Angular interview questions

For someone with 2 years of experience in Angular, the interview questions typically range from basic concepts to intermediate-level topics. Here’s a categorized list of commonly asked questions:

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Basic Concepts

1. What is Angular, and how is it different from AngularJS?
2. What are the building blocks of Angular?
3. Explain the difference between components and directives.
4. What is data binding, and what are its types?
5. What is the purpose of Angular CLI? Can you name some CLI commands?
6. What is a TypeScript, and why is it used in Angular?

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Components and Templates

1. What is a component? How do you create and use it?
2. Explain the role of @Input() and @Output() decorators.
3. What is the difference between template-driven forms and reactive forms?
4. How do you handle events in Angular templates?
5. What are lifecycle hooks? Can you name and explain their purposes?
6. How would you implement conditional rendering in an Angular template?

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Directives

1. What are Angular directives, and how are they classified?
2. What is the difference between structural and attribute directives?
3. How do you create a custom directive?
4. Explain the purpose of ngIf, ngFor, and ngClass.

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Services and Dependency Injection

1. What is a service in Angular, and how do you create one?
2. What is dependency injection, and how does Angular implement it?
3. What is the purpose of a singleton service?
4. Explain the difference between providedIn: 'root' and providing a service in a specific module.

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Routing

1. How does routing work in Angular?
2. What is the purpose of RouterModule and Routes?
3. How do you implement lazy loading in Angular?
4. What are route guards, and what types of guards are available?
5. How do you pass parameters in Angular routes?

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Forms

1. What is the difference between reactive forms and template-driven forms?
2. How do you perform form validation in Angular?
3. How do you bind form data to the component?

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HTTP and Observables

1. How do you make HTTP requests in Angular?
2. What is the purpose of the HttpClient module?
3. What is an observable, and how is it used in Angular?
4. What is the difference between Promise and Observable?

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Angular Modules

1. What is the purpose of NgModules?
2. Explain the difference between feature modules and shared modules.
3. How do you organize an Angular application into modules?

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Performance and Optimization

1. What is AOT (Ahead-of-Time) compilation, and why is it important?
2. How does Angular handle tree shaking?
3. What are some ways to optimize an Angular application?

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Miscellaneous

1. What are Angular Pipes? How do you create a custom pipe?
2. What is the difference between Angular’s ngOnInit and constructor?
3. What is the purpose of Angular’s zone.js?
4. What are interceptors, and how are they used in Angular?
5. Explain the role of Change Detection in Angular.
6. What is the role of the trackBy function in ngFor?

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Practical Scenarios

1. How would you debug an Angular application?
2. How would you handle a slow-loading Angular application?
3. Describe a project you worked on and your role in the team.
4. How do you manage version control in Angular projects?

Would you like me to create short notes or examples for any of these topics?

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Here are comprehensive answers to all the Angular interview questions you listed, organized by topic:

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Basic Concepts

1. What is Angular, and how is it different from AngularJS?

   • Angular is a TypeScript-based front-end framework for building dynamic single-page applications. AngularJS (v1.x) is based on JavaScript and uses a two-way binding architecture. Angular offers improved performance with Ahead-of-Time (AOT) compilation, component-based architecture, and dependency injection.

2. What are the building blocks of Angular?

   • The primary building blocks of Angular are:
     • Modules (e.g., NgModules)
     • Components
     • Templates
     • Directives
     • Services
     • Dependency Injection
     • Pipes
     • Routing

3. Explain the difference between components and directives.

   • Components control views, have a template and styles, and are the main building blocks of UI.
   • Directives modify the behavior or appearance of DOM elements.
     • Structural Directives: Change DOM structure (e.g., *ngIf, *ngFor).
     • Attribute Directives: Change the appearance or behavior of an element (e.g., ngClass, ngStyle).

4. What is data binding, and what are its types?

   • Data binding links UI and application logic. Types:
     • Interpolation: {{data}} (one-way from component to view).
     • Property Binding: [property]="expression" (one-way).
     • Event Binding: (event)="handler()" (one-way from view to component).
     • Two-Way Binding: [(ngModel)]="property" (two-way).

