Angular Interview Questions & Answers

Angular is a TypeScript-based open-source web application framework led by Google. Unlike AngularJS (Angular 1.x), Angular is a complete rewrite with improved performance, better mobile support, and more modular architecture.

The core building blocks include Components, Directives, Services, Modules, Templates, Routing, Dependency Injection, and Pipes.

TypeScript is a superset of JavaScript that adds optional static typing, classes, and other features. Angular uses TypeScript to provide better tooling, type checking, and more robust code.

A decorator is a design pattern and a function that defines how a class, method, or property should be processed and modified. In Angular, decorators like @Component, @Injectable, and @NgModule are used to configure and enhance classes.

Angular CLI (Command Line Interface) is a tool that helps developers create, develop, test, and maintain Angular applications. It provides commands for generating components, services, and other application artifacts.

Angular has two compilation modes: JIT (Just-in-Time) compilation, which compiles code in the browser at runtime, and AOT (Ahead-of-Time) compilation, which pre-compiles code during the build process for better performance.

Modules in Angular are containers for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. They help organize and encapsulate related components, directives, pipes, and services.

Zone.js is a library that helps Angular track asynchronous operations and automatically trigger change detection. It provides a mechanism to intercept and track async operations like setTimeout, promises, and event listeners.

Angular provides built-in internationalization support through its i18n tools, allowing developers to easily mark translatable text, generate translation files, and switch between different languages and locales.

The 'main.ts' file is the entry point of an Angular application. It bootstraps the root module (typically AppModule) and determines whether to use JIT or AOT compilation.

A component is a TypeScript class decorated with @Component that controls a portion of the screen called a view. It contains the component's logic, defines its structure with an HTML template, and can have its own styles.

There are three types of directives: Component Directives (which have a template), Structural Directives (like *ngIf and *ngFor that modify DOM layout), and Attribute Directives (like ngStyle that change appearance or behavior of an element).

Lifecycle hooks include ngOnChanges, ngOnInit, ngDoCheck, ngAfterContentInit, ngAfterContentChecked, ngAfterViewInit, ngAfterViewChecked, and ngOnDestroy. They allow developers to tap into different stages of a component's lifecycle.

@Input() allows data to be passed into a component from a parent component, while @Output() allows a component to emit events to its parent component, typically using EventEmitter.

To create a custom directive, use the @Directive decorator, define a selector, and implement the directive's logic. You can use ElementRef, Renderer2, and @HostListener for manipulating elements and handling events.

ViewEncapsulation determines how styles are applied to a component. The three modes are Emulated (default, styles are scoped), Native (uses browser's shadow DOM), and None (styles are global).

@Component is a type of directive with a template, used to create UI components. @Directive doesn't have a template and is used to add behavior to existing elements.

ng-template is a template definition element that doesn't render directly, while ng-container is a logical container that can hold structural directives without adding extra DOM elements.

You can use services with shared state, use a state management library like NgRx, or use a complex event bus pattern to communicate between unrelated components.

Content projection allows you to insert content from a parent component into a child component's template using . It helps create reusable and flexible component designs.

Angular supports four types of data binding: Interpolation ({{ }}), Property Binding ([ ]), Event Binding (( )), and Two-way Binding ([(ngModel)]).

One-way data binding updates the view when the model changes (property binding), while two-way data binding synchronizes data between the model and the view in both directions.

Event binding is done using parentheses () and the event name. For example, (click)='handleClick()' binds a click event to a method in the component class.

The $event object represents the DOM event that triggers an event binding. It provides access to event details like target, value, and other event-specific properties.

Pipes transform displayed values within a template. Built-in pipes include date, currency, uppercase, lowercase, and percent. Custom pipes can also be created for specific transformations.

Change detection is the process of synchronizing a component's view with its data model. Angular uses a unidirectional data flow and checks for changes to update the view efficiently.

Create a custom pipe by defining a class decorated with @Pipe and implementing the PipeTransform interface. The transform method defines the pipe's logic for transforming input values.

