Angular Interview Questions & Answers

Observables and Promises are both used for handling asynchronous operations in Angular, but they have different characteristics:

• Observables: Based on the RxJS library, Observables are lazy, cancellable, and can emit multiple values over time. They support various operators to manipulate and transform the emitted values. Observables are useful when dealing with multiple values, real-time updates, or complex data manipulation.

• Promises: Promises are eager, non-cancellable, and can emit only a single value or error. They have a simpler API with fewer built-in features. Promises are useful for one-time operations, such as HTTP requests, where only a single value or error is expected.

Angular's Change Detection mechanism is responsible for updating the view whenever the underlying data changes. It uses a unidirectional data flow and a component tree structure. By default, Angular runs change detection whenever an event, an HTTP request, or a timeout/interval occurs.

There are two change detection strategies in Angular:

• Default: Angular checks for changes in the entire component tree, starting from the root component, and updates the view whenever necessary. This strategy is the most straightforward but can be inefficient for large applications.

• OnPush: Angular checks for changes only when the component's input properties change or when an event is emitted from the component. This strategy can significantly improve performance by reducing unnecessary change detection runs but requires more careful handling of data update

Content projection (previously called transclusion in AngularJS) is a technique that allows developers to insert external content into an Angular component's template. This helps create reusable and customizable components.

To implement content projection in Angular, you can use the <ng-content> tag in the component's template. Any external content placed between the component's opening and closing tags will be projected into the <ng-content> slot when the component is rendered.

Angular decorators are special functions used to annotate and modify classes, properties, methods, and parameters to provide additional metadata and functionality.

Some of the most common decorators used in Angular include:

• @Component: Used to define a component class, with metadata like selector, template, and styles.

• @Directive: Used to define a directive class, with metadata like selector and providers.

• @Injectable: Used to define a service class, indicating that it can be injected as a dependency.

• @Input: Used to define a property as an input binding, allowing data to be passed from a parent component.

• @Output: Used to define a property as an output binding, allowing data to be emitted to a parent component.

• @HostBinding: Used to bind a property to a host element attribute.

• @HostListener: Used to bind a method to a host element event.

State management in Angular refers to the organization and handling of data and state across an application. As applications grow in complexity, managing state can become challenging, especially when multiple components and services need to access and modify the same data.

There are several state management libraries and techniques for Angular:

• NgRx: A popular state management library based on Redux and RxJS, NgRx follows the principles of unidirectional data flow, immutability, and the use of actions, reducers, and effects. It provides a predictable and maintainable way to manage state in large applications.

• Akita: Akita is a simpler and more flexible state management library that uses the concepts of stores, queries, and entities. It aims to reduce boilerplate code and provide a more straightforward approach to state management, while still offering powerful features like caching, undo/redo, and persistence.

MobX: MobX is a reactive state management library that focuses on making it easy to manage state through observable properties and computed values. It automatically tracks dependencies and updates the UI when necessary. Although MobX is not specific to Angular, it can be used in Angular applications with the help of the ‘mobx-angular’ integration library.

Internationalization (i18n) is the process of designing and preparing an application to support multiple languages and regions. Localization (L10n) is the process of adapting an application for a specific language or region by translating text and formatting data.

In Angular, you can implement i18n and L10n using the following steps:

• Mark text in your templates for translation using the ‘i18n’ attribute.

• Use Angular's built-in pipes, like ‘DatePipe’, ‘CurrencyPipe’,  ‘PercentPipe’, to format data according to the user's locale.

• Extract marked text using the Angular CLI's ‘ng xi18n’ command, which generates a translation file (XLIFF or XMB format).

• Translate the extracted text in the translation file.

• Configure your build process to create localized versions of your application using the translated files with the help of Angular CLI's ‘ng build’ command and the ‘--localize’ flag.

• Serve the appropriate localized version of your application based on the user's language or locale settings.

