Top Angular Interview Questions for 2025

Question: What is change detection in Angular?

Answer:

Change Detection in Angular is a mechanism that tracks changes in the application’s state and updates the view (DOM) accordingly. In Angular, whenever the application state changes, the framework checks which parts of the UI need to be updated and makes the necessary changes to the DOM. This process is called change detection.

Angular’s change detection system ensures that the view (UI) is always in sync with the data model. It is a core part of Angular’s two-way data binding and reactivity model.

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How Change Detection Works in Angular:

Angular uses an implicit change detection mechanism that automatically updates the view when a change occurs in the component’s data model. This can happen in response to user actions, HTTP responses, or other triggers.

1. Change Detection Cycle:

   • The change detection cycle is triggered whenever Angular checks if any bound data in a component has changed. It runs through a series of steps to identify which views need to be updated.

2. Component Tree:

   • Angular maintains a tree of components, and change detection runs over this tree to check if any of the component’s inputs or internal state have changed.
   • Each component has its own change detection strategy and scope in the tree.

3. Detection Mechanism:

   • Angular’s change detection system uses dirty checking. It compares the current value of a model or property with its previous value. If the value has changed, Angular updates the view for that component and its children.
   • The change detection is often triggered by events like user input, HTTP responses, or timer-based operations.

4. Zones and NgZone:

   • Angular uses Zones (specifically NgZone) to manage asynchronous operations. This helps Angular track when asynchronous events (like HTTP requests or user interactions) occur, so it can trigger change detection automatically after such events are completed.

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Types of Change Detection Strategies:

Angular provides two types of change detection strategies that can be used to optimize performance:

1. Default Change Detection (CheckAlways):

   • This is the default strategy for Angular components.
   • In this mode, Angular checks the component and its child components on every event cycle. It checks all components in the tree for changes, regardless of whether the component’s data has changed or not.
   • While simple, this can be inefficient in large applications with many components, especially if many components are not changing frequently.

   How it works:

   • Angular runs a check on the component, then propagates the check to its child components recursively.
   • If any data-bound property in the component changes, Angular will update the view and propagate changes.

2. OnPush Change Detection:

   • The OnPush strategy is an optimization technique used to limit the frequency of change detection checks, improving performance.
   • With OnPush, Angular only checks a component when:
     • Its input properties change.
     • An event occurs within the component (such as a button click).
     • A manual trigger happens (such as calling markForCheck() or detectChanges()).

   How it works:

   • The component’s view is updated only when its input properties or its internal state explicitly change. Angular does not check the component on every change detection cycle unless one of these conditions is met.

   Usage Example:

   @Component({
     selector: 'app-my-component',
     changeDetection: ChangeDetectionStrategy.OnPush,
     templateUrl: './my-component.component.html',
   })
   export class MyComponent {
     @Input() data: any;
   }

   In this example, Angular will only check MyComponent for changes if the data input property changes or if an event (e.g., user interaction) occurs within the component.

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Triggers for Change Detection:

1. User Input: When users interact with the application (clicking buttons, typing in forms, etc.), Angular triggers change detection to update the view.

2. HTTP Requests: When the application receives a response from an HTTP request, Angular triggers change detection to update the view with the new data.

3. Timers/Intervals: Events like setTimeout or setInterval also trigger Angular’s change detection.

4. Component Lifecycle Hooks: Lifecycle hooks like ngOnChanges, ngDoCheck, and ngAfterViewChecked can trigger or control when change detection occurs.

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Manual Control Over Change Detection:

Sometimes, developers may need more control over change detection, especially in complex or performance-sensitive applications. Angular provides the ChangeDetectorRef service to manually trigger change detection or detach certain parts of the component tree from Angular’s change detection cycle.

1. markForCheck(): This method tells Angular that a component and its ancestors should be checked during the next change detection cycle. It is used when the component is using OnPush strategy but has an asynchronous operation or external event that modifies the data.

