI (Command Line Interface) and include the --aot flag when running the build command:

ng build --aot

By leveraging AOT, Angular applications can deliver improved performance and a better user experience.

9. what is providers in angular

In Angular, providers is a metadata property used in the configuration of a service. Services in Angular are a way to organize and share code across different parts of an application. They play a crucial role in promoting modularity, reusability, and maintainability.

The providers array is used in various Angular components such as @NgModule, @Component, and @Directive to register a service with the Angular dependency injection system. The dependency injection system is responsible for creating instances of services and making them available for injection into components, directives, and other services.

Here's a brief overview of how providers are used in Angular:

  • Registering a Service:

    • When you create a service in Angular, you typically decorate it with the @Injectable() decorator.
    • You then include the service in the providers array of an Angular module (@NgModule), a component (@Component), or a directive (@Directive).
  • Dependency Injection:

    • Angular's dependency injection system injects the registered service instances into the components or other services that request them.

Example in @NgModule:

import { NgModule } from '@angular/core'; import { MyService } from './my.service'; @NgModule({ providers: [MyService], // other module configurations... }) export class MyModule { }

Example in @Component:

import { Component } from '@angular/core'; import { MyService } from './my.service'; @Component({ selector: 'app-my-component', providers: [MyService], // other component configurations... }) export class MyComponent { }

In these examples, MyService is registered as a provider in the respective Angular module or component. This makes an instance of MyService available for injection into the components or services within the scope of that module or component.

By using the providers property, you can control the scope and lifecycle of services within your Angular application.

10. how you will protect route

In Angular, protecting routes typically involves implementing a mechanism to restrict access to certain routes based on user authentication or authorization. Here are common approaches to protect routes in Angular:

11. Route Guards:

Angular provides route guards, which are interfaces that can be implemented to control navigation to and from a route. There are different types of route guards:

  • CanActivate: Checks if a route can be activated.
  • CanActivateChild: Checks if children of a route can be activated.
  • CanDeactivate: Checks if a route can be deactivated.
  • CanLoad: Checks if a lazy-loaded module can be loaded.

Example of CanActivate guard:

import { Injectable } from '@angular/core'; import { CanActivate, ActivatedRouteSnapshot, RouterStateSnapshot, UrlTree, Router } from '@angular/router'; import { Observable } from 'rxjs'; import { AuthService } from './auth.service'; @Injectable({ providedIn: 'root', }) export class AuthGuard implements CanActivate { constructor(private authService: AuthService, private router: Router) {} canActivate( route: ActivatedRouteSnapshot, state: RouterStateSnapshot ): boolean | UrlTree | Observable<boolean | UrlTree> | Promise<boolean | UrlTree> { if (this.authService.isAuthenticated()) { return true; } else { // Redirect to the login page if not authenticated return this.router.createUrlTree(['/login']); } } }

You can then use this guard in your routing configuration:

const routes: Routes = [ { path: 'protected', component: ProtectedComponent, canActivate: [AuthGuard], }, ];

12. Role-Based Authorization:

You can extend route guards to include role-based authorization checks. For example, you might have an AuthService with a method like hasRole('admin'), and you can use this method in your guard to check if the user has the required role.

13. Lazy Loading and Preloading:

Use lazy loading to load certain parts of your application only when needed. Additionally, you can implement preloading strategies to load modules in the background after the initial page load.

14. Route Resolvers:

Implement route resolvers to fetch data before activating a route. This can be used to ensure that the user has the necessary data or permissions before navigating to a protected route.

These methods can be used individually or in combination, depending on the specific requirements of your application.

15. how SEO can be done

Search Engine Optimization (SEO) for Angular applications involves ensuring that search engines can properly crawl and index your content. Angular applications are typically Single Page Applications (SPAs), and traditional web crawlers may face challenges when navigating SPAs. Here are some strategies to enhance SEO for Angular applications:

16. Server-Side Rendering (SSR):

  • Implementing server-side rendering generates HTML on the server and sends it to the client, providing a fully rendered page to search engines.
  • Angular Universal is a technology that enables server-side rendering for Angular applications.

17. Prerendering:

  • Prerendering involves creating static HTML pages for your Angular app during the build process.
  • Tools like prerender-spa-plugin can be used to generate static HTML versions of your pages, which can improve search engine indexing.

18. Use the Meta Tags Appropriately:

  • Include relevant meta tags in the <head> section of your HTML to provide information to search engines.
  • Important meta tags include title, description, and keywords.

19. Dynamic Rendering:

  • Implement dynamic rendering to serve different content to search engine crawlers than what is served to regular users.
  • This can be achieved using services like or by configuring your server to detect user-agent strings.

20. Sitemap:

  • Create a sitemap.xml file that lists all the URLs you want search engines to crawl.
  • Submit the sitemap to search engines through their webmaster tools.

21. Robots.txt:

  • Use the robots.txt file to provide directives to web crawlers about which pages should not be crawled.
  • Ensure that critical pages are not blocked by robots.txt.