Angular pipes are used to transform data in templates before displaying it to the user. They allow for more readable and reusable code by separating data transformation logic from the template.

There are two types of pipes in Angular:

Pure pipes: Pure pipes are only executed when their input values or arguments change. They are more efficient because Angular caches the result of the transformation and reuses it for the same input values. Examples of built-in pure pipes include ‘DatePipe’, ‘CurrencyPipe’, ‘UpperCasePipe’, ‘LowerCasePipe’.
Impure pipes: Impure pipes are executed on every change detection cycle, regardless of whether their input values or arguments have changed. They can be less efficient but are necessary for handling dynamic data, such as real-time updates. An example of a built-in impure pipe is the ‘AsyncPipe’.

Several techniques can be used to optimize the performance of an Angular application:

• AOT Compilation: Ahead-of-Time (AOT) compilation compiles Angular templates and components into efficient JavaScript code during the build process, reducing runtime overhead and improving initial load performance.
• Lazy Loading: Lazy loading is the process of loading feature modules on-demand, which reduces the initial bundle size and speeds up the application's initial load time.

• Bundle Optimization: Using tools like Webpack or the Angular CLI, you can optimize the size of your application bundles through techniques like minification, tree-shaking, and code splitting. This results in smaller and faster-loading bundles, improving the overall performance of your application.
• Change Detection Optimization: By using the OnPush change detection strategy and implementing immutable data structures, you can reduce the number of unnecessary change detection cycles, improving the performance of your application.
• Caching and Memoization: By caching the results of expensive operations or using memoization techniques, you can avoid redundant computations and improve the overall performance of your application.
• Optimizing Angular Pipes: By using pure pipes wherever possible and minimizing the use of impure pipes, you can reduce the overhead of change detection and improve the performance of your application.

Custom directives in Angular allow you to extend the functionality of HTML elements and components by adding custom behavior.

To create a custom directive, follow these steps:

1. Create a new class and decorate it with the ‘@Directive’ decorator, providing a CSS selector that will be used to apply the directive.
2. Implement the desired functionality within the directive class. You can use ‘ElementRef’ to access the host element and ‘Renderer2’ to manipulate the DOM in a platform-agnostic way.
3. Register the directive in the ‘declarations’ array of an Angular module.
4. Use the directive in your templates by adding the specified selector to the relevant elements or components.

                      import { Pipe, PipeTransform } from '@angular/core';

                      @Pipe({
                        name: 'searchFilter'
                      })
                      export class SearchFilterPipe implements PipeTransform {
                        transform(items: any[], searchTerm: string): any[] {
                          if (!items || !searchTerm) {
                            return items;
                          }

                          return items.filter(item =>
                            item.toLowerCase().includes(searchTerm.toLowerCase())
                          );
                        }
                      }

                    

To use this custom pipe in a template, you can simply apply it to an ‘*ngFor’ directive:

  <input [(ngModel)]="searchTerm" placeholder="Search">
<ul>
  <li *ngFor="let item of items | searchFilter: searchTerm">{{ item }}</li>
</ul>

This custom pipe will filter the list of items based on the search term entered by the user.

Dependency Injection (DI) is a design pattern that promotes the separation of concerns and enhances modularity, maintainability, and testability of an application. In Angular, DI is implemented through an injector system that manages the creation and sharing of instances of various classes (services, components, etc.).

Angular's DI system allows developers to decouple the creation and consumption of dependencies, making it easier to swap out implementations, reuse components and services, and mock dependencies in tests. By using Angular's @Injectable decorator and declaring dependencies in constructors, developers can rely on the DI system to handle the creation and provision of the required instances.

Reactive Forms and Template-Driven Forms are two approaches to handling forms in Angular, with different trade-offs:

• Reactive Forms: Reactive Forms are more powerful and flexible, offering better control over validation, dynamic form creation, and form state management. They are built using reactive programming principles with RxJS, and the form logic is defined in the component class. Reactive Forms are more suitable for complex forms and applications that require fine-grained control over form behavior.