The async pipe subscribes to an Observable or Promise and returns the latest emitted value. It automatically handles subscription and unsubscription.

Template reference variables (prefixed with #) provide direct access to DOM elements or directive instances within a template. They can be used to access element properties and methods.

ngIf completely removes elements from the DOM, while the hidden property merely hides elements but keeps them in the DOM. ngIf is more performant for conditionally rendering elements.

A service is a TypeScript class decorated with @Injectable that encapsulates business logic, data fetching, and cross-component functionality. Services promote code reusability and separation of concerns.

Dependency Injection is a design pattern where dependencies are provided to a class from external sources rather than created within the class. Angular's DI system manages service instantiation and injection.

Angular supports class providers, value providers, factory providers, and existing providers. These define how services are created and instantiated within the dependency injection system.

Create a service using the Angular CLI command 'ng generate service' or manually by creating a TypeScript class with the @Injectable decorator and specifying the providedIn metadata.

The @Injectable decorator marks a class as available to be provided and injected as a dependency. It allows Angular's dependency injection system to manage the service's lifecycle.

Services can be injected at different levels: root (singleton), module, and component levels. The injection level determines the service's scope and lifetime.

providedIn 'root' creates a single, application-wide singleton service, while providing a service in a module creates a service scoped to that module's injector.

Angular uses HttpClient from @angular/common/http for making HTTP requests. It provides methods like get(), post(), put(), delete() and supports RxJS observables for handling responses.

HTTP interceptors allow you to intercept and modify HTTP requests and responses globally. They can be used for adding headers, handling errors, logging, and transforming data.

Services can be mocked using dependency injection in tests. You can create mock implementations of services or use jasmine spies to simulate service behavior during unit testing.

Routing is the mechanism of navigating between different components in an Angular application. It allows users to move between views without reloading the entire page.

Set up routing by importing RouterModule, defining routes in an array, configuring the module with RouterModule.forRoot() or RouterModule.forChild(), and using in the template.

Route parameters are dynamic values in the URL that can be passed between routes. They are defined using the ':paramName' syntax and accessed using ActivatedRoute service.

Lazy loading is a technique where modules are loaded on demand, improving initial load time. It's implemented using loadChildren property in route configuration and Angular's dynamic import syntax.

ActivatedRoute provides information about a route associated with a component loaded in an outlet. It allows access to route parameters, query parameters, and route data.

Route guards (CanActivate, CanDeactivate, CanLoad) are interfaces that determine if a route can be activated, deactivated, or loaded based on certain conditions like authentication.

forRoot() is used in the root module to register router service providers, while forChild() is used in feature modules to register child routes without duplicating router providers.

Query parameters are managed using NavigationExtras when navigating programmatically, and can be accessed using ActivatedRoute's queryParams observable.

Wildcard routes (using '**') are used to handle undefined routes and redirect users to a default or 404 page when no matching route is found.

Child routes are nested routes within a parent route. They allow you to create more complex routing structures and load child components within a parent component's router-outlet.

NgRx is a state management library based on Redux principles. It uses actions, reducers, and selectors to manage application state in a predictable and immutable way.

Core NgRx concepts include Store (global state), Actions (events), Reducers (state transformation functions), Effects (side effects), and Selectors (state querying).

Redux provides a centralized, immutable state with strict unidirectional data flow, while service-based management is more flexible but can lead to less predictable state changes.

Actions are created using the createAction() function from @ngrx/store. They define the type and optional payload for state-changing events.

Effects handle side effects in NgRx, such as API calls. They listen for specific actions, perform async operations, and dispatch new actions based on the results.

Async operations are typically handled using Effects with RxJS operators like switchMap, mergeMap, and catchError to manage API calls and state updates.

Selectors are pure functions used to select and derive data from the store. They can compose complex state queries and provide memoization for performance optimization.

Immutability means not modifying the original state directly. Instead, create a new state object with changes, which helps in tracking state changes and optimizing performance.

Error states are typically managed by creating specific error actions, updating the store with error information, and using Effects to handle error scenarios.

