As a leading front-end framework for enterprise-grade web applications, Angular empowers developers to build robust, scalable, and maintainable SPAs (Single Page Applications). Recruiters must identify professionals skilled in TypeScript, component architecture, and Angular’s reactive patterns, ensuring delivery of performant and maintainable codebases.
This resource, "100+ Angular Interview Questions and Answers," is tailored for recruiters to simplify the evaluation process. It covers topics from Angular basics to advanced concepts like RxJS, state management, and performance optimization, including dependency injection, routing, forms, and module architecture.
Whether hiring for Front-End Developers, Angular Developers, or Full-Stack Engineers, this guide enables you to assess a candidate’s:
For a streamlined assessment process, consider platforms like WeCP, which allow you to:
✅ Create customized Angular assessments aligned to role requirements and experience levels.
✅ Include hands-on coding tasks, such as building components, services, or reactive forms within a simulated IDE.
✅ Proctor assessments remotely with AI-based integrity monitoring.
✅ Leverage AI-powered grading to evaluate code quality, logic, and adherence to Angular’s architectural standards.
Save time, improve hiring precision, and confidently recruit Angular professionals who can build scalable, high-performance applications from day one.
Angular is a platform and framework for building single-page client applications using HTML, CSS, and TypeScript. Developed and maintained by Google, Angular is a component-based framework that uses dependency injection to create modular, reusable, and maintainable applications.
Angular provides a set of tools and libraries to facilitate the development of complex web applications, including routing (to manage navigation), forms (template-driven and reactive forms), HTTP client (to interact with RESTful APIs), pipes (to transform data in templates), and directives (to modify the DOM).
The framework follows the MVC (Model-View-Controller) design pattern, but Angular's architecture is more focused on components, which act as the building blocks of an application.
With two-way data binding, lazy loading, and strong support for unit testing, Angular is a powerful choice for developing modern web apps with rich user interfaces. It is typically used for building enterprise-grade applications, progressive web apps (PWAs), and complex dynamic interfaces.
Angular has several key features that make it a robust framework for building modern web applications:
Angular and AngularJS refer to two different versions of the framework, with AngularJS being the original version (1.x), while Angular (starting from version 2.x) is a complete rewrite of AngularJS. The primary differences between AngularJS and Angular are:
A component in Angular is a fundamental building block of the application. It is used to define the structure, behavior, and styling of a part of the user interface (UI). Each component in Angular is made up of three key parts:
Every Angular component is associated with a selector, which is the HTML tag that references the component in the template. A component can also have input and output bindings to communicate with other components.
Example of a simple Angular component:
@Component({
selector: 'app-my-component',
templateUrl: './my-component.component.html',
styleUrls: ['./my-component.component.css']
})
export class MyComponent {
title: string = 'Hello, Angular!';
changeTitle(newTitle: string) {
this.title = newTitle;
}
}
In Angular, a module is a logical unit of code that groups related components, directives, pipes, and services. Angular modules help in organizing an application into cohesive blocks of functionality and improve maintainability.
An Angular module is defined using the @NgModule decorator. The most important properties of an Angular module are:
Example:
@NgModule({
declarations: [AppComponent, OtherComponent],
imports: [BrowserModule],
providers: [AppService],
bootstrap: [AppComponent]
})
export class AppModule {}
Modules help in organizing large applications, enabling features like lazy loading, modularization, and scoping of services.
TypeScript is a superset of JavaScript developed by Microsoft. It adds static typing to JavaScript, meaning that variables and functions can be typed explicitly, allowing for better tooling, error checking, and code refactoring. TypeScript also supports features like classes, interfaces, and generics, making it more suitable for building large-scale applications.
In Angular, TypeScript is used for:
Data binding in Angular is a mechanism for synchronizing the data between the component (TypeScript class) and the view (HTML template). It allows the dynamic updating of the UI when the data changes, as well as capturing user input and updating the component's properties accordingly.
There are four types of data binding in Angular:
Interpolation ({{}}): It is used to bind component data to the view (e.g., display a property value).
<h1>{{ title }}</h1>Property Binding ([ ]): It binds a property of an HTML element to a component property.
<img [src]="imageUrl">
Event Binding (( )): It binds an event (such as click, keyup) from the view to the component.
<button (click)="onClick()">Click me</button>
Two-Way Binding ([( )]): It binds both a property and an event. Changes in the input field will update the component’s data, and changes in the component’s data will reflect in the input field.
<input [(ngModel)]="username">
The main types of data binding in Angular are:
Interpolation: Allows you to embed component properties into HTML. It’s mostly used for displaying values in the template.
<p>{{ componentValue }}</p>
Property Binding: Binds an element property to a component property. It helps in dynamically setting HTML element properties.
