Angular --### **Important Angular Interview Topics (With Explanation, Why & Where to Use in Projects)**

Important Angular Interview Topics (With Explanation, Why & Where to Use in Projects)

If you're preparing for an Angular interview, focus on these core topics. Below is a detailed breakdown of key concepts, why they are important, and where they are used in real-world projects.

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1. Components & Lifecycle Hooks

What?

• Components are the building blocks of an Angular application.
• Lifecycle hooks (ngOnInit, ngOnChanges, ngAfterViewInit, etc.) allow you to tap into different phases of a component’s existence.

Why?

• Helps manage component initialization, updates, and destruction effectively.

Where Used?

• Fetching API data (ngOnInit).
• Reacting to input property changes (ngOnChanges).
• Handling DOM manipulations after view rendering (ngAfterViewInit).

Example:

export class MyComponent implements OnInit {
  ngOnInit() {
    console.log('Component initialized');
  }
}

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2. Directives (ngIf, ngFor, Custom Directives)

What?

• Structural directives (*ngIf, *ngFor, *ngSwitch) manipulate the DOM.
• Attribute directives ([ngClass], [ngStyle]) modify element behavior.
• Custom directives extend Angular functionality.

Why?

• Helps dynamically render UI elements based on conditions and loops.
• Improves code reusability.

Where Used?

• ngIf for conditionally displaying elements.
• ngFor to iterate over lists.
• Custom directives for reusable behaviors (e.g., adding tooltips, animations).

Example:

<div *ngIf="isLoggedIn">Welcome, User!</div>
<ul>
  <li *ngFor="let user of users">{{ user.name }}</li>
</ul>

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3. Services & Dependency Injection (DI)

What?

• Services are used to share data and logic across components.
• Dependency Injection (DI) helps inject these services where needed.

Why?

• Promotes code reusability and maintainability.
• Ensures single instances (Singleton pattern) of shared logic.

Where Used?

• API calls (HttpClient in services).
• Managing user authentication, data sharing between components.

Example:

@Injectable({ providedIn: 'root' })
export class UserService {
  constructor(private http: HttpClient) {}

  getUsers() {
    return this.http.get('/api/users');
  }
}

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4. Routing & Navigation (RouterModule)

What?

• Handles navigation between pages without full page reload.

Why?

• Enables SPA (Single Page Applications).
• Improves user experience.

Where Used?

• Multi-page apps (e.g., e-commerce sites with Home, Product, Cart pages).

Example:

const routes: Routes = [
  { path: 'home', component: HomeComponent },
  { path: 'product/:id', component: ProductComponent }
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule {}

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5. Forms (Template-driven & Reactive Forms)

What?

• Template-driven forms use directives like ngModel.
• Reactive forms use FormControl, FormGroup, Validators.

Why?

• Improves form validation and state management.

Where Used?

• User login, registration, contact forms.

Example (Reactive Form):

this.userForm = new FormGroup({
  name: new FormControl('', Validators.required),
  email: new FormControl('', [Validators.required, Validators.email])
});

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6. Observables & RxJS

What?

• Observables (from, of, interval) handle asynchronous data.
• RxJS (map, filter, switchMap) helps manage complex async operations.

Why?

• Used for API calls, event handling, real-time data streams.

Where Used?

• Handling HTTP requests (HttpClient).
• Live search, WebSocket data streaming.

Example:

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

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7. State Management (NgRx, BehaviorSubject)

What?

• NgRx uses Redux-like state management.
• BehaviorSubject allows reactive data sharing.

Why?

• Avoids excessive API calls.
• Manages global application state efficiently.

Where Used?

• Shopping cart data, authentication state.

Example (Using BehaviorSubject for state sharing):

private user = new BehaviorSubject<User | null>(null);
user$ = this.user.asObservable();

setUser(user: User) {
  this.user.next(user);
}

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8. Lazy Loading Modules

What?

• Loads modules on demand instead of at startup.

Why?

• Improves app performance by reducing initial load time.

Where Used?

• Large applications with multiple feature modules.

Example:

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

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9. Interceptors & HTTP Handling

What?

• Interceptors modify HTTP requests/responses globally.

Why?

• Used for authentication tokens, logging, error handling.

Where Used?

• Adding JWT tokens to API requests.
• Handling global error messages.

Example:

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

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10. Security Best Practices (XSS, CSRF, JWT, Sanitization)

What?

• Angular has built-in security mechanisms (DOM sanitization, CSP).

Why?

• Prevents XSS (Cross-Site Scripting) and CSRF (Cross-Site Request Forgery).

Where Used?

• Sanitizing user-generated content, securing API calls.

Example (Sanitization):

this.safeHTML = this.sanitizer.bypassSecurityTrustHtml(userInput);

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Conclusion

Topic	Why Important?	Where Used?
Components & Lifecycle Hooks	Manage component initialization & updates	UI components, API calls
Directives (ngIf, ngFor)	Dynamic UI manipulation	Looping lists, conditional rendering
Services & Dependency Injection	Reuse logic, inject dependencies	API calls, global state management
Routing & Navigation	Page navigation	Multi-page apps, authentication
Forms (Reactive & Template-driven)	User input handling	Login, Registration, Contact forms
Observables & RxJS	Handle async operations	API calls, live search
State Management (NgRx, BehaviorSubject)	Manage global app state	Shopping cart, user authentication
Lazy Loading Modules	Optimize app performance	Large applications
Interceptors & HTTP Handling	Modify API requests globally	JWT authentication, logging
Security (XSS, CSRF, JWT)	Protect from attacks	API security, user-generated content

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