5. What is the purpose of Angular CLI? Can you name some CLI commands?

   • CLI automates app creation, testing, and deployment. Common commands:
     • ng new <app-name>: Creates a new Angular app.
     • ng serve: Runs the application locally.
     • ng generate <component|service> <name>: Generates components or services.
     • ng build: Builds the project for deployment.
     • ng test: Runs unit tests.

6. What is TypeScript, and why is it used in Angular?

   • TypeScript is a superset of JavaScript that adds static typing. Angular uses TypeScript for better tooling, maintainability, and early error detection.

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Components and Templates

1. What is a component? How do you create and use it?

   • A component is the fundamental building block of Angular's UI. It controls a part of the UI with its template and logic.
     • Create: ng generate component <name>
     • Use: Add the selector (e.g., <app-name></app-name>) in the HTML.

2. Explain the role of @Input() and @Output() decorators.

   • @Input(): Pass data from a parent to a child component.
   • @Output(): Emit events from a child to a parent using EventEmitter.

3. What is the difference between template-driven forms and reactive forms?

   • Template-Driven Forms: Define forms in the HTML template; simpler for small forms.
   • Reactive Forms: Define forms in the component class using FormGroup and FormControl; better for dynamic and complex forms.

4. How do you handle events in Angular templates?

   • Use event binding syntax: (event)="handlerFunction()". Example: <button (click)="onClick()">Click Me</button>

5. What are lifecycle hooks? Can you name and explain their purposes?

   • Lifecycle hooks allow developers to tap into component events. Examples:
     • ngOnInit(): Executes after the component is initialized.
     • ngOnChanges(): Executes when input properties change.
     • ngOnDestroy(): Executes just before the component is destroyed.

6. How would you implement conditional rendering in an Angular template?

   • Use *ngIf:

    <div *ngIf="condition">Content</div>
   

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Directives

1. What are Angular directives, and how are they classified?

   • Directives are instructions to manipulate the DOM.
     • Structural Directives: Modify DOM structure (e.g., *ngIf, *ngFor).
     • Attribute Directives: Modify element behavior or appearance (e.g., ngStyle, ngClass).

2. What is the difference between structural and attribute directives?

   • Structural: Change the DOM layout (e.g., add/remove elements).
   • Attribute: Change the appearance or behavior of an element.

3. How do you create a custom directive?

   • Use @Directive decorator. Example:

    @Directive({
      selector: '[appHighlight]'
    })
    export class HighlightDirective {
      constructor(el: ElementRef) {
        el.nativeElement.style.backgroundColor = 'yellow';
      }
    }
   

4. Explain the purpose of ngIf, ngFor, and ngClass.

   • ngIf: Conditionally adds/removes elements.
   • ngFor: Iterates over a list.
   • ngClass: Dynamically applies classes.

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Services and Dependency Injection

1. What is a service in Angular, and how do you create one?

   • A service contains reusable business logic. Create it using CLI:

    ng generate service serviceName
   

2. What is dependency injection, and how does Angular implement it?

   • DI is a design pattern where dependencies are provided to a class rather than created by it. Angular implements DI using the injector hierarchy.

3. What is the purpose of a singleton service?

   • A singleton service ensures that a single instance of the service is shared across the app.

4. Explain the difference between providedIn: 'root' and providing a service in a specific module.

   • providedIn: 'root': Makes the service available app-wide.
   • Providing in a module: Restricts availability to that module.

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Routing in Angular

1. How does routing work in Angular?

   • Angular routing allows navigation between views (components) within a single-page application (SPA).
   • The Router listens to URL changes and maps them to a specific component using route configuration.
   • Example:

    const routes: Routes = [
      { path: 'home', component: HomeComponent },
      { path: 'about', component: AboutComponent },
      { path: '', redirectTo: 'home', pathMatch: 'full' }
    ];
    @NgModule({
      imports: [RouterModule.forRoot(routes)],
      exports: [RouterModule]
    })
    export class AppRoutingModule {}
   

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1. What is the purpose of RouterModule and Routes?
   • RouterModule: Provides the necessary services and directives for routing (e.g., <router-outlet>, routerLink).
   • Routes: Defines the mapping of URL paths to components. It is an array of objects, where each object represents a route.

