125+ Angular Questions and Answers for Job Interview Success

Angular Interview Questions come up in almost every frontend coding interview, and many candidates trip over core concepts during live coding rounds. Have you ever built a feature but struggled to explain dependency injection, change detection, RxJS observables, lifecycle hooks, routing, forms, unit testing, or state management with NgRx when an interviewer asks? This article gathers practical Angular Coding Interview questions with clear answers and concise code examples so you can confidently crack Angular interviews at any level, fresher, experienced, or senior, and secure your desired job with thorough, ready-to-use answers.

To help with that, Interview Coder offers an undetectable coding assistant for interviews that provides tailored practice questions, model answers, and step-by-step code guidance, allowing you to rehearse answers, identify weak spots, and walk into interviews calmer and more prepared.

Top 71 Angular Interview Questions and Answers

1. What Is Angular? Clear Definition and Where It Fits

Angular is an open-source framework for building dynamic single-page web applications. It utilizes TypeScript as its primary language and employs a component-based architecture. Angular provides two-way data binding, dependency injection, a powerful CLI, routing, and build tooling to create scalable and maintainable apps for web and hybrid platforms.

2. Main Features of Angular You Should Know

Two way data binding that keeps the model and view synchronized. Dependency injection to manage and provide services across components. Modularization to split an app into cohesive feature modules. Template-driven views for declarative UI. Built in HTTP and RxJS integration to consume RESTful APIs.

3. Latest Angular Version and Release Notes

Angular 19.0.0 was released on November 19, 2024. It promotes standalone components, directives, and pipes by default, reduces reliance on NgModules, and introduces improvements in rendering and state management.

4. Major Updates in Angular 19 That Matter

Standalone components made default, simplified module surface area. Incremental hydration support in preview for faster SSR. Route-level render mode to tune rendering per route. Linked signals and resource API to improve state and async flows. Automatic unused import detection, improved HMR, zoneless change detection in development, and Material enhancements.

5. Differences Between Angular 18 and Angular 19 in Practice

Angular 19 makes standalone components the default instead of optional. It provides incremental hydration compared to Angular 18’s SSR improvements. Signals evolved into linked signals for a complex state. Angular 19 emphasizes fine-grained reactivity, reduces Zone.js reliance, and adds automatic detection of unused imports, as well as improved incremental builds.

6. Why Angular Was Introduced and What Problem It Solves

Angular was created to simplify the development of complex single-page applications by giving a structured framework: components for UI, DI for testability, templating for clear view logic, and tooling for builds and testing. It reduces boilerplate and helps teams scale front-end codebases.

7. Types of Compilation Angular Provides

JIT (Just in Time) compiles in the browser at runtime and speeds up development cycles. AOT (Ahead of Time) compiles templates and TypeScript at build time, producing smaller bundles and a faster startup suitable for production.

8. What Is a Component in Angular and a Short Example

A component controls a portion of the UI and its associated logic. Components are reusable units composed of a class, template, styles, and metadata.

Example:

import { Component, Input } from '@angular/core';@Component({ selector: 'app-header', templateUrl: './header.component.html', styleUrls: ['./header.component.css']})export class HeaderComponent { @Input() title: string; @Input() links: { name: string, url: string }[];}

9. Purpose of the @Component Decorator Explained

@Component marks a class as an Angular component and supplies metadata:

• The selector that places the element in templates
• The template or templateUrl, and the style files
• Providers for dependency injection, enabling encapsulated view logic and styles.

10. What Is a Module in Angular and a Minimal Example

An NgModule groups related components, directives, pipes, and services, and defines the compilation context.

Example:

@NgModule({ declarations: [AppComponent], imports: [BrowserModule], providers: [], bootstrap: [AppComponent]})export class AppModule { }

11. Angular CLI: The Developer Productivity Tool

Angular CLI automates project creation, serving, code generation, and production builds. Key commands:

• ng new to scaffold
• ng serve to run
• ng generate to create components or services
• ng build to produce optimized output

12. What Is a Directive in Angular with a Sample

Directives attach behavior to DOM elements. They can change appearance or respond to events. Attribute directives modify elements while structural directives change the DOM structure.

Example:

import { Directive, ElementRef, Renderer2, HostListener, Input } from '@angular/core';@Directive({ selector: '[appHoverBackground]' })export class HoverBackgroundDirective { @Input('appHoverBackground') hoverColor: string; constructor(private el: ElementRef, private renderer: Renderer2) {} @HostListener('mouseenter') onMouseEnter() { this.changeBackgroundColor(this.hoverColor || 'yellow'); } @HostListener('mouseleave') onMouseLeave() { this.changeBackgroundColor(null); } private changeBackgroundColor(color: string) { this.renderer.setStyle(this.el.nativeElement, 'backgroundColor', color); }}

13. What Is a Service in Angular and a Typical Use

A service is a class that encapsulates reusable logic and can be injected into components. Typical uses are HTTP calls, caching, and business logic.

Example:

import { Injectable } from '@angular/core';import { HttpClient } from '@angular/common/http';import { BehaviorSubject } from 'rxjs';import { tap } from 'rxjs/operators';@Injectable({ providedIn: 'root' })export class DataService { private dataSubject = new BehaviorSubject<any>(null); data$ = this.dataSubject.asObservable(); constructor(private http: HttpClient) {} fetchData() { return this.http.get('https://api.example.com/data').pipe( tap(data => this.dataSubject.next(data)) ); }}

14. Two-Way Data Binding Explained with Usage

Two-way binding keeps a component property and a form control synchronized so changes in one reflect in the other. Use [(ngModel)] for template-driven or Reactive Forms with valueChanges for reactive scenarios.