• Template-Driven Forms: Template-Driven Forms are simpler and more declarative, with form logic and validation defined directly in the template. They rely on Angular's two-way data binding and directives such as ngModel. Template-Driven Forms are more suitable for simple forms and applications that don't require advanced form features.

Performance optimization in Angular can be approached from several angles, including bundle size optimization, change detection optimization, and runtime performance improvement.

• Bundle size optimization:
1. Use Angular's built-in lazy loading feature to load feature modules on-demand, reducing the initial bundle size.
2. Use the AOT (Ahead-of-Time) compiler to compile your templates during the build process instead of at runtime.
3. Utilize tools like Webpack's code splitting and tree shaking to eliminate unused code and create smaller, more efficient bundles.
4. Enable build optimizations such as minification, compression, and dead code elimination provided by the Angular CLI.

• Change detection optimization:
1. Use the OnPush change detection strategy for components with simple or static data to minimize unnecessary change detection cycles.
2. Minimize the use of complex expressions or function calls in templates, as they can negatively impact change detection performance.
3. Utilize pure pipes to transform data in templates, as they are only re-evaluated when their input changes.

• Runtime performance improvement:
1. Optimize data handling in components and services by using RxJS operators to minimize data processing and avoid unnecessary recalculations.
2. Use Angular's trackBy function in *ngFor loops to improve performance when rendering large lists by minimizing DOM manipulation.

Leverage Angular's built-in performance tools, such as the enableProdMode function and the ng.profiler in the Angular DevTools, to identify and address performance bottlenecks.

Higher-Order Observables are Observables that emit other Observables, creating a nested structure. They are often the result of using operators like map to transform Observables that emit asynchronous actions.

To manage Higher-Order Observables, you can use flattening operators like switchMap, mergeMap, and concatMap, which combine the nested Observables into a single Observable:

• switchMap: When a new inner Observable is emitted, switchMap unsubscribes from the previous inner Observable and starts emitting values from the new one. This is useful for canceling outdated asynchronous requests, like search queries or HTTP requests.
• mergeMap: mergeMap subscribes to all emitted inner Observables and merges their values into a single Observable. This is useful for handling multiple simultaneous asynchronous actions, like multiple file uploads.
• concatMap: concatMap subscribes to the inner Observables in the order they are emitted, waiting for each one to complete before subscribing to the next. This is useful for handling asynchronous actions that must be executed in a specific order, like a queue of tasks.

Server-Side Rendering (SSR) with Angular Universal involves rendering Angular applications on the server and sending the pre-rendered HTML to the client. This can improve the perceived performance, especially for the initial load, and enhance SEO by making the application more accessible to search engine crawlers.

To implement SSR with Angular Universal:

1. Install the necessary dependencies and configure your Angular project to support server-side rendering.
2. Create a server-side app module that imports your main app module and uses the ServerModule from @angular/platform-server.
3. Set up a Node.js server (e.g., using Express) that handles incoming requests, renders the Angular application using the renderModuleFactory function from @angular/platform-server, and sends the pre-rendered HTML to the client.
4. Configure your build process to create both the client-side and server-side bundles using the Angular CLI and tools like Webpack.

Benefits of using SSR with Angular Universal:

• Improved initial load performance: Users see the pre-rendered content faster, enhancing the perceived performance of the application.
• Better Search Engine Optimization (SEO): Search engine crawlers can more easily index the pre-rendered content, improving the visibility of your application in search results.
• Enhanced social sharing: Pre-rendered content can be more easily scraped by social media platforms, improving the appearance of shared links.

Trade-offs of using SSR:

• Increased server load: The server must render the application for each incoming request, which can increase resource usage and impact scalability.
• Increased complexity: Implementing SSR adds complexity to your application's architecture and build process, which may increase development and maintenance efforts.