Example:

   const routes: Routes = [
     { path: 'dashboard', component: DashboardComponent },
     { path: 'profile', component: ProfileComponent }
   ];

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1. How do you implement lazy loading in Angular?
   • Lazy loading loads feature modules only when their associated route is accessed, reducing the initial bundle size.
   • Steps:
     1. Create a feature module (e.g., AdminModule).
     2. Define routes within the feature module.
     3. Use the loadChildren property in the app's routes to point to the feature module.

Example:

   const routes: Routes = [
     { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) }
   ];

In AdminModule:

   const adminRoutes: Routes = [
     { path: '', component: AdminDashboardComponent }
   ];
   @NgModule({
     imports: [RouterModule.forChild(adminRoutes)],
     exports: [RouterModule]
   })
   export class AdminRoutingModule {}

---

1. What are route guards, and what types of guards are available?
   • Route Guards control navigation to and from routes based on conditions.
   • Types of Guards:
     • CanActivate: Determines if a route can be activated.
     • CanActivateChild: Determines if child routes can be activated.
     • CanDeactivate: Determines if a user can leave a route.
     • Resolve: Pre-fetches data before the route is activated.
     • CanLoad: Determines if a module can be lazy-loaded.

Example of CanActivate:

   @Injectable({
     providedIn: 'root'
   })
   export class AuthGuard implements CanActivate {
     canActivate(): boolean {
       return isAuthenticated(); // Custom logic
     }
   }

Apply guard in routes:

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

---

1. How do you pass parameters in Angular routes?

   • Route Parameters:
     • Define a route with a parameter:

      const routes: Routes = [
        { path: 'profile/:id', component: ProfileComponent }
      ];
   

 - Access the parameter in the component:

   ```typescript
   constructor(private route: ActivatedRoute) {}
   ngOnInit() {
     this.route.params.subscribe(params => {
       console.log(params['id']);
     });
   }
   ```

• Query Parameters:

  • Pass query parameters:

     <a [routerLink]="['/profile']" [queryParams]="{ id: 123 }">Profile</a>
  

  • Access query parameters in the component:

     constructor(private route: ActivatedRoute) {}
     ngOnInit() {
       this.route.queryParams.subscribe(params => {
         console.log(params['id']);
       });
     }
  

---

Forms

1. What is the difference between reactive forms and template-driven forms?

   • Reactive Forms:
     • Defined programmatically in the component using FormGroup and FormControl.
     • More scalable and testable.
     • Ideal for dynamic forms.
   • Template-Driven Forms:
     • Defined in the template using directives like ngModel.
     • Simpler, ideal for small forms.

2. How do you perform form validation in Angular?

   • Reactive Forms: Add validators programmatically:

    this.myForm = new FormGroup({
      name: new FormControl('', [Validators.required, Validators.minLength(3)])
    });
   

• Template-Driven Forms: Use validation directives in the template:
  

   <input name="name" ngModel required minlength="3" />
  

1. How do you bind form data to the component?
   • Reactive Forms: Use FormGroup and FormControl bindings.
   • Template-Driven Forms: Use [(ngModel)]="property" for two-way binding.

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HTTP and Observables

1. How do you make HTTP requests in Angular?

   • Use the HttpClient module:

    this.http.get('api/url').subscribe(data => console.log(data));
   

2. What is the purpose of the HttpClient module?

   • Simplifies HTTP requests and handles tasks like request headers, responses, and error handling. It also supports observables.

3. What is an observable, and how is it used in Angular?

   • Observables are streams of data that can emit multiple values over time. Angular uses observables for asynchronous operations like HTTP calls and event handling.

4. What is the difference between Promise and Observable?

   • Promise: Emits a single value and is not cancellable.
   • Observable: Emits multiple values, can be cancellable, and supports operators like map and filter.

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Angular Modules

1. What is the purpose of NgModules?

   • NgModules group components, directives, pipes, and services into cohesive blocks to organize and modularize an Angular application.

2. Explain the difference between feature modules and shared modules.

   • Feature Modules: Contain components, services, and directives specific to a feature.
   • Shared Modules: Contain reusable components, pipes, and directives to be imported into other modules.