Example:

<input [(ngModel)]="userInput" />export class UserInputComponent { userInput = ''; updateInput(value: string) { this.userInput = value; }}

15. Angular Lifecycle Hooks and When to Use Them

Lifecycle hooks enable you to act at specific moments in the component's lifecycle. ngOnInit runs once after inputs are initialized. ngOnChanges reacts to input changes. ngOnDestroy cleans up before removal. Others include ngDoCheck, ngAfterContentInit, and ngAfterViewInit for fine-grained control.

16. Difference Between Angular and AngularJS Summarized

Angular is component-based and uses TypeScript. AngularJS adheres to the MVC pattern, utilizing scopes and watchers in JavaScript. Angular has AOT, better performance, and a more modern toolchain, whereas AngularJS is older and less suited for mobile applications.

17. What Is Data Binding in Angular Templates

Data binding connects the component model to the view, enabling the UI to reflect changes in the model. It can be one-way or two-way and includes property binding, event binding, and interpolation.

18. One-Way Binding Versus Two-Way Binding at a Glance

One-way binding moves data in a single direction, either from component to view or from view to component. Two-way binding synchronizes the component and view so both update when either changes.

Syntax examples:

{{ value }} or [property] and [(ngModel)] respectively.

19. String Interpolation in Templates

Interpolation inserts component values into HTML using double curly braces {{ expression }}, letting templates display dynamic data.

Example:

<div>{{ message }}</div>

20. Types of Directives in the AngularJS Era

AngularJS recognized element directives, attribute directives, CSS class directives, and comment directives as extension points for HTML behavior.

21. Factory Method Pattern in AngularJS Explained

A factory in AngularJS is a function that returns an object or function instance. Each injection of a factory yields the same object reference by default, allowing you to attach logic to the created object and use it across controllers or services.

22. The Digest Cycle in AngularJS and Change Detection Basics

Digest is AngularJS’s loop that compares old and new scope values and updates the view when differences appear. It runs after events or when explicitly called via $apply, keeping the model and view in sync.

23. Dependency Injection in Angular and Benefits

Dependency injection supplies components or services with required dependencies instead of instantiating them directly. This improves modularity, testability, and separation of concerns by centralizing the creation and lifecycle of shared services.

24. How to Create a Service Step by Step

Use the CLI or create a class with @Injectable and provide it at the root or module level.

Example:

import { Injectable } from '@angular/core';import { HttpClient } from '@angular/common/http';@Injectable({ providedIn: 'root' })export class DataFetchingService { private apiUrl = 'https://api.example.com/data'; constructor(private http: HttpClient) {} fetchData() { return this.http.get<any>(this.apiUrl); }}

25. What Is the Angular Router and What It Enables

The router maps URLs to components, letting you build single-page navigation. It supports route parameters, nested routes, guards, lazy loading, and route resolvers.

26. Scope in Angular and the Modern Alternative

In AngularJS, the scope was the context for model data. Modern Angular replaced scope with component properties and inputs, making state explicit and easier to reason about.

27. What DOM Does Angular Use and How Change Detection Updates It

Angular updates the real DOM but optimizes updates with change detection so only affected parts are re-rendered. Components can also use Shadow DOM for style encapsulation.

28. How to Embed Bootstrap in Angular Projects

Install via npm and add styles to angular.json, or reference a CDN from index.html. npm installation integrates with the build pipeline and is the recommended approach.

29. Passing Data Between Components: Practical Techniques

Use @Input to receive values from a parent and @Output with EventEmitter to send events back. For non-related components, use a shared service with observables or pass state through the router.

30. Lazy Loading Explained with Practical Benefit

Lazy loading defers loading a feature module until the user navigates to its route, which reduces initial bundle size and speeds up app startup.

Example route:

{ path: 'feature', loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule) }

31. MVVM Architecture Applied to Angular

Model holds application data and services, View is the template, and ViewModel is the component that mediates between model and view. Components expose state and handlers that the template binds to.

32. Angular Lifecycle Hooks Listed with Use Cases

ngOnInit initializes data, ngOnChanges react to input mutations, ngDoCheck adds custom change detection, ngAfterContentInit and ngAfterContentChecked for projected content, ngAfterViewInit and ngAfterViewChecked for child views, and ngOnDestroy to unsubscribe and clean resources.

33. What Is a Pipe and When to Implement a Custom One

Pipes transform displayed data in templates without changing the underlying model. Use built-in pipes for dates, currency, or uppercase, or create a custom pipe for repeated formatting logic.

34. Angular Universal and Server-Side Rendering

Angular Universal renders Angular apps on the server for faster first paint and improved SEO by delivering pre-rendered HTML to the client.

35. Performance Optimization Checklist for Angular Apps

Use AOT builds, enable lazy loading, adopt OnPush change detection where appropriate, tree-shake dead code, minify assets, use trackBy in ngFor lists, and add service workers for caching.

36. What Angular Interceptors Do and a Common Use Case

Interceptors inspect and modify HTTP requests and responses. Common uses include adding auth headers, logging, or handling global HTTP errors.

37. Purpose of NgZone and How It Interacts with Change Detection

NgZone tracks asynchronous tasks and triggers Angular change detection when they complete, keeping the view synchronized after async operations like timers or HTTP calls.

38. Difference Between @Input and @Output Concise

@Input receives data from a parent into a child component. @Output emits events from a child to a parent using EventEmitter to notify about actions.

39. Practical Authentication Flow in Angular

Use JWT-based tokens stored securely, interceptors to add tokens to requests, and route guards to protect routes. Keep token refresh and logout flows centralized in an AuthService.

Example guard check:

@Injectable({ providedIn: 'root' })export class AuthService { isLoggedIn() { return !!localStorage.getItem('userToken'); } }

40. Standalone Components in Angular 19 Explained

Standalone components run without NgModules, reducing boilerplate. They simplify imports and improve tree shaking by making components more self-contained and easier to reuse.