2. detectChanges(): This method forces Angular to check the component and its children for changes immediately.

3. detach(): This method detaches the component from Angular’s change detection mechanism. The component will no longer be checked unless you explicitly call detectChanges().

Example of Using ChangeDetectorRef:

import { ChangeDetectorRef, Component } from '@angular/core';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
})
export class MyComponent {
  constructor(private cdRef: ChangeDetectorRef) {}

  ngOnInit() {
    // Detach the component from the change detection cycle
    this.cdRef.detach();
  }

  updateData() {
    // Manually trigger change detection when needed
    this.cdRef.detectChanges();
  }
}

---

Performance Considerations:

1. Avoid Unnecessary Change Detection:

   • Use the OnPush strategy wherever possible to avoid checking components that don’t need updates.
   • Reduce the number of components that rely on default change detection to optimize performance.

2. Immutable Data Structures:

   • Use immutable data structures or patterns like Object.freeze() and Immutable.js to make it easier for Angular to detect changes, especially with the OnPush strategy.

3. Track By for ngFor:

   • When rendering lists with *ngFor, use the trackBy function to tell Angular how to track individual items in the list. This reduces the amount of DOM manipulation required when the list changes.

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Conclusion:

Change Detection in Angular is the mechanism that ensures the view (DOM) stays in sync with the data model. It checks for changes in component data and updates the view accordingly. Angular offers two change detection strategies, Default and OnPush, with OnPush being a performance optimization. Change detection is triggered by user input, HTTP responses, asynchronous events, and component lifecycle hooks. Developers can manually control change detection using the ChangeDetectorRef service for advanced use cases. Optimizing change detection is essential for building performant Angular applications, especially when dealing with complex or large-scale apps.

Question: How do you optimize the performance of an Angular application?

Answer:

Optimizing the performance of an Angular application is crucial, especially as the complexity of the application increases. There are several strategies and best practices you can adopt to improve the speed, responsiveness, and scalability of your Angular application. These optimizations can target various aspects, such as change detection, rendering, network performance, and application structure.

Here are key strategies to optimize the performance of an Angular application:

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1. Change Detection Optimization:

Use OnPush Change Detection Strategy:

• By default, Angular uses the Default Change Detection Strategy, which checks the entire component tree for changes on every cycle. This can be inefficient for large applications.

• The OnPush strategy reduces the frequency of change detection by only checking a component when:

  • Its input properties change.
  • An event is triggered inside the component.
  • Manually triggered change detection via markForCheck() or detectChanges().

  How to implement OnPush:

  @Component({
    selector: 'app-my-component',
    changeDetection: ChangeDetectionStrategy.OnPush,
    templateUrl: './my-component.component.html',
  })
  export class MyComponent {
    @Input() data: any;
  }

Use ChangeDetectorRef for Manual Control:

• You can gain more control over change detection using ChangeDetectorRef. For example, detach components that don’t require frequent checks or manually trigger change detection when necessary.
  • detectChanges(): To explicitly trigger change detection in a component.
  • detach(): To remove a component from the change detection tree.
  • markForCheck(): To mark a component for checking in the next cycle.

---

2. Lazy Loading Modules:

• Lazy loading allows Angular to load modules only when they are needed rather than loading everything upfront. This helps reduce the initial load time of the application and improves performance.

  How to implement lazy loading:

  • Configure lazy-loaded modules in the Angular router by using the loadChildren property.

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

---

3. AOT (Ahead-of-Time) Compilation:

• AOT Compilation compiles the Angular application during the build phase rather than at runtime. This reduces the size of the application bundle and speeds up the initial rendering time.

  How to enable AOT:

  • Use the Angular CLI --aot flag when building or serving the application.

  ng build --prod

  • AOT reduces the amount of JavaScript that needs to be downloaded and parsed by the browser, improving load time and performance.

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4. Tree Shaking and Bundle Optimization:

Tree Shaking:

• Tree shaking is a process where unused code is removed from the final bundle. Angular’s AOT compiler and bundlers like Webpack enable tree shaking, reducing the size of the output bundle.