3. How do you organize an Angular application into modules?

   • Create feature modules for individual functionalities, a shared module for reusable code, and core modules for singleton services.

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Performance and Optimization

1. What is AOT (Ahead-of-Time) compilation, and why is it important?

   • AOT compiles Angular HTML and TypeScript into JavaScript at build time, reducing runtime overhead and improving performance.

2. How does Angular handle tree shaking?

   • Tree shaking removes unused modules and code during the build process, reducing bundle size.

3. What are some ways to optimize an Angular application?

   • Use lazy loading for routes.
   • Minimize change detection with OnPush.
   • Optimize template bindings.
   • Enable AOT compilation.
   • Use lightweight libraries.

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Miscellaneous

1. What are Angular Pipes? How do you create a custom pipe?

   • Pipes transform data in templates. Create a custom pipe:

    @Pipe({ name: 'customPipe' })
    export class CustomPipe implements PipeTransform {
      transform(value: string): string {
        return value.toUpperCase();
      }
    }
   

2. What is the difference between Angular’s ngOnInit and the constructor?

   • Constructor: Used for dependency injection.
   • ngOnInit: Executes after component initialization, ideal for initialization logic.

3. What is the purpose of Angular’s zone.js?

   • zone.js intercepts and keeps track of asynchronous tasks, enabling Angular's change detection.

4. What are interceptors, and how are they used in Angular?

   • Interceptors intercept HTTP requests and responses for tasks like adding headers or handling errors.

    @Injectable()
    export class AuthInterceptor implements HttpInterceptor {
      intercept(req: HttpRequest<any>, next: HttpHandler) {
        const authReq = req.clone({ setHeaders: { Authorization: 'Bearer token' } });
        return next.handle(authReq);
      }
    }
   

5. Explain the role of Change Detection in Angular.

   • Change detection updates the DOM whenever data changes in the component. Strategies:
     • Default: Checks the entire component tree.
     • OnPush: Only checks when input properties change.

6. What is the role of the trackBy function in ngFor?

   • Optimizes rendering by tracking items using a unique identifier. Prevents unnecessary DOM updates.

---

Practical Scenarios

1. How would you debug an Angular application?

   • Use browser dev tools, Angular DevTools, and console.log().
   • Debug HTTP calls with network tools.
   • Check errors in the console and investigate stack traces.

2. How would you handle a slow-loading Angular application?

   • Implement lazy loading and AOT compilation.
   • Optimize bindings and use the OnPush change detection strategy.
   • Minimize heavy computations in templates.

3. Describe a project you worked on and your role in the team.

   • Prepare a concise description of a project, highlighting your role in implementing features, solving problems, or optimizing performance.

4. How do you manage version control in Angular projects?

   • Use Git for version control. Follow practices like branching, committing changes with meaningful messages, and using pull requests for code review.

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Let's focus on advanced concepts and real-world scenarios:

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Architecture and Best Practices

1. How do you design a scalable Angular application architecture?
   • Answer:
     • Use a modular architecture with feature modules for specific functionality.
     • Implement a shared module for reusable components and pipes.
     • Use a core module for singleton services.
     • Apply lazy loading for optimizing large-scale applications.
     • Follow SOLID principles for component and service design.

---

1. How do you handle state management in Angular applications?
   • Answer:
     • Use libraries like NgRx, Akita, or services with BehaviorSubjects.
     • Implement NgRx for complex applications with features like selectors, effects, and reducers.
     • Use the OnPush change detection strategy for performance.
     • Ensure immutability by using libraries like immer or plain JavaScript techniques.

---

1. How do you ensure consistent coding standards in a team project?
   • Answer:
     • Use Angular Style Guide for consistent structure.
     • Enforce linting rules with ESLint or TSLint.
     • Implement prettier for code formatting.
     • Set up pre-commit hooks with Husky to run linters and unit tests.
     • Conduct regular code reviews and encourage pair programming.

---

Performance Optimization

1. How do you optimize the performance of an Angular application?
   • Answer:
     • Enable Ahead-of-Time (AOT) compilation for faster rendering.
     • Use lazy loading and preloading strategies for modules.
     • Optimize template bindings and minimize watchers with ChangeDetectionStrategy.OnPush.
     • Avoid large DOM manipulations and use Angular's trackBy in ngFor directives.
     • Cache HTTP requests with RxJS operators like shareReplay.