41. Using Typed Forms for Safer Form Handling

Typed reactive forms let you declare exact types for FormGroup and FormControl values, so TypeScript helps prevent runtime errors and improves developer tooling when working with form values.

42. Purpose of the Signal API for Reactive State

Signals provide a reactive primitive that tracks dependencies and updates the view when state changes, simplifying state management and reducing the need for manual change detection.

43. How the Inject Function Works in Modern Angular

Inject lets you obtain dependencies from the DI container outside of constructors, usable in factories and lifecycle contexts where constructor injection is not practical.

44. Improvements to Standalone Component Testing

Standalone testing reduces setup by letting tests import the component directly with its dependencies, lowering boilerplate and speeding up unit test runs.

45. Functional Components and When to Use Them

Functional components are lightweight render-only constructs without class state. Use them for simple presentational pieces where lifecycle hooks and DI are not required.

46. Ahead-of-Time Compilation Benefits Explained

AOT compiles templates during build, producing optimized JavaScript, faster app startup, smaller runtime bundles, and earlier template error detection.

47. What Ivy Is and the Advantages It Brings

Ivy is the modern Angular renderer and compiler that produces smaller bundles, speeds up incremental builds, and enables advanced tree shaking and improved debugging.

48. Angular Elements for Web Component Integration

The Angular Elements package components as custom elements that can run in any HTML environment, enabling the reuse of Angular UI in non-Angular apps and micro front-end setups.

49. Resolver Concept and How It Helps Routing

A Resolver prefetches route data before activation, so components receive required data on init, reducing flicker and avoiding loading indicators inside the routed component.

50. Angular Versus React Compared Straight to the Point

Angular is a complete framework with built-in routing, DI, forms, and CLI, and favors TypeScript. React is a UI library focused on rendering with JSX and relies on third-party libraries for routing and state management.

51. How Angular Expressions Differ from JavaScript Expressions

Angular expressions are a constrained subset of JavaScript used safely in templates without side effects, while JavaScript expressions execute fully in the application logic layer.

52. Purpose of NgModule and How It Organizes App

NgModule defines a compilation boundary, declares components, imports other modules, provides services, and bootstraps the application entry point to keep concerns separated.

53. Template-Driven Versus Reactive Forms Comparison

Template-driven forms rely on directives and two-way binding for simple forms. Reactive forms utilize explicit FormGroup and FormControl objects, are synchronous, and are well-suited for complex validation and dynamic forms.

54. What Angular Guards Do and Common Types

Guards control navigation. CanActivate decides if a route can be entered, CanDeactivate checks if navigation away is allowed, and CanLoad prevents lazy modules from loading unless the conditions are met.

55. Creating Custom Validators in Angular Reactive Forms

Implement a ValidatorFn that returns a ValidationErrors object or null and attach it to the control. For async checks, return an Observable resolving to errors or null.Example async validator:uniqueEmailValidator(control: AbstractControl) { const emailsInUse = ['test@example.com', 'user@example.com']; return of(emailsInUse.includes(control.value)).pipe( delay(500), map(isInUse => (isInUse ? { emailInUse: true } : null)) );}

56. Purpose of Angular Animations and How to Use Them

Angular animations offer a declarative API for animating element state transitions, including fading, sliding, and complex motions, thereby enhancing perceived performance and user experience.

57. Dynamic Components and Runtime Creation

Dynamic components are instantiated and attached to the view at runtime using the ComponentFactoryResolver or ViewContainerRef's createComponent method, which is helpful for dialogs, dynamic forms, or plugin systems.

58. What Angular Material Provides for UI

Angular Material provides pre-built UI components that adhere to Material Design, including buttons, inputs, dialogs, and navigation, thereby accelerating the development of consistent and accessible UI.

59. Eager Loading Explained

Eager loading loads modules and resources at application bootstrap, rather than on demand, which is helpful when most app features are needed immediately and startup time is acceptable.

60. Purpose of Renderer2 and Why to Use It

Renderer2 abstracts DOM manipulation across platforms and environments like server-side rendering, preventing direct DOM access and improving security and testability.

61. Difference Between AOT and JIT Refocused

AOT compiles templates at build time, giving faster startup and smaller bundles for production. JIT compiles in the browser at runtime, making it convenient during development for faster iteration.

62. Benefits of Web Workers in Angular

Web Workers offload CPU-intensive tasks to background threads, allowing the main UI thread to remain responsive and improving perceived performance for large computations or data processing.

63. Data Binding Types in Angular Summarized

Four main bindings:

• Interpolation for display
• Property binding for element properties
• Event binding for handling user actions
• Two-way binding for synchronized model and view updates

64. What Impure Pipes Are and Trade-Offs

Impure pipes run on every change detection cycle and can update their output when inputs change without a new reference; however, they can negatively impact performance if used extensively.

65. What Pure Pipes Are and Why They Are Efficient

Pure pipes run only when input references change, making them efficient for static or immutable data transformations.

66. PipeTransform Interface and Creating a Custom Pipe

Implement PipeTransform with a transform method that accepts input and optional args and returns the transformed value. Attach @Pipe metadata to register it for templates.

67. Scenario: Fetching Data from Multiple APIs Concurrently

Use RxJS forkJoin to run multiple HTTP calls and receive all results once each observable completes.

Example:

const api1$ = this.http.get('https://api1.example.com');const api2$ = this.http.get('https://api2.example.com');forkJoin([api1$, api2$]).subscribe(([r1, r2]) => this.processData(r1, r2));

68. Scenario: Use Lazy Loading to Restore App Performance

Move the feature code into its own module and load it with the router using loadChildren, so that non-critical code does not burden the initial bundle.

69. Scenario: Form validation with Reactive Forms and Async Checks

Combine synchronous validators, such as Validators.email, with asynchronous validators that call the server. Attach the async validator as the third parameter when creating the control to validate uniqueness.