<img [src]="imageUrl" alt="Dynamic image">
Event Binding: Allows the view to listen to events like click, keyup, etc., and call methods in the component in response.
<button (click)="handleClick()">Click me</button>
Two-Way Binding: This is a combination of property binding and event binding. The ngModel directive is used to bind the data in both directions (model to view and view to model).
<input [(ngModel)]="username">
Directives in Angular are special markers in the DOM that tell Angular to attach a behavior to an element, component, or another directive. They can be used to modify the DOM structure, style elements, or create reusable components.
There are three types of directives in Angular:
Structural Directives: These change the structure of the DOM by adding or removing elements. Common examples include *ngFor (looping through collections) and *ngIf (conditionally adding/removing elements).
<div *ngIf="showMessage">This is a message!</div>Attribute Directives: These change the appearance or behavior of an element. Examples include ngClass, ngStyle, or custom directives that manipulate the DOM.
<div [ngClass]="{ 'active': isActive }">Styled Content</div>
ngOnInit() is a lifecycle hook in Angular, which is called once after the component's data-bound properties are initialized. It is part of the OnInit interface and is typically used to initialize data or perform tasks that should occur after the component's view has been fully initialized.
Some typical use cases for ngOnInit():
Example:
export class AppComponent implements OnInit {
data: any;
ngOnInit() {
this.loadData();
}
loadData() {
// Load data from service
this.data = ['Item 1', 'Item 2'];
}
}
This hook is ideal for handling any initialization logic that depends on the component's inputs and must be done after Angular has set those inputs.
In Angular, a template is the view layer of an Angular component, where you define the structure and layout of the user interface. It is written using HTML along with Angular's templating syntax to add dynamic behavior, data binding, event handling, and conditional rendering. Templates allow developers to express the UI declaratively, meaning you describe what you want to render, rather than how to render it.
A template consists of:
Example of an Angular component with a template:
@Component({
selector: 'app-example',
template: `
<h1>{{ title }}</h1>
<button (click)="changeTitle()">Change Title</button>
`
})
export class ExampleComponent {
title = 'Hello Angular';
changeTitle() {
this.title = 'Title Changed';
}
}
In the template above, {{ title }} binds the component's title property to the view, and the (click) binding calls the changeTitle() method on button click.
In Angular, a service is a class that typically contains logic for performing tasks that are not directly related to user interface rendering, such as handling data operations, business logic, and interaction with external APIs or other backend services. Services help to separate concerns, promoting the reuse of logic and keeping your components lean and focused on managing the view and user interaction.
Key characteristics of Angular services:
Example of a simple service in Angular:
@Injectable({
providedIn: 'root'
})
export class UserService {
private apiUrl = 'https://api.example.com/users';
constructor(private http: HttpClient) {}
getUserData() {
return this.http.get(this.apiUrl);
}
}
In this example, UserService is a service that makes an HTTP request to get user data from an API. The HttpClient service is injected into the service constructor, enabling it to fetch data from a remote server.
In Angular, services are often provided using the @Injectable() decorator, and the providedIn: 'root' metadata ensures the service is available globally in the application.
To create a new component in Angular, the Angular CLI (Command Line Interface) provides a simple command to generate the component and all necessary files, including the TypeScript class, HTML template, and CSS styles.
Here’s how you can create a component:
Run the following command to create a new component:
ng generate component component-name
Alternatively, you can use the shorthand version:
ng g c component-name
This command will generate four files for the component:
Example:Suppose we want to create a component named user-profile.Run the following command:
ng g c user-profile
Manual Creation: If you prefer to manually create a component, you'll need to create the .ts, .html, and .css files, and use the @Component decorator to define the component.Example:
@Component({
selector: 'app-user-profile',
templateUrl: './user-profile.component.html',
styleUrls: ['./user-profile.component.css']
})
export class UserProfileComponent {
user = { name: 'John Doe', age: 30 };
}
The selector specifies the tag that will be used to embed this component in the HTML template, and the templateUrl and styleUrls link to the component's HTML and CSS files.
The *ngIf directive is a structural directive in Angular that conditionally includes or excludes a part of the DOM based on a given expression. If the expression evaluates to true, the element is added to the DOM; if it evaluates to false, the element is removed.
Syntax:
<div *ngIf="condition">This is visible if condition is true</div>
<div *ngIf="isVisible">This content will only appear if 'isVisible' is true.</div>
In the corresponding TypeScript file:
export class MyComponent {
isVisible = true;
}
ngIf with else:
You can also use the else keyword to specify an alternative template when the condition is false.