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1. What tools and techniques do you use for debugging Angular applications?
   • Answer:
     • Use Augury or Angular DevTools for component tree debugging.
     • Debug HTTP calls with browser DevTools' network tab.
     • Add console.log or Angular’s DebugElement for runtime checks.
     • Use RxJS debugging techniques with tap or libraries like RxJS Spy.
     • Monitor performance using tools like Lighthouse or Webpack Bundle Analyzer.

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Advanced Topics

1. How does Angular handle dependency injection at a deeper level?
   • Answer:
     • Angular uses a hierarchical dependency injection system.
     • Providers declared in AppModule are shared across the application.
     • Providers in lazy-loaded modules create their own instances.
     • Multi-provider tokens allow multiple values for the same injection token.
     • Use @Inject to resolve tokens and @Optional() for optional dependencies.

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1. Explain the Angular Compiler's role in optimizing performance.
   • Answer:
     • The Angular compiler pre-compiles templates and components during the build process using AOT.
     • It reduces runtime errors and improves security by detecting issues early.
     • Generates optimized JavaScript code, enabling tree shaking to remove unused code.

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1. What is the difference between Zone.js and Angular's Change Detection?
   • Answer:
     • Zone.js: Tracks asynchronous operations (e.g., HTTP calls, events) to trigger change detection.
     • Change Detection: Updates the DOM based on changes in the application state.
     • Use ChangeDetectorRef to fine-tune or manually trigger change detection.

---

Testing

1. How do you ensure robust testing for large Angular applications?
   • Answer:
     • Write unit tests using Jasmine and Karma for components, services, and pipes.
     • Write integration tests using Angular Testing Utilities like TestBed.
     • Use end-to-end (E2E) testing with Protractor or Cypress.
     • Mock dependencies using libraries like Jest or Angular’s testing utilities.
     • Maintain high code coverage and integrate testing in CI/CD pipelines.

---

1. What strategies do you use for mocking HTTP requests in Angular tests?

   • Answer:

     • Use Angular’s HttpTestingController to mock HTTP requests in unit tests.
     • Example:

     it('should fetch data', () => {
       service.getData().subscribe(data => {
         expect(data).toEqual(mockData);
       });
       const req = httpTestingController.expectOne('api/data');
       req.flush(mockData);
     });
     

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Real-World Challenges

1. How do you handle application security in Angular?
   • Answer:
     • Sanitize user inputs with Angular’s DomSanitizer.
     • Avoid cross-site scripting (XSS) by using Angular's template bindings ({{}}) and avoiding direct DOM manipulations.
     • Use Angular's built-in CSRF protection.
     • Secure HTTP calls with HTTPS and add authentication tokens to headers.

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1. How do you handle version upgrades in large Angular projects?
   • Answer:
     • Use Angular Update Guide for a step-by-step upgrade plan.
     • Upgrade Angular CLI and dependencies incrementally.
     • Run unit tests and e2e tests after each upgrade.
     • Refactor deprecated APIs gradually.

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1. How do you manage feature toggles in Angular applications?
   • Answer:
     • Use configuration files or services to toggle features dynamically.
     • Leverage NgRx Store or custom state management for feature toggles.
     • Hide or show UI elements based on feature toggle flags.

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Enterprise-Level Practices

1. What is micro-frontend architecture, and how can Angular implement it?
   • Answer:
     • Micro-frontends split large applications into smaller, independently deployable modules.
     • Use frameworks like Module Federation in Webpack to load Angular modules dynamically.
     • Ensure communication between micro-frontends using shared services or libraries.

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1. How do you manage large-scale forms with dynamic validation in Angular?
   • Answer:
     • Use Reactive Forms for dynamic validation.
     • Build dynamic form controls using a configuration object.
     • Validate inputs using custom validators.
     • Leverage FormArray for handling dynamic collections of form controls.

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These questions emphasize real-world applications, advanced concepts, and best practices that are expected from a senior Angular developer. Let me know if you'd like further examples or a deeper dive into any topic!