Example:

emailForm = this.fb.group({ email: ['', [Validators.required, Validators.email], [this.uniqueEmailValidator.bind(this)]]});

70. Scenario: Debugging Change Detection Failures

Check if the component uses OnPush. If so, ensure you replace mutated objects with new references or call ChangeDetectorRef.markForCheck to notify Angular to re-evaluate the component.

71. Scenario: Protecting Routes with Authentication Guards

Implement CanActivate to check authentication in a guard. If the user is not authenticated, redirect to login via the Router and return false to block the route.

Example:

@Injectable({ providedIn: 'root' })export class AuthGuard implements CanActivate { constructor(private authService: AuthService, private router: Router) {} canActivate(): boolean { if (this.authService.isAuthenticated()) { return true; } this.router.navigate(['/login']); return false; }}

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20 Angular Interview Questions and Answers for Freshers

1. Why Client-Side Frameworks Exist: Simplifying Complex Web Apps

Client-side frameworks like Angular emerged because vanilla JavaScript and jQuery became increasingly difficult to maintain as app logic grew. They enforce the separation of concerns, splitting UI and logic into components, and provide built-in tools for data flow, routing, and state management. For single-page applications, frameworks accelerate development and maintain code organization.

2. How an Angular App Boots and Renders: App Entry Flow

angular.json holds build options; its main option points to src/main.ts. main.ts creates a browser platform and calls platformBrowserDynamic().bootstrapModule(AppModule). AppModule (app.module.ts) declares components and bootstraps AppComponent. index.html contains <app-root>, which Angular replaces with the root component’s template.

The flow is:

• angular.json → main.ts → AppModule → AppComponent → rendered HTML.

3. Advantages of Angular Out of the Box: What You Get Immediately

Angular bundles routing, HTTP client, RxJS patterns, forms, and dependency injection. It uses HTML templates for declarative UI and TypeScript for static typing. Google’s long-term backing ensures ecosystem stability and consistent tooling.

4. Angular vs React: Core Differences to Expect

• Data binding: Angular supports two-way binding; React favors one-way, immutable updates.
• Dependency injection: Built into Angular; in React, you add libraries or patterns.
• Scope: Angular is a full framework with CLI, modules, and opinionated tools; React is a UI library relying on external choices.
• Language: Angular uses TypeScript; React typically uses JavaScript (though TypeScript is common).

5. AngularJS vs Angular: What Changed

• Architecture: AngularJS used controllers and scope; Angular uses component-based architecture.
• Language: AngularJS used JavaScript; modern Angular uses TypeScript.
• Mobile and structure: Angular is better suited for large apps and mobile browsers.
• Syntax: Angular uses property binding [prop] and event binding (event) rather than ng-directives.

6. Angular Expressions vs JavaScript Expressions: Scope and Features

Angular template expressions evaluate against the component scope, not the global window object. They avoid displaying undefined values and restrict complex logic, avoiding loops and try/catch statements within templates. Use pipes to transform display, for example {{ message | lowercase }}.

7. Single Page Applications: One Page, Dynamic Navigation

A SPA loads a shell page once and updates the DOM dynamically as users navigate. JavaScript manipulates the view without requiring full-page reloads, resulting in faster interactions and a seamless user experience.

8. Angular Templates: View Code for Components

A template is HTML that tells Angular how to render a component. You can provide it inline via template: `<div>...</div>` or via templateUrl pointing to an HTML file. Templates can include bindings, directives, and pipes to shape the view.

9. Directives: Reusable Behavior You Attach to DOM

Directives are classes with @Directive or @Component metadata. Types:

• Component directives: Components with template and styles.
• Structural directives: Change DOM structure; marked with * (for example, *ngIf, *ngFor).
• Attribute directives: Change element appearance or behavior; example custom directive that sets background color:import { Directive, ElementRef } from '@angular/core';@Directive({ selector: '[appBlueBackground]' })export class BlueBackgroundDirective { constructor(el: ElementRef) { el.nativeElement.style.backgroundColor = 'blue'; }}

10. Components, Modules, Services: The Core Building Blocks

• Components: UI units declared with @Component; template, styles, and class logic.
• Modules (NgModule): Group declarations, imports, and exports; root modules import BrowserModule, feature modules import CommonModule.
• Services: Reusable singleton logic provided via @Injectable({ providedIn: 'root' }) and injected where needed using DI. Use ng generate component/module/service via the Angular CLI.

11. Scope: The Context for Expressions (AngularJS Concept)

In AngularJS, scope is an object that represents the model for a view and forms a hierarchical structure mirroring the DOM; it propagates events and watches expressions. In modern Angular, component class properties replace scope for template binding.

12. Data Binding: Connect the Template and Component

Four main types:

• Interpolation: {{ value }} for one-way display.
• Property binding: [property]="value" to set DOM properties.
• Event binding: (event)="handler()" to respond to DOM events.
• Two-way binding: [(ngModel)]="value" combines property and event binding.

13. Two-Way Data Binding: Sync Model and View With ngModel

Two-way binding keeps the component and template in sync.

Example:

<input [(ngModel)]="data" type="text"><h2>You entered: {{ data }}</h2>Import FormsModule in the module to enable ngModel.

14. Decorators: Metadata That Tells Angular What a Class Does

Decorators start with @ and attach metadata:

• Class decorators: @Component, @NgModule, @Injectable to define purpose.
• Property decorators: @Input, @Output for inputs and events.
• Method decorators: Augment or register methods.
• Parameter decorators: Annotate constructor parameters for DI.

15. Annotations: Metadata Applied to Classes (AngularJS Note)

Annotations are metadata entries added to classes; the compiler stores them so code can instantiate helpers with that metadata. In AngularJS, you could define custom annotation names; they act as configuration used at runtime.