<div *ngIf="isVisible; else noContent">Content is visible.</div>
<ng-template #noContent><div>Content is not visible.</div></ng-template>
In this case, if isVisible is false, the noContent template will be displayed instead.
The *ngFor directive is another structural directive in Angular, used for iterating over a collection (like an array or list) and rendering an element for each item in the collection. It works similarly to a loop in traditional programming languages.
Syntax:
<div *ngFor="let item of items">
{{ item }}
</div>
Example:
<ul>
<li *ngFor="let user of users">
{{ user.name }}
</li>
</ul>
In the corresponding TypeScript file:
export class MyComponent {
users = [{ name: 'John' }, { name: 'Jane' }];
}
The *ngFor will create a <li> for each object in the users array and display the name property.
ngFor with index:
You can also access the index of the iteration using the index variable.
<div *ngFor="let item of items; let i = index">
{{ i }}: {{ item }}
</div>
The Angular CLI (Command Line Interface) is a powerful tool provided by the Angular team to help developers create, manage, build, test, and deploy Angular applications from the command line. The CLI abstracts many of the tasks that developers would typically need to do manually, such as generating components, services, modules, and routing configurations, as well as setting up build configurations and running tests.
Key features of Angular CLI:
To install Angular CLI:
npm install -g @angular/cli
To create a new Angular project using Angular CLI, follow these steps:
Install Angular CLI (if not already installed): You need to install Angular CLI globally on your machine if you don’t have it installed.
npm install -g @angular/cli
Create a New Project: Run the ng new command to create a new Angular project. You can specify the name of your project as the first argument.
ng new my-angular-app
Navigate to the Project Directory:
cd my-angular-app
Serve the Application: Run the Angular development server to see the application in the browser:
ng serve
Routing in Angular allows navigation between different views or pages in a single-page application (SPA). The Angular Router is a powerful tool that enables developers to define routes for different components and handle navigation between them.
Key concepts of Angular routing:
Example of Routing in Angular:
Defining Routes:In app-routing.module.ts:
const routes: Routes = [
{ path: 'home', component: HomeComponent },
{ path: 'about', component: AboutComponent },
{ path: '', redirectTo: '/home', pathMatch: 'full' }
];
Using RouterOutlet:In app.component.html:
<router-outlet></router-outlet>
Navigating via Links:In the template:
<a routerLink="/about">Go to About</a>Navigating Programmatically:In the component:
constructor(private router: Router) {}
goToHome() {
this.router.navigate(['/home']);
}
Both ngModel and ngModelGroup are part of Angular's FormsModule used for working with forms in Angular. They are related but serve different purposes:
Example:
<input [(ngModel)]="username" />
Example:
<form #form="ngForm">
<div ngModelGroup="address">
<input ngModel name="street" />
<input ngModel name="city" />
</div>
</form>
ngModelGroup is typically used when you need to organize a group of form controls under a single name for easier validation and management.
Angular components have a set of lifecycle hooks that allow developers to hook into different stages of a component's lifecycle. These hooks allow you to add custom behavior when certain events occur during the component's creation, update, and destruction.
Key lifecycle hooks in Angular:
Each lifecycle hook allows developers to implement logic at a particular point in the component’s lifecycle, providing a powerful way to manage the component's behavior.
Dependency Injection (DI) is a design pattern in Angular used to manage how objects or services are created and injected into components or other services. Instead of creating dependencies manually within a class, Angular provides an inversion of control, where dependencies (such as services, values, or objects) are injected into a class by an external source, often by the Angular framework itself. This promotes modularity, code reusability, and testability.
How DI Works in Angular:
Example:
Imagine a UserService that depends on HttpClient to fetch user data:
@Injectable({
providedIn: 'root'
})
export class UserService {
constructor(private http: HttpClient) {}
getUsers() {
return this.http.get('https://api.example.com/users');
}
}
Now, in your component, you inject UserService using Angular’s DI:
@Component({
selector: 'app-user-list',
template: `<ul><li *ngFor="let user of users">{{ user.name }}</li></ul>`
})
export class UserListComponent {
users: any[];
constructor(private userService: UserService) {}
ngOnInit() {
this.userService.getUsers().subscribe(users => this.users = users);
}
}
The UserService instance is automatically injected into the UserListComponent constructor, so you don't have to create an instance manually.
An Observable is a core concept in RxJS (Reactive Extensions for JavaScript), which is used extensively in Angular for handling asynchronous operations like HTTP requests, event handling, and user input.
Key Points about Observables:
Example of using an Observable:
import { Observable } from 'rxjs';
let numbers = new Observable(observer => {
observer.next(1);
observer.next(2);
observer.next(3);
observer.complete();
});
numbers.subscribe({
next: value => console.log(value), // 1, 2, 3
complete: () => console.log('Done')
});
In Angular, observables are often used with HTTP requests or event handling.