22. Navigation and Internal Linking:

  • Implement clear navigation and internal linking within your Angular app.
  • Ensure that important pages are accessible through regular HTML links to improve crawlability.

23. Lazy Loading:

  • Implement lazy loading for your routes to ensure that the initial page load is fast.
  • Lazy loading can also improve the perceived performance of your application.

24. Canonical URLs:

  • Use canonical URLs to indicate the preferred version of a page, especially when dealing with duplicate content.
  • Specify the canonical URL in the <head> section of your HTML.

25. Enable HTML5 Mode:

  • Angular applications typically use hash fragments in URLs (/#/route). Enabling HTML5 mode removes the hash and uses regular URLs (/route), which can be more SEO-friendly.

26. Social Media Markup:

  • Implement Open Graph Protocol and Twitter Card tags to control how your content appears when shared on social media platforms.

27. Monitor and Test:

  • Regularly monitor your site's performance in search engine results.
  • Use tools like Google Search Console to identify and fix potential SEO issues.

By combining these strategies, you can improve the search engine visibility of your Angular application and enhance its overall SEO performance.

28. what is directive. create a custom directive

In Angular, a directive is a special type of Angular component that allows you to extend or create custom behavior for elements in the DOM (Document Object Model). Directives are used to add, modify, or remove HTML elements and attributes in a declarative way.

There are three types of directives in Angular:

  • Component Directives: These are the directives that are components with a template.

  • Attribute Directives: These directives change the appearance or behavior of an element, component, or another directive.

  • Structural Directives: These directives alter the structure of the DOM by adding, removing, or manipulating elements.

Now, let's create a simple custom attribute directive in Angular. This example will create a directive called appHighlight that changes the background color of an element when the mouse hovers over it.

  • Create the Directive:
// highlight.directive.ts import { Directive, ElementRef, HostListener, Input } from '@angular/core'; @Directive({ selector: '[appHighlight]' }) export class HighlightDirective { @Input('appHighlight') highlightColor: string = 'yellow'; // Default color is yellow constructor(private el: ElementRef) {} @HostListener('mouseenter') onMouseEnter() { this.highlight(this.highlightColor); } @HostListener('mouseleave') onMouseLeave() { this.highlight(null); // Remove the highlight on mouse leave } private highlight(color: string | null) { = color; } }
  1. Use the Directive:

Now, you can use the appHighlight directive in your component's template:

<!-- some-component.component.html --> <div appHighlight [highlightColor]="'lightblue'"> Hover over me to see the highlight! </div>

In this example, the appHighlight directive is applied to a <div> element, and the highlightColor input is set to 'lightblue'. When you hover over the <div>, the background color changes to light blue.

Remember to add your directive to the declarations array in the module where it is used.

// some.module.ts import { NgModule } from '@angular/core'; import { HighlightDirective } from './highlight.directive'; @NgModule({ declarations: [HighlightDirective], // other module configurations... }) export class SomeModule { }

This is a basic example, but you can create more complex directives to encapsulate and reuse specific behavior across your Angular application.

29. what is filter in angularJS. create a custom filter

In Angular, filters are used to format and manipulate the data displayed in the view. Angular provides built-in filters like currency, date, and uppercase, but you can also create custom filters to suit specific needs.

Angular custom filters are implemented using TypeScript classes and are typically used in conjunction with Angular pipes. A filter class should implement the PipeTransform interface, which requires the implementation of the transform method.

Here's an example of creating a custom filter to capitalize the first letter of each word in a string:

  • Create the Filter Class:
// capitalize-first-letter.pipe.ts import { Pipe, PipeTransform } from '@angular/core'; @Pipe({ name: 'capitalizeFirstLetter' }) export class CapitalizeFirstLetterPipe implements PipeTransform { transform(value: string): string { if (!value) return value; return value.split(' ').map(word => word.charAt(0).toUpperCase() + word.slice(1)).join(' '); } }

In this example, the CapitalizeFirstLetterPipe class implements the PipeTransform interface with the transform method. The method takes a string as input, splits it into words, capitalizes the first letter of each word, and then joins them back into a string.

  1. Use the Custom Filter in a Component:
// some.component.ts import { Component } from '@angular/core'; @Component({ selector: 'app-some', template: ` <div> Original Text: {{ originalText }} </div> <div> Capitalized Text: {{ originalText | capitalizeFirstLetter }} </div> ` }) export class SomeComponent { originalText: string = 'hello world'; }
  1. Add the Filter to a Module:
// some.module.ts import { NgModule } from '@angular/core'; import { CapitalizeFirstLetterPipe } from './capitalize-first-letter.pipe'; @NgModule({ declarations: [CapitalizeFirstLetterPipe], // other module configurations... }) export class SomeModule { }

In this example, the custom filter capitalizeFirstLetter is used in the template of the SomeComponent to capitalize the first letter of each word in the originalText string.

Remember to add the custom filter to the declarations array of the module where it is used.

Custom filters provide a way to encapsulate specific data manipulation logic, making it reusable across different parts of your Angular application.