16. Pure Pipes: Deterministic, Called on Input Change Only

Pure pipes are stateless functions. Angular executes a pure pipe only when its input reference changes, so results are cached across change detection cycles until inputs change.

17. Impure Pipes: Run Every Change Detection Cycle

Set pure: false to mark a pipe as impure. Angular invokes impure pipes on every change detection cycle, allowing them to react to mutated inputs or external state changes. Use them sparingly for lists or mutable data.

18. PipeTransform Interface: How to Implement a Custom Pipe

Pipes implement PipeTransform and define transform(value, ...args).

Example skeleton:

import { Pipe, PipeTransform } from '@angular/core';@Pipe({ name: 'transformpipe' })export class TransformpipePipe implements PipeTransform { transform(value: unknown, ...args: unknown[]): unknown { return null; }}

19. Parent to Child Data: @Input Example

Parent component:

@Component({ selector: 'app-parent', template: `<app-child [childMessage]="parentMessage"></app-child>` })export class ParentComponent { parentMessage = 'message from parent'; }

Child component:

@Component({ selector: 'app-child', template: `Say {{ childMessage }}` })export class ChildComponent { @Input() childMessage: string; }

20. Simple TypeScript Class With Constructor and Method

class IB { name: string; constructor(message: string) { this.name = message; } greet() { return 'Hello, ' + this.name + ' How are you'; }}let msg = new IB('IB');

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25 Angular Interview Questions and Answers for the Experienced

1. MVVM: Model View ViewModel: Separation That Scales

Model holds data and shapes. View is the template and DOM. The ViewModel sits between them, owning the presentation logic and adapting the model data for the view. In Angular, a component class often acts as the ViewModel, exposing inputs, outputs, state, and methods, while the template serves as the View. Keep templates declarative and move behavior, API calls, and heavy logic into the component or services to keep testing simple and behavior predictable.

Example Pattern

Component subscribes to a service that returns an Observable model, maps it to view state, and exposes simple getters for the template. Use OnPush and pure functions to maintain a stable ViewModel surface and reduce unnecessary re-rendering.

2. Bootstrapping module: Simplifying Complex Web Apps

AppModule (or any root NgModule) is the bootstrapping module that declares bootstrap components and imports runtime dependencies. The bootstrap array lists the root component(s) the platform should render.

Example skeleton:

import { BrowserModule } from '@angular/platform-browser';import { NgModule } from '@angular/core';@NgModule({ declarations: [AppComponent], imports: [BrowserModule], bootstrap: [AppComponent]})export class AppModule {}

For large apps, keep the root module small by delegating features to lazy-loaded modules and avoiding the import of global feature modules into AppModule, thereby maintaining a small bundle size.

3. Change Detection Mechanism: How Updates Flow Through the Tree

Angular walks the component tree and checks bindings when a change signal occurs. By default, Zone triggers a global check. Use ChangeDetectionStrategy.OnPush to limit checks to when inputs change by reference or when an event originates from the component.

For manual control, use ChangeDetectorRef:constructor(cd: ChangeDetectorRef) {}// force a checkcd.markForCheck();// run change detection immediately for this component subtreecd.detectChanges();

For expensive trees, detach child branches and call detectChanges selectively. TrackBy in *ngFor prevents complete DOM re-creation. When using OnPush, update immutable inputs or call markForCheck after async updates from services.

4. AOT Compilation: Compile Templates Before the Browser

Ahead-of-Time compiles templates and metadata at build time, producing smaller, faster start bundles and surfacing template errors earlier. Use' ng build --configuration production' to enable AOT, optimization, dead code elimination, and minification. AOT reduces runtime work and improves security by embedding templates in JS. Combine AOT with tree shaking and differential loading to further shrink the payload size.

5. HTTP Interceptors: A Single Place for Cross-Cutting HTTP Logic

Interceptors wrap HttpClient requests and responses and are ideal for auth headers, logging, metrics, and token refresh.

Typical token refresh pattern:

intercept(req, next) { const token = auth.getToken(); if (token) req = req.clone({ setHeaders: { Authorization: `Bearer ${token}` }}); return next.handle(req).pipe( catchError(err => { if (err.status === 401) { return auth.refreshToken().pipe( switchMap(newToken => { const retry = req.clone({ setHeaders: { Authorization: `Bearer ${newToken}` }}); return next.handle(retry); }) ); } return throwError(err); }) );}

Register interceptors as multiple providers. Avoid long-running logic inside interceptors; prefer delegating to services to keep the pipeline fast.

6. Transpiling: TypeScript to JavaScript and Build Control

Transpiling converts TypeScript into JavaScript at a target ECMAScript level. tsconfig.json controls target, module format, downleveling, and source maps. Keep targeting modern browsers if they support it to reduce polyfills and bundle size.

Monitor emitted code size:

• emitDecoratorMetadata and downlevelIteration can inflate output
• set compilerOptions to match runtime constraints

7. ngOnInit: Initialize After Inputs Are Set

ngOnInit runs once after Angular sets the component inputs. Use it for wiring Observables, starting data fetches, and invoking lightweight setup. Avoid heavy synchronous computation in the constructor; reserve constructors for DI. For async initialization, subscribe to Observables with takeUntil(this.destroy$) or use the async pipe to avoid manual unsubscribe and memory leaks.

8. Angular Material: Component Library and Accessibility Tools

Angular Material provides UI primitives and the CDK for accessibility, overlays, drag-and-drop, and virtual scrolling. Import only the modules you need rather than a bulk MaterialModule to reduce bundle size. For performance, use mat-virtual-scroll-viewport for long lists and prefer OnPush with Material components.