In Angular, HTTP requests are handled using the HttpClient module, which provides methods to perform RESTful operations like GET, POST, PUT, DELETE, etc. These requests return Observables, which can be subscribed to in components or services.
Steps to Handle HTTP Requests:
import { HttpClientModule } from '@angular/common/http';
@NgModule({
declarations: [AppComponent],
imports: [HttpClientModule],
bootstrap: [AppComponent]
})
export class AppModule {}
@Injectable({
providedIn: 'root'
})
export class UserService {
private apiUrl = 'https://api.example.com/users';
constructor(private http: HttpClient) {}
getUsers() {
return this.http.get(this.apiUrl);
}
addUser(user: any) {
return this.http.post(this.apiUrl, user);
}
}
@Component({
selector: 'app-user-list',
template: `<ul><li *ngFor="let user of users">{{ user.name }}</li></ul>`
})
export class UserListComponent {
users: any[];
constructor(private userService: UserService) {}
ngOnInit() {
this.userService.getUsers().subscribe(users => this.users = users);
}
}
import { catchError } from 'rxjs/operators';
import { of } from 'rxjs';
getUsers() {
return this.http.get(this.apiUrl).pipe(
catchError(error => {
console.error('Error fetching users:', error);
return of([]); // Return empty array in case of error
})
);
}
The HttpClientModule is an Angular module that provides an easy-to-use API for making HTTP requests to external services (REST APIs, databases, etc.) in Angular applications. It provides the HttpClient service, which is used to perform GET, POST, PUT, DELETE, and other HTTP operations.
Key points about HttpClientModule:
To use HttpClient, you need to:
Example:
import { HttpClientModule } from '@angular/common/http';
@NgModule({
imports: [HttpClientModule]
})
export class AppModule {}
To create a service in Angular, you can either manually create a service class or use the Angular CLI.
Using Angular CLI:
ng generate service user
This will create a user.service.ts file with the following structure:
import { Injectable } from '@angular/core';
@Injectable({
providedIn: 'root' // The service is available app-wide
})
export class UserService {
constructor() { }
}
Manually:
Example:
import { Injectable } from '@angular/core';
@Injectable()
export class UserService {
constructor() {}
getUser() {
return { name: 'John', age: 25 };
}
}
Finally, you can inject the service into a component to use it.
The @Input() and @Output() decorators are used for communication between Angular components.
@Input(): This decorator allows a parent component to pass data to a child component.Example:
@Component({
selector: 'app-child',
template: `<div>{{ parentData }}</div>`
})
export class ChildComponent {
@Input() parentData: string;
}
The parent component can bind a value to the child component's @Input() property like this:
<app-child [parentData]="parentValue"></app-child>
@Output(): This decorator allows a child component to emit an event to its parent component. It’s typically used for sending data or notifying the parent about an action.Example:
@Component({
selector: 'app-child',
template: `<button (click)="sendData()">Send Data</button>`
})
export class ChildComponent {
@Output() dataEmitter = new EventEmitter<string>();
sendData() {
this.dataEmitter.emit('Hello, Parent!');
}
}
The parent component listens for this event using the (dataEmitter) syntax:
html
Copy code
<app-child (dataEmitter)="receiveData($event)"></app-child>
Angular’s change detection mechanism is responsible for keeping the model and the view in sync. When a model changes, Angular updates the view to reflect those changes. It uses a strategy called dirty checking to determine if the model has changed.
There are two types of change detection strategies:
Change Detection Process:
You can manually trigger change detection using ChangeDetectorRef or ApplicationRef.
ngContent is used in Angular to implement content projection, which is a mechanism that allows you to insert dynamic content into a component’s template. It is similar to slots in web components or custom elements.
With ngContent, you can insert arbitrary content from the parent component into the child component’s view.
Example of ngContent:
Parent component:
<app-card>
<h1>Dynamic Content</h1>
<p>This content will be projected into the card component.</p>
</app-card>
Child component (app-card):
<div class="card">
<ng-content></ng-content>
</div>
The content inside the <app-card> tag in the parent component will be projected into the ng-content tag of the child component.
An Angular Pipe is a way to transform data in the template, allowing developers to modify the data before displaying it to the user. Pipes can transform strings, numbers, dates, or arrays into desired formats.
Pipes are used within template expressions, and they are composed using the | symbol.
Common Angular Pipes:
Example:
<p>{{ userName | uppercase }}</p>
You can also create custom pipes by implementing the PipeTransform interface.
The async pipe is a special Angular pipe used to subscribe to Observables or Promises in the template. It automatically handles subscription and unsubscription, making it convenient for working with asynchronous data streams without the need for explicit subscription logic in the component.