Bundle Optimization:

• Use Webpack (via Angular CLI) to optimize bundles by splitting the code into smaller chunks.
  • Code Splitting: Angular CLI does this automatically for lazy-loaded modules. It splits the application code into smaller chunks so only the necessary code is loaded.
  • Minification: The Angular CLI automatically minifies your code when you build for production, reducing file sizes.
  To enable production builds:

  ng build --prod

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5. Reduce the Number of HTTP Requests:

Use HTTP Interceptors:

• HTTP interceptors can be used to optimize network requests, handle caching, and reduce unnecessary calls.

  Caching: Implement caching strategies to avoid making the same requests repeatedly (e.g., using a service worker or an HTTP cache).

  Example of an HTTP interceptor:

  @Injectable()
  export class CacheInterceptor implements HttpInterceptor {
    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
      // Check for cached data before making a new HTTP request
      const cachedResponse = this.cacheService.get(req.url);
      if (cachedResponse) {
        return of(cachedResponse);
      }
      return next.handle(req).pipe(
        tap(response => {
          this.cacheService.set(req.url, response);
        })
      );
    }
  }

Batch HTTP Requests:

• Minimize HTTP requests by combining multiple requests into a single request when possible (using the forkJoin operator for multiple observables).

---

6. Optimize Template Rendering:

Track By with ngFor:

• When rendering lists with *ngFor, use the trackBy function to tell Angular how to track individual items in the list. This reduces the number of DOM manipulations required when the list changes, improving performance.

  Example:

  <div *ngFor="let item of items; trackBy: trackById">
    {{ item.name }}
  </div>

  trackById(index: number, item: any): number {
    return item.id;
  }

Avoid Complex Expressions in Templates:

• Avoid performing complex logic or calculations directly in the template expressions. These can trigger change detection frequently. Instead, move logic to the component class.

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7. Optimize Image and Asset Loading:

Lazy Load Images:

• Lazy-load images so that they are only loaded when they are about to be displayed in the viewport. This can greatly reduce the initial page load time.
  • Use the loading="lazy" attribute for images to enable native lazy loading.

Use WebP Format for Images:

• Consider using WebP images, which provide superior compression and smaller file sizes compared to JPEG and PNG formats.

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8. Use Service Workers and Caching for Offline Support:

• Service workers can cache resources and API responses to provide offline capabilities and improve performance. This is especially useful for progressive web apps (PWAs).

  • Use Angular’s PWA package to automatically configure service workers for caching:

  ng add @angular/pwa

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9. Minimize and Compress CSS and JavaScript Files:

• CSS and JavaScript should be minified and compressed in production to reduce the size of the files being served to the client. The Angular CLI automatically handles minification when building with the --prod flag.

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10. Use Web Workers for Heavy Computations:

• For computationally intensive tasks, use Web Workers to offload work from the main thread and avoid blocking the UI thread. This ensures the application remains responsive.

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11. Optimize Font Loading:

• Avoid rendering blocking issues with fonts by:
  • Using font-display: swap in your CSS to ensure text is rendered while the font is loading.
  • Use Google Fonts or Font Awesome with the preload or async attributes.

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12. Profiling and Benchmarking:

• Use Angular DevTools and Chrome DevTools to profile and benchmark your Angular application. Look for performance bottlenecks and optimize areas such as:
  • Change detection performance.
  • Memory leaks.
  • Slow rendering.

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Conclusion:

Optimizing the performance of an Angular application involves a combination of strategies that target different parts of the application, including change detection, module loading, HTTP requests, asset management, and rendering. By using techniques such as lazy loading, OnPush change detection, AOT compilation, tree shaking, HTTP caching, and optimizing images and assets, you can significantly improve the performance and responsiveness of your Angular application, especially for large and complex applications. Always monitor performance using profiling tools and aim for gradual improvements across different layers of your application.

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