9. RouterState: Read the Active Route Tree and Data

RouterState is the hierarchical snapshot of the activated routes with consumed URL segments, params, queryParams, and resolved data. Use ActivatedRoute to read params and data via observable streams:

• this.route.paramMap.pipe(map(p => p.get('id'))).subscribe(...)

Avoid relying solely on a snapshot when you expect the route to change while the component is active. For the global state, consider ngrx/router-store to mirror router navigation in an international store.

10. RouterLink: Declarative Navigation With Options

routerLink binds routes to anchors and elements. Use parameterized links, relative navigation, and query param handling:<a [routerLink]="['/users', user.id]" [queryParams]="{tab: 'profile'}">Profile</a>

For active classes, use routerLinkActive and routerLinkActiveOptions to control exact matching. Use queryParamsHandling: 'preserve' in programmatic navigation to carry existing query params when appropriate.

11. Lifecycle Hooks: When to Run What Code

Key hooks and practical use:

• ngOnChanges(changes): React to input changes using SimpleChanges.
• ngOnInit: Set up once after inputs arrive.
• ngDoCheck: Run custom change detection; avoid heavy work here
• ngAfterViewInit / ngAfterViewChecked: Access child views and DOM-dependent logic
• ngOnDestroy: Cleanup subscriptions, stop intervals, and remove event handlers.

Pattern: Create a private destroy$ = new Subject<void>(); then in ngOnDestroy call destroy$.next(); destroy$.complete(); Use takeUntil(destroy$) to avoid leaks.

12. Component Decorator: Metadata That Shapes Behavior

@Component metadata controls selector, template, styles, changeDetection, encapsulation, and providers.

Example:@Component({ selector: 'user-card', templateUrl: './user-card.html', changeDetection: ChangeDetectionStrategy.OnPush, providers: [UserLocalService]})

Class-level provider scopes change service lifetime. Prefer providedIn: 'root' for application singletons and component providers when you need isolated instances.

13. Property Decorators: @Input, @Output and Patterns for Advanced Cases

Use @Input for inbound data; prefer setter inputs when you need transformation or validation:

• @Input() set config(value: Config) { this._cfg = normalize(value); }

Use @Output as EventEmitter, but do not treat it like a Subject for component internals. Export strongly typed events:

• @Output() save = new EventEmitter<SavePayload>();

When receiving complex objects, prefer immutable patterns so OnPush change detection works reliably.

14. Method Decorators: HostListener, HostBinding, and Practical Uses

HostListener maps host and global events to component methods:

@HostListener('window:resize', ['$event'])onResize(e: Event) { this.size = (e.target as Window).innerWidth; }

Use HostBinding to reflect component state into host element classes and attributes. For cross-cutting behavior, write small decorators that wrap methods, but keep side effects explicit to aid testability.

15. Class Decorators: Marking Roles and DI Scope

Decorators like @Injectable, @Directive, @Pipe, and @NgModule tell Angular how to treat a class. Use @Injectable({ providedIn: 'root' }) for tree-shakable services. For services that must be per module or per component, provide them at that scope rather than the root to limit lifetime and resource use.

16. Parameterized Pipes: Pass Args to Customize Behavior

Parameterized pipes accept arguments after a colon.

Example:{{ birthday | date: 'dd/MM/yyyy' }}

When building custom pipes, decide on purity. Pure pipes recalc only on object identity change; impure pipes run every change detection cycle and may harm performance. Prefer pure pipes and immutable inputs for predictable execution.

17. Pipes: Transform Values With Minimal Impact

Built-in pipes include date, currency, uppercase, and async. The async pipe is handy with Observables and Promises because it manages subscription and unsubscription:

• <div *ngIf="user$ | async as user">{{ user.name }}</div>

Avoid heavy computation inside pipes. If a transformation is expensive, compute it in the component or memoize results to prevent repeated execution during change detection.

18. Dependency Injection: Providers, Injectors, and Testing Patterns

Angular’s DI system supplies class instances based on provider configuration. Key points:

• providedIn: 'root' creates app singletons.
• Component providers create child injectors, enabling instance isolation.
• Use InjectionToken for primitive or interface-based dependencies.
• Use @Optional, @SkipSelf, and @Self to control resolution.

For unit tests, TestBed.overrideProvider and inject let you swap implementations easily. For factories that require runtime values, use' useFactory' with' deps' and multiple providers when aggregating plugins.

19. Observables vs Promises: When to Choose Which

Observables are lazy, cancellable, and can emit multiple values. Promises are eager, single-value, and always async. Use Observables for event streams, long-lived async flows, HTTP results when you want operators or cancellation, and for form valueChanges.

For one-off operations that do not need cancellation or composition, a Promise can be simpler. Remember to avoid nested subscriptions; instead, compose streams using pipeable operators.

20. String Interpolation and Property Binding: Two Ways to Feed the View

String interpolation ({{ }}) evaluates expressions and inserts text. Property binding ([property]) sets DOM properties. Use property binding when interacting with DOM element properties like disabled, src, or value:

• <img [src]="photo.url" />

When injecting HTML, use [innerHTML] but treat it as sanitized by Angular; use DomSanitizer only when you explicitly trust content and need to bypass security checks.

21. RxJS: Operators, Patterns, and Performance Guards

Use the right operator for intent:

• switchMap: Cancel previous inner observable when a new source arrives (ideal for typeahead).
• concatMap: Queue inner observables and preserve order.
• mergeMap: Run inner observables concurrently.
• exhaustMap: Ignore new emissions while processing one.

Combine shareReplay({ refCount: true, bufferSize: 1 }) to multicast a cold Observable while allowing garbage collection when no subscribers remain. Use takeUntil for unsubscription and prefer the async pipe where possible.

For Reactive Forms, integrate valueChanges with operators:

this.searchControl.valueChanges.pipe( debounceTime(300), distinctUntilChanged(), switchMap(q => this.searchService.query(q))).subscribe(...)