Example:
If you have an observable users$ in your component, instead of subscribing manually, you can use the async pipe in the template:
<ul>
<li *ngFor="let user of users$ | async">{{ user.name }}</li>
</ul>
The async pipe automatically subscribes to users$ and updates the view when new data arrives. It also handles the unsubscription when the component is destroyed.
In Angular, you can bind events using the Event Binding syntax, which allows you to listen to DOM events and trigger methods or expressions in your component.
Event binding syntax is as follows:
<button (click)="onClick()">Click Me</button>
Here:
Passing Parameters to Event Handlers:
You can also pass parameters to event handler methods:
<button (click)="onClick($event)">Click Me</button>
In your component, you can handle the $event (which contains information about the event):
onClick(event: MouseEvent) {
console.log('Button clicked!', event);
}
Event Binding with Method Expressions:
You can use expressions directly in the event binding if you don't need to define a separate method:
<button (click)="count = count + 1">Increment</button>
Both components and directives are fundamental building blocks of Angular applications, but they serve different purposes.
Example of a component:
@Component({
selector: 'app-user',
template: `<h1>{{ userName }}</h1>`
})
export class UserComponent {
userName: string = 'John Doe';
}
Example of an attribute directive:
@Directive({
selector: '[appHighlight]'
})
export class HighlightDirective {
constructor(private el: ElementRef) {
el.nativeElement.style.backgroundColor = 'yellow';
}
}
Validation in Angular forms can be done in two ways: Template-Driven Forms and Reactive Forms. Both have mechanisms to apply validations, but they are used in different contexts.
In Template-Driven Forms:
You use built-in directives such as ngModel with validation attributes like required, minlength, etc.
Example:
<form #myForm="ngForm">
<input name="username" [(ngModel)]="username" required minlength="5">
<div *ngIf="myForm.submitted && myForm.controls.username.errors">
<span *ngIf="myForm.controls.username.errors.required">Username is required</span>
<span *ngIf="myForm.controls.username.errors.minlength">Username must be at least 5 characters</span>
</div>
<button [disabled]="!myForm.valid">Submit</button>
</form>
In this example:
In Reactive Forms:
Reactive forms provide more flexibility by defining validation directly in the form model (in TypeScript).
Example:
import { FormBuilder, Validators } from '@angular/forms';
constructor(private fb: FormBuilder) {}
form = this.fb.group({
username: ['', [Validators.required, Validators.minLength(5)]]
});
In the template:
<form [formGroup]="form" (ngSubmit)="onSubmit()">
<input formControlName="username">
<div *ngIf="form.get('username').invalid && form.get('username').touched">
<span *ngIf="form.get('username').hasError('required')">Username is required</span>
<span *ngIf="form.get('username').hasError('minlength')">Username must be at least 5 characters</span>
</div>
<button [disabled]="form.invalid">Submit</button>
</form>
In reactive forms, you get more control over the form state and validation, especially in complex forms.
Example:
<form #form="ngForm">
<input name="username" [(ngModel)]="username" required>
</form>
Example:
import { FormGroup, FormBuilder, Validators } from '@angular/forms';
this.form = this.fb.group({
username: ['', [Validators.required, Validators.minLength(5)]]
});
Summary:
Angular Modules are a way to organize an Angular application into cohesive blocks of functionality. An Angular application is made up of one or more modules, and each module can contain components, services, pipes, and directives that are related to a specific part of the application.
Key points about Angular modules:
Example:
@NgModule({
declarations: [AppComponent, UserComponent],
imports: [CommonModule, HttpClientModule],
providers: [UserService],
bootstrap: [AppComponent]
})
export class AppModule {}Modules help with:
Lazy Loading is a technique in Angular where feature modules are loaded only when they are needed, rather than when the application starts. This helps improve the application’s initial load time and overall performance.
With lazy loading, Angular will load only the necessary code for the current route and delay loading other code until the user navigates to a route that requires it.
To implement lazy loading in Angular:
Example:
// app-routing.module.ts
const routes: Routes = [
{ path: 'user', loadChildren: () => import('./user/user.module').then(m => m.UserModule) }
];
This configuration ensures that the UserModule is only loaded when the user navigates to the /user route.
In Angular, a subscription is used to subscribe to an Observable, such as when making HTTP requests, listening to events, or handling async data.
When you subscribe to an observable, it begins to emit values. To avoid memory leaks or unnecessary background tasks, it’s essential to unsubscribe when the component or service is destroyed.