30. what all lifecycle it has

In Angular, components and directives have a lifecycle managed by the Angular framework. These lifecycles provide hooks that allow you to perform actions at various stages of the component or directive lifecycle. Here are the main lifecycle hooks for Angular components:

31. OnInit:

  • Hook Method: ngOnInit
  • Description: Called after the component has been initialized. It is a good place to perform initialization logic.

32. OnChanges:

  • Hook Method: ngOnChanges
  • Description: Called when one or more input properties change. It receives a SimpleChanges object that contains information about the changes.

33. DoCheck:

  • Hook Method: ngDoCheck
  • Description: Called during every change detection run. It allows you to implement custom change detection logic.

34. AfterContentInit:

  • Hook Method: ngAfterContentInit
  • Description: Called after Angular has fully initialized the content that the component or directive manages.

35. AfterContentChecked:

  • Hook Method: ngAfterContentChecked
  • Description: Called after Angular checks the content that the component or directive manages.

36. AfterViewInit:

  • Hook Method: ngAfterViewInit
  • Description: Called after Angular has fully initialized the view of the component or directive.

37. AfterViewChecked:

  • Hook Method: ngAfterViewChecked
  • Description: Called after Angular checks the view of the component or directive.

38. OnDestroy:

  • Hook Method: ngOnDestroy
  • Description: Called just before the component or directive is destroyed. It is a good place to perform cleanup tasks.

These lifecycle hooks allow you to tap into different stages of the component or directive lifecycle and execute custom logic. For example, you might use ngOnInit for initialization tasks, ngOnChanges to respond to input changes, and ngOnDestroy to clean up resources before a component is destroyed.

Here's an example of a component implementing some of these lifecycle hooks:

import { Component, Input, OnInit, OnChanges, OnDestroy } from '@angular/core'; @Component({ selector: 'app-lifecycle-example', template: '...', }) export class LifecycleExampleComponent implements OnInit, OnChanges, OnDestroy { @Input() data: string; ngOnInit() { // Initialization logic } ngOnChanges() { // Handle input changes } ngOnDestroy() { // Cleanup tasks before component is destroyed } }

Understanding and utilizing these lifecycle hooks is essential for managing the behavior and performance of your Angular components and directives.

39. How you changed your application from angularJS to angular

Migrating from AngularJS (often referred to as Angular 1.x) to Angular (2 and above) involves a significant transition, as Angular and AngularJS are different frameworks with distinct architectures. Here's a general guide on how you might approach this migration:

40. Understand the Differences:

  • Angular and AngularJS have different architectures, syntax, and concepts. Familiarize yourself with the Angular documentation and understand the changes you'll need to make.

41. Assess the Application:

  • Evaluate the size and complexity of your AngularJS application. Smaller applications may benefit from a complete rewrite, while larger applications may prefer a gradual migration.

42. Choose a Migration Strategy:

  • There are several migration strategies, including the "Big Bang" (complete rewrite) and "Incremental" (gradual migration). The choice depends on your application's complexity, timeline, and business requirements.

43. Set Up a New Angular Project:

  • Create a new Angular project using the Angular CLI or another setup method. This project will house the new Angular code.

44. Identify Common Components:

  • Identify common components, services, and features between AngularJS and Angular. These shared elements can be used as a starting point for the migration.

45. Implement Angular Components:

  • Begin reimplementing components in Angular, starting with simpler ones. Focus on converting templates, styles, and basic functionality.

46. Establish Communication:

  • Establish communication between AngularJS and Angular components. This is crucial for a gradual migration. AngularJS components can be upgraded and coexist with Angular components during the transition.

47. Upgrade Services:

  • Migrate services and business logic from AngularJS to Angular. Services can be shared between AngularJS and Angular components.

48. Migrate Routing:

  • If your AngularJS application uses routing, migrate it to Angular's routing system. Adjust the navigation structure as needed.

49. Testing:

  • Write tests for your Angular components using Angular testing tools. Ensure that existing tests for AngularJS components are still valid.

50. Gradual Removal of AngularJS:

  • As components and features are migrated, gradually remove AngularJS dependencies. Eventually, you can phase out the entire AngularJS codebase.

51. Optimize and Enhance:

  • Take advantage of Angular features, such as Ahead-of-Time (AOT) compilation, lazy loading, and improved performance optimizations.

52. Update Dependencies:

  • Ensure that third-party libraries and dependencies used in the AngularJS application are compatible with Angular.

53. Documentation and Training:

  • Update documentation and provide training for the development team on Angular concepts and best practices.

54. Testing and QA:

  • Rigorous testing is crucial. Test the Angular application thoroughly to catch any issues during the migration.

55. Deploy and Monitor:

  • Deploy the new Angular application and monitor its performance. Be ready to address any issues that may arise in a production environment.

Remember that migrating from AngularJS to Angular is a complex process, and the specific steps may vary based on the application's architecture and requirements. It's essential to thoroughly plan, test, and iterate during the migration process. Additionally, consider seeking guidance from the Angular community and resources provided by the Angular team.

Published On: 2024-01-17