22. View Encapsulation: Style Containment Strategies

Encapsulation controls whether component styles affect others. Emulated scopes styles with generated attributes and is the default. ShadowDom uses native shadow DOM when available. None applies styles globally. Libraries prefer Emulated or ShadowDom to avoid leaking styles. Use CSS variables and scoped classes when you want predictable overrides without disabling encapsulation.

23. Eager and Lazy Loading: Balance Startup Speed and Responsiveness

Eager loading loads modules at bootstrap; lazy loading splits code into chunks and loads them on demand:{ path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) }

Use preloading strategies (PreloadAllModules or custom) to load critical lazy modules after initial render. Analyze bundles with source-map-explorer and tune route splits to avoid large initial payloads. Guards and resolvers still run with lazy modules; design them to minimize blocking HTTP calls.

24. Server-Side Rendering With Angular Universal: Render on the Server

Angular Universal renders pages on the server for faster first paint and improved SEO. Adapt code for isPlatformBrowser checks before using window or document. Use TransferState to avoid duplicate HTTP requests:

• Fetch data on the server,
• Attach it to the page
• Read it from the client to skip a second fetch

Keep server logic synchronous where possible and avoid long blocking operations in server builds.

25. Script Tags Within Templates: Sanitization and XSS Protection

Angular sanitizes potentially dangerous HTML. If you bind HTML that includes a <script> tag via [innerHTML], Angular strips the script content to prevent XSS. If you must inject trusted HTML, use DomSanitizer.bypassSecurityTrustHtml sparingly and only for content you control. Prefer structured data or safe templates over raw HTML to eliminate attack vectors and keep the app secure.

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10+ Senior Angular Questions and Answers

1. Parent ↔ Child Communication: Patterns Beyond @Input/@Output

Ask which pattern fits the component. Use @Input() / @Output() or signals for simple data and events. Use a shared service with a signal or Subject when many siblings or deep trees need the same state. For reusable form-like components, use custom two-way binding or implement ControlValueAccessor.

Examples and trade-offs:

• @Input() / @Output(): simple, testable, aligns with Angular change detection. Good for one-to-one relationships.
• Shared service (signal/Subject): centralizes state, reduces prop drilling, but increases coupling and lifecycle management. Use providedIn or a local provider to control instance scope.
• Custom two-way binding: implement an @Input() plus an @Output() with the Change suffix so the template can use [(value)]. This is ergonomic for library consumers and keeps change propagation explicit.
• model(): use when you want syntactic sugar over manual two-way binding and want tight integration with signals or ngModel.
• ControlValueAccessor: implement when the child is a custom form control that must work with reactive forms or template-driven forms. It provides writeValue, registerOnChange, registerOnTouched. Use when the component will be used inside FormControl/FormGroup to preserve validators, touched/dirty state, and form APIs.

When not to use certain tricks:

• ViewChild for data passing is fine for tight parent/child references but hurts reuse if you rely on implementation details.
• Local storage, cookies or global events for intra-app wiring creates hidden dependencies and testing pain.

Question for you: Does this component need to be used inside forms, or will it be a simple data emitter? That question guides the choice between Output, model(), and ControlValueAccessor.

2. NgZone and Opting Out of Angular Change Detection

NgZone wraps async work so Angular runs change detection after async callbacks. For heavy or high-frequency non-UI work, runOutsideAngular() avoids repeated change detection cycles. Re-enter with NgZone.run() only when the DOM or bindings must update.

When to opt out:

• High-frequency listeners: Scroll, mousemove, continuous telemetry.
• Passive analytics and trackers that do not affect UI.
• Integrations with third-party libraries that emit many events but do not require UI updates.

Example pattern:

• Run the listener outside Angular and push minimal updates into the Angular zone when needed. This reduces the CPU usage by change detection and keeps the UI responsive.

Trade-offs and pitfalls:

• If you forget to re-enter the zone for UI updates, the view will not refresh.
• Manual zone management requires discipline and clear comments in code to prevent maintainers from introducing bugs.
• Memory leaks from detached listeners are easy to create; always arrange teardown.

3. InjectionToken: Typed Keys for Non-Class DI

InjectionToken gives a typed identifier for values or interfaces that are not classes. Use it for config objects, endpoints, feature flags, or library hooks.

Common patterns:

• Library configuration: Export const API_ENDPOINT = new InjectionToken<string>('API_ENDPOINT'); consumer provides useValue or useFactory.
• Multi-providers: Allow multiple plugins to contribute handlers via multi: true.
• App initialization: Replace deprecated APP_INITIALIZER with provideAppInitializer to run boot-time logic that may fetch config or set up monitoring.

Practical notes:

• Prefer typed tokens so TypeScript enforces shapes.
• Use factories when the provided value needs runtime computation or depends on other injected tokens.

4. Resolution Modifiers: Optional, Self, SkipSelf, Host

When calling inject(SomeToken, { ... }), resolution modifiers control where Angular searches.

Modifiers and use cases:

• Optional: Return undefined instead of throwing when nothing is provided. Use for optional integrations.
• Self: Restrict lookup to the current injector. Useful inside directives that must only affect the element they sit on.
• SkipSelf: Skip the current injector and search parents. Useful when you want a parent-supplied instance and want to avoid a local override.
• Host: Limit lookup to the component host boundary; prevents climbing past the host component.

Real examples:

• A RequiredMarkerDirective injects NgControl with { optional: true, self: true } to check validators on the same element.
• FormControlName uses @SkipSelf() to find the parent ControlContainer so controls bind to the enclosing form.

When to use:

• Mostly in libraries and advanced directives, where you must avoid accidental captures or ensure the proper scope for instances.