Example of Subscription:
import { Observable } from 'rxjs';
const observable = new Observable(observer => {
observer.next('Hello');
});
const subscription = observable.subscribe(value => {
console.log(value); // Output: 'Hello'
});
Unsubscribing:
To unsubscribe, call the unsubscribe() method on the subscription:
subscription.unsubscribe();
In Angular, you typically unsubscribe in the ngOnDestroy() lifecycle hook to avoid memory leaks:
ngOnDestroy() {
this.subscription.unsubscribe();
}
Alternatively, use the async pipe in templates to automatically manage subscriptions.
Angular helps protect applications from Cross-Site Scripting (XSS) attacks through its built-in security mechanisms, particularly sanitizing user input.
import { DomSanitizer, SafeHtml } from '@angular/platform-browser';
constructor(private sanitizer: DomSanitizer) {}
getSafeHtml(unsafeHtml: string): SafeHtml {
return this.sanitizer.bypassSecurityTrustHtml(unsafeHtml);
}
Angular's default [sanitization strategy] prevents XSS attacks by making sure that any untrusted HTML content is not directly rendered.
The @angular/forms module provides functionality for handling user input through forms in Angular. It contains APIs to work with both template-driven and reactive forms. This module enables features such as:
The module includes services, directives, and classes like FormControl, FormGroup, Validators, ReactiveFormsModule, and FormsModule.
To use it, import the relevant form module in your app:
import { ReactiveFormsModule } from '@angular/forms';
@NgModule({
imports: [ReactiveFormsModule]
})
export class AppModule {}
The ng serve command is used to run an Angular application in development mode. It compiles the application and serves it on a local development server, allowing you to view the application in the browser.
Features of ng serve:
You can use ng serve with optional flags, such as:
Example:
ng serve --port 4201 --open
This will serve your Angular app on http://localhost:4201 and open it in the default browser.
Both ngOnInit() and constructor() are important parts of an Angular component’s lifecycle, but they serve different purposes.
Example:
constructor(private myService: MyService) {
// Used for DI and setting up properties
}Example:
ngOnInit(): void {
// This is where you can fetch data or perform other initialization tasks
this.fetchData();
}
In summary:
Angular provides several lifecycle hooks that allow developers to run custom logic during specific stages of a component's lifecycle. Here’s a list of key lifecycle hooks:
Example:
ngOnChanges(changes: SimpleChanges) {
console.log(changes);
}
Example:
ngOnDestroy() {
console.log("Component is being destroyed");
}
Each of these lifecycle hooks provides opportunities for executing code at various points during the lifecycle of a component or directive.
An Angular decorator is a special kind of function that modifies the behavior of a class or its members. Decorators are used to add metadata to classes, methods, properties, or parameters in Angular.
Some commonly used Angular decorators:
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent { }
@NgModule({
declarations: [AppComponent],
imports: [BrowserModule],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule { }
@Injectable({
providedIn: 'root'
})
export class MyService { }
@Input() userName: string;
@Output() update = new EventEmitter<string>();
@ViewChild(MyComponent) child: MyComponent;
A Factory Provider is a way of defining custom logic to create an instance of a service in Angular. Instead of using a simple class to provide a service, a factory function is used to create the service instance.
A factory provider is useful when:
Example:
export function loggerFactory() {
return new LoggerService('my-app');
}
@NgModule({
providers: [
{ provide: LoggerService, useFactory: loggerFactory }
]
})
export class AppModule {}
In this example, the loggerFactory function is used to create the LoggerService instance.
Change detection in Angular is the process through which Angular checks the application’s model and updates the view whenever the model changes. Angular has two change detection strategies:
Example:
@Component({
selector: 'app-user',
changeDetection: ChangeDetectionStrategy.OnPush,
templateUrl: './user.component.html'
})
export class UserComponent { }
ngZone is an Angular service that allows Angular to track asynchronous operations (such as HTTP requests, timers, and promises) and update the view when the operation completes. It ensures that Angular is aware of any changes made outside Angular’s change detection context, like setTimeout or setInterval.
Why it's important:
You can interact with ngZone directly to control when change detection runs or bypass it entirely.
import { NgZone } from '@angular/core';
constructor(private ngZone: NgZone) {}
someAsyncOperation() {
this.ngZone.run(() => {
// This triggers change detection when the async operation completes
});
}
Example:
@ViewChild(MyComponent) child: MyComponent;
Example:
@ContentChild(MyDirective) contentChild: MyDirective;
There are several techniques for optimizing Angular application performance:
The trackBy function in Angular's ngFor directive helps Angular track items in lists more efficiently by identifying each item uniquely, preventing unnecessary re-rendering when the list changes.
By default, Angular re-renders every item in the list when the data changes. With trackBy, you can specify a unique identifier for each item, improving performance for large lists.