The ControlValueAccessor interface allows you to create custom form controls that can be used with both Reactive Forms and Template-Driven Forms. By implementing the interface methods (writeValue, registerOnChange, registerOnTouched, and setDisabledState), your custom form control can integrate seamlessly with Angular's form APIs and validation system.

Example use case: Creating a custom date picker control that provides a more user-friendly UI for selecting dates and integrates with Angular's form validation and state management.

• Nested Routes: To implement nested routes, you can define child routes within your route configuration using the children property. This allows you to create hierarchical navigation structures and reuse components for different routes.
• Route Guards: Route guards are services that implement the CanActivate, CanDeactivate, CanLoad, or Resolve interfaces. They can be used to control access to routes, prevent navigation away from unsaved changes, or preload data for a route. You can attach route guards to your routes using the canActivate, canDeactivate, canLoad, or resolve properties in your route configuration.
• Lazy Loading: Lazy loading allows you to load feature modules on-demand, reducing the initial bundle size and improving application load times. To implement lazy loading, use the loadChildren property in your route configuration and provide a function that imports the module using the dynamic import() syntax.

• Global Error Handling: You can implement a global error handler by extending the ErrorHandler class from @angular/core and overriding its handleError method. This allows you to catch and handle all unhandled exceptions in your application. You can then register your custom error handler in your app module's providers.
• Custom Error Pages: By using Angular's Router, you can create custom error pages (e.g., 404 Not Found) and route users to these pages when a navigation error occurs. You can also use a catch-all wildcard route to display a custom error page when no matching route is found.
• Error Reporting Services: Integrating third-party error reporting services, such as Sentry or Rollbar, can help you monitor and track errors in your application. These services provide real-time error tracking, notifications, and detailed error reports, making it easier to identify and fix issues.

                        import { Injectable } from '@angular/core';
                        import { BehaviorSubject } from 'rxjs';

                        @Injectable({
                          providedIn: 'root'
                        })
                        export class SharedService {
                          private messageSource = new BehaviorSubject<string>('Initial message');
                          currentMessage$ = this.messageSource.asObservable();

                          updateMessage(message: string): void {
                            this.messageSource.next(message);
                          }
                        }
                    

This shared service uses an RxJS BehaviorSubject to manage a shared state (a message in this case) between sibling components. To use this service in your components, inject it into the constructor and subscribe to the currentMessage$ Observable to receive updates. To update the shared message, call the updateMessage method with the new message.

Example usage in Component A:

                      @Component({...})
                          export class ComponentA implements OnInit {
                            constructor(private sharedService: SharedService) {}

                            ngOnInit(): void {
                              this.sharedService.currentMessage$.subscribe(message => {
                                console.log('Component A received:', message);
                              });
                            }

                            updateSharedMessage(): void {
                              this.sharedService.updateMessage('Message from Component A');
                            }
                          }

                    

By using the shared service, both Component A and Component B can communicate and share data with each other, even though they are sibling components.

• 
  Internationalization (i18n) is the process of designing and preparing your application to support multiple languages and cultures. Localization (l10n) is the process of adapting the application for a specific language or culture by translating text, formatting dates and numbers, and adjusting layout and images.
  Angular provides built-in support for i18n through the @angular/localize package. To implement internationalization and localization in an Angular application:

  1. Configure your application to use the @angular/localize package by running the ng add @angular/localize command.
  2. Use the i18n attribute in your templates to mark elements or attributes containing translatable text. Provide a unique ID and description to help translators understand the context.
  3. Extract the marked text into translation files (XLIFF, XMB, or JSON) using the Angular CLI's ng extract-i18n command.
  4. Translate the extracted text into the desired languages and save the translations in separate files.
  5. Configure the Angular CLI to build multiple versions of your application, each with a different language or locale. Use the localize option in your angular.json file to specify the translation files for each build configuration.
  Optionally, use Angular's built-in pipes (e.g., DatePipe, CurrencyPipe, DecimalPipe, and PercentPipe) to format dates, numbers, and currencies according to the current locale.