5. Why trackBy Matters in Lists

Always provide a stable key for list rendering. Without it, Angular will recreate DOM nodes on every array change, and you lose input focus, animations, and mounting state.

Rules and pitfalls:

• Use a unique id property, e.g., trackBy id.
• Do not use the entire object as a key: object references change when new data is added.
• Avoid $index for dynamic lists: removing an item shifts later indexes and forces re-rendering. $index is okay for static lists.

New control-flow syntax:

• The @for() construct now requires a track key, which encourages correct, predictable behavior by default.

6. Providers vs ViewProviders and Template Function vs Pure Pipe

• providers: service is available to the component, its view, child components, and projected content.
• viewProviders: service is available only to the component and its view, but not to content projected with ng-content. Use viewProviders when you want an instance private to the component implementation and not accessible to the projected content or external children.

Concrete Scenario

You implement a PaymentButtonComponent that uses an abstract PaymentService. When creating the component dynamically, you may provide PaymentService at the component injector to change the implementation at runtime. Use viewProviders when projected children must not see the local implementation.

Template performance:

• Pure pipes cache results and only re-evaluate when inputs change. Use them for formatting or derived values that are stable between change detection runs.
• Functions called in templates execute on every change detection cycle. Heavy computations there cause performance problems. If you must compute in a template, either use a pure pipe, precompute in the component, or use a memoized computed signal.

Interactive question: Do your templates contain heavy computation or repeated function calls? If yes, refactor to pipes or signals.

Convincing a Team to Adopt Signals

Give measurable benefits and an incremental plan. Signals offer synchronous read access, built-in batching, and simpler local state patterns. Demonstrate a concrete pilot:

• Convert a small feature to signals
• Measure simpler code
• Improve performance

Use toSignal/fromSignal adapters to interoperate with RxJS. Use RxJS for complex asynchronous or streaming operations where operators excel.

Migration strategy:

• Start with UI-local state and small components.
• Wrap HTTP streams with toSignal for view usage.
• Keep RxJS in services where operators like switchMap are indispensable.
• Run performance and maintainability comparisons to enable the team to make an informed choice.

7. Diamond Problem in Observables vs Signals

The diamond problem occurs when multiple derived streams depend on shared bases. Using combineLatest or other RxJS combinators can emit multiple times for a single logical update when base sources change in sequence.Example behavior:

• Two signals updated consecutively are batched; an effect reading both runs once.
• The same two values presented through toObservable and combineLatest can emit twice, once per base update.

Mitigations in RxJS

Use operators like auditTime, sample, or buffer to batch emissions, or design upstream to emit a single combined payload. Each fix trades immediacy for fewer emissions, so choose based on user expectations.

8. Effect and Untracked: When to React and When to Disable Tracking

Use effect when you need to run imperative logic in response to signals: syncing DOM APIs, bridging to non-reactive libraries, or sending analytics events. Use untracked inside an effect to read or mutate signals without creating a dependency, which avoids infinite loops when the effect also writes to a signal.Practical pattern:

• Use afterRenderEffect when you must access DOM nodes after render.
• Wrap only the parts that must be excluded from tracking inside the untracked area. That keeps dependencies clear and prevents accidental cycles.

Safety and Cleanup

Use DestroyRef.onDestroy or store teardown logic inside the effect cleanup callback to avoid leaks. Always guard writes that could re-trigger the same effect.

9. Life-Cycle Hooks in a Signal-First App

Signals replace many common hook use cases, but hooks still matter for compatibility and clarity.Mapping hooks to signals:

• ngOnInit: often replaced by constructor logic or an effect for reactive initialization.
• ngOnChanges: replaced by computed or effect watching specific input signals.
• ngAfterViewInit / ngAfterContentInit: replaced by effects that depend on viewChild/contentChild signals or afterRenderEffect for DOM work.
• ngOnDestroy: use DestroyRef.onDestroy to run cleanup.

When to Keep Hooks

Use hooks when integrating with third-party libraries or when a lifecycle hook makes the intent clearer for maintainers unfamiliar with signals.

10. Higher-Order Observables Explained

A higher-order observable emits inner observables. Operators determine how inner subscriptions are handled.Operator choices and effects:

• switchMap: Unsubscribes the previous inner observable when a new outer value arrives. Best for live search, where only the latest input matters.
• mergeMap: Subscribes to all inner observables concurrently. Use for fire-and-forget parallel work.
• concatMap: Queues inner observables and preserves order. Use when order matters.
• exhaustMap: Ignores new outer values while an inner observable is active. Good for preventing duplicate submissions.

Select an operator based on your cancellation, ordering, and concurrency requirements. Which one matters most depends on user expectations for responsiveness and correctness.

11. share() vs shareReplay() and scan() + expand() for Pagination

• share(): multicasts the source to current subscribers, does not replay past values. Good when only live emission matters.
• shareReplay({ bufferSize: 1, refCount: true }): multicasts and replays the latest value to new subscribers while automatically unsubscribing when no subscribers exist, if refCount is true. Use it for caching HTTP responses or derived state that should be available to late subscribers.

Configuration notes:

• BufferSize controls the number of past values to replay.
• RefCount determines whether the shared subscription stays active without subscribers.

What scan() + expand() Do in Pagination

• expand(): Recursively projects each emission into a new inner observable, enabling chained or multi-page fetching in a single stream.
• scan(): Accumulates results over time, merging new pages into the existing array without dropping previous data.

Pattern behavior:

• startWith provides an initial state, allowing the UI to render an empty or loading state.
• exhaustMap prevents concurrent pagination runs, thereby avoiding race conditions caused by multiple scroll events.
• Expand can fetch additional pages automatically up to a limit, and scan reduces the stream to a combined list for the UI to consume.

Example result: A single signal that represents the accumulated message list while the underlying observable pipeline handles pagination, retries, and error handling.

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