Example:
<ul>
<li *ngFor="let item of items; trackBy: trackById">{{ item.name }}</li>
</ul>
trackById(index: number, item: any): number {
return item.id; // Unique identifier for each item
}
This way, Angular only updates items whose data has changed, instead of re-rendering the entire list.
Angular interceptors are services that implement the HttpInterceptor interface and allow you to intercept and modify HTTP requests and responses globally, before they are sent or after they are received.
Common use cases for interceptors:
Example:
@Injectable()
export class AuthInterceptor implements HttpInterceptor {
intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
const clonedReq = req.clone({
setHeaders: { Authorization: 'Bearer ' + this.authService.getToken() }
});
return next.handle(clonedReq);
}
}
To register the interceptor, add it to the providers array in the module:
@NgModule({
providers: [
{ provide: HTTP_INTERCEPTORS, useClass: AuthInterceptor, multi: true }
]
})
export class AppModule {}
Interceptors provide a powerful way to centralize HTTP logic, such as authentication or error handling.
Both Observables and Promises are used for handling asynchronous operations, but they have significant differences in their behavior, use cases, and capabilities.
Example:
const promise = new Promise((resolve, reject) => {
setTimeout(() => resolve("Success"), 1000);
});
promise.then(result => console.log(result)); // Logs "Success" after 1 second
Example:
import { Observable } from 'rxjs';
const observable = new Observable(subscriber => {
setTimeout(() => subscriber.next("First Value"), 1000);
setTimeout(() => subscriber.next("Second Value"), 2000);
});
observable.subscribe(value => console.log(value));
// Logs "First Value" after 1 second, then "Second Value" after 2 seconds
Key Differences:
In Angular, Observables are often used for asynchronous operations like HTTP requests, handling streams of user inputs, or handling events.
RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using Observables, making it easier to compose asynchronous or callback-based code using operators such as map, filter, merge, etc.
In Angular, RxJS plays a central role because:
Common operators include:
Example of using RxJS with Angular HTTP Client:
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class DataService {
constructor(private http: HttpClient) {}
fetchData(): Observable<any> {
return this.http.get('https://api.example.com/data').pipe(
map(response => response['data']),
catchError(error => {
console.error('Error fetching data', error);
throw error;
})
);
}
}
RxJS helps manage async data streams efficiently and is a powerful tool to make Angular applications more reactive and maintainable.
Angular provides built-in form validators, but custom validators can be defined when more complex validation is required. Custom validators are functions that return an error object or null if the validation passes. Custom validators can be used in both template-driven forms and reactive forms.
Custom Validator for Template-Driven Forms:
import { AbstractControl, ValidationErrors } from '@angular/forms';
// Custom Validator
export function forbiddenNameValidator(control: AbstractControl): ValidationErrors | null {
const forbiddenNames = ['admin', 'root'];
return forbiddenNames.includes(control.value) ? { forbiddenName: { value: control.value } } : null;
}
To apply the custom validator in a template-driven form:
<input [(ngModel)]="username" name="username" [ngModelOptions]="{standalone: true}" [ngModel]="username" [forbiddenNameValidator]="true">
Custom Validator for Reactive Forms:
import { FormGroup, FormBuilder, Validators } from '@angular/forms';
// Define a form with a custom validator
this.form = this.fb.group({
username: ['', [Validators.required, forbiddenNameValidator]]
});
Custom validators can return an error object (such as { forbiddenName: { value: 'admin' } }) if the validation fails, or null if the input is valid.
Example:
ngOnInit() {
console.log('Component initialized!');
// Initialize data or perform actions on inputs
}
Example:
ngAfterViewInit() {
console.log('View and child views initialized!');
// Access or manipulate DOM elements in the view
}
Key Differences:
Lazy loading is a design pattern used to load modules or components only when they are needed (on demand) rather than loading them upfront when the application starts. This can significantly improve the application's initial load time by splitting the application into smaller chunks.
To implement lazy loading in Angular, use the Angular Router to define routes that load modules only when the user navigates to a certain route.
Example:
In the app-routing.module.ts, use loadChildren to load the module lazily:
const routes: Routes = [
{
path: 'feature',
loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
}
];
In this example, the FeatureModule will be loaded only when the user navigates to the /feature route, reducing the initial bundle size.
An NgModule is a fundamental building block in Angular that provides a mechanism to group related components, directives, pipes, and services together. Modules help organize the codebase into logical, reusable blocks, and they define the boundaries of functionality within the app.
Key features of NgModule:
Example of an NgModule:
@NgModule({
declarations: [AppComponent],
imports: [BrowserModule],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule { }
By using NgModules, Angular provides a way to logically organize and modularize an application, making it easier to scale, maintain, and reuse code.