NgRx provides centralized state management, predictable state changes, easier debugging, performance optimization, and better scalability for complex applications.

Angular supports two types of forms: Template-driven Forms (created in the template with ngModel) and Reactive Forms (created programmatically using FormBuilder and FormGroup).

Template-driven forms are created in the template with two-way data binding, while Reactive forms are more explicit, created in the component class with more control and validation.

Form validation can be done using built-in validators like required, minLength, maxLength, and pattern, or by creating custom validators in both template-driven and reactive forms.

FormBuilder is a service that provides convenient methods for generating form controls. It simplifies the creation of complex reactive forms by reducing boilerplate code.

Form controls include FormControl (individual input), FormGroup (collection of controls), and FormArray (dynamic collection of controls). They provide methods for validation and value tracking.

Custom validators are functions that return null if validation passes or a validation error object. They can be created for both template-driven and reactive forms.

ngModel is used in template-driven forms for two-way data binding. It creates a FormControl instance from a domain model and updates the form state.

Form submission is handled using (ngSubmit) event in template-driven forms and by checking form validity in the component method for reactive forms.

Reactive forms provide observables like valueChanges and statusChanges that emit new values when the form's value or validation status changes.

Cross-field validation involves validating multiple form controls together. In reactive forms, this can be achieved by creating custom validators at the form group level.

Change Detection Strategy determines how and when Angular checks for changes in components. OnPush strategy reduces change detection cycles by checking components only when input references change.

Performance optimization techniques include using OnPush change detection, lazy loading, AOT compilation, minimizing watchers, using pure pipes, and avoiding complex expressions in templates.

AOT compilation converts Angular HTML and TypeScript code into efficient JavaScript code during the build process, resulting in faster rendering and smaller bundle sizes.

trackBy helps optimize rendering performance in lists by tracking list items using a unique identifier. It reduces unnecessary DOM manipulations when list items change.

Performance-related decorators include @Component's changeDetection property for OnPush strategy, and @Injectable's providedIn for efficient service management.

Prevent memory leaks by unsubscribing from observables, using async pipe, avoiding manual DOM manipulations, and implementing ngOnDestroy lifecycle hook for cleanup.

Tree-shaking is an optimization technique that eliminates unused code during the build process, reducing bundle size and improving application performance.

Code splitting involves breaking the application into smaller chunks that can be loaded on demand, improving initial load time and overall performance.

Build optimizations include minification, uglification, dead code elimination, and using production mode with Angular CLI's --prod flag.

Use Chrome DevTools, Angular DevTools, Lighthouse, and performance profiling to analyze rendering time, memory usage, and identify performance bottlenecks.

Angular primarily uses Jasmine for writing tests and Karma as the test runner. For end-to-end testing, Protractor is commonly used, though Cypress is becoming more popular.

Unit tests for components are written using TestBed, which creates a testing module. Tests typically check component creation, method behavior, and interactions with services.

In Angular testing, dependencies can be mocked or stubbed using TestBed.configureTestingModule() and providing mock services to isolate the component being tested.

A test fixture is a wrapper around a component that allows you to create a testing environment, access the component instance, and trigger change detection during testing.

Services are tested by creating an instance, mocking dependencies, and verifying method behaviors. HttpClientTestingModule is used for testing HTTP services.

Testing utilities include ComponentFixture, async, fakeAsync, tick, and TestBed, which help simulate component lifecycle, handle asynchronous operations, and create testing environments.

E2E testing involves testing the entire application flow using tools like Protractor or Cypress. These tests simulate user interactions and verify application behavior from a user's perspective.

HTTP requests are mocked using HttpClientTestingModule and HttpTestingController, which allow you to define expected requests and provide mock responses.

Code coverage reports show the percentage of code executed during testing. In Angular, this is generated using Istanbul and can be configured in the Karma configuration.

Debugging techniques include using browser DevTools, source maps, Angular DevTools extension, console.log(), breakpoints, and the debugger statement in TypeScript.