Data can be passed between Angular components in several ways:
Example:
// Parent Component
@Component({ selector: 'app-parent' })
export class ParentComponent {
message = 'Hello from Parent';
}
// Child Component
@Component({ selector: 'app-child' })
export class ChildComponent {
@Input() message: string;
}
<!-- Parent Template -->
<app-child [message]="message"></app-child>
Example:
// Child Component
@Component({ selector: 'app-child' })
export class ChildComponent {
@Output() notifyParent = new EventEmitter<string>();
sendMessage() {
this.notifyParent.emit('Hello from Child');
}
}
// Parent Component
@Component({ selector: 'app-parent' })
export class ParentComponent {
onMessageReceived(message: string) {
console.log(message);
}
}
<!-- Parent Template -->
<app-child (notifyParent)="onMessageReceived($event)"></app-child>
Angular Guards are used to control access to routes, allowing you to protect routes based on certain conditions. There are several types of guards in Angular:
Example:
canActivate(): boolean {
return this.authService.isAuthenticated();
}Example:
canDeactivate(): boolean {
return confirm('Are you sure you want to leave without saving?');
}Example:
canLoad(): boolean {
return this.authService.hasPermission();
}Example:
resolve(route: ActivatedRouteSnapshot): Observable<any> {
return this.dataService.getData();
}You can handle errors in Angular HTTP requests by using the catchError operator from RxJS. This allows you to catch HTTP errors and perform custom error handling.
Example:
import { HttpClient } from '@angular/common/http';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class DataService {
constructor(private http: HttpClient) {}
fetchData() {
return this.http.get('https://api.example.com/data').pipe(
catchError(error => {
console.error('Error occurred:', error);
return throwError('Something went wrong!'); // Or a custom error handling logic
})
);
}
}
In this example, catchError intercepts the error and provides a fallback or custom error message.
The HttpClientModule is an Angular module that provides a simplified API for making HTTP requests. It is part of Angular's @angular/common/http package and allows you to interact with remote APIs using Observables.
To use HttpClientModule:
Example:
import { HttpClientModule } from '@angular/common/http';
import { HttpClient } from '@angular/common/http';
@NgModule({
imports: [HttpClientModule],
})
export class AppModule { }
@Injectable({
providedIn: 'root'
})
export class DataService {
constructor(private http: HttpClient) {}
getData() {
return this.http.get('https://api.example.com/data');
}
}
The HttpClient simplifies HTTP operations, making it easier to handle responses, errors, and HTTP methods (GET, POST, PUT, DELETE, etc.).
In Angular, HttpClient and Http are both used to make HTTP requests, but HttpClient is the newer, preferred API that offers several improvements over the older Http API.
Example:
// HttpClient (preferred):
import { HttpClient } from '@angular/common/http';
this.httpClient.get('/api/data').subscribe(response => {
console.log(response);
});
// Http (deprecated):
import { Http } from '@angular/http';
this.http.get('/api/data').subscribe(response => {
console.log(response.json());
});
The Injector in Angular is a core concept of the dependency injection (DI) system. It is responsible for creating and providing instances of services (or other dependencies) to Angular components, directives, pipes, and services. The Injector manages the lifecycle of services and determines how and when they are instantiated and injected into the class constructors.
Example:
@Injectable({
providedIn: 'root' // Service provided at the root level of the injector
})
export class MyService {
constructor() { }
}
Here, Angular will use the root injector to provide the instance of MyService throughout the application.
Angular's routing mechanism allows you to define routes that map URLs to components. The Router provides navigation and enables dynamic updates of views without reloading the entire page.
Key Concepts in Angular Routing:
Example:
const routes: Routes = [
{ path: 'home', component: HomeComponent },
{ path: 'about', component: AboutComponent }
];
@NgModule({
imports: [RouterModule.forRoot(routes)],
exports: [RouterModule]
})
export class AppRoutingModule {}
<a routerLink="/home">Home</a>
<a routerLink="/about">About</a>
<router-outlet></router-outlet> <!-- Routed component will be inserted here -->
An Angular Resolver is a service that is used to pre-fetch data before a route is activated. Resolvers are commonly used to fetch data from an API or perform other asynchronous tasks before the component for a specific route is loaded. This ensures that the component is rendered with the necessary data from the start.
Key Use Cases for Resolvers:
Example:
@Injectable({
providedIn: 'root'
})
export class DataResolver implements Resolve<any> {
constructor(private dataService: DataService) {}
resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<any> {
return this.dataService.fetchData();
}
}
const routes: Routes = [
{
path: 'data',
component: DataComponent,
resolve: {
data: DataResolver
}
}
];
Now, DataComponent will only be activated once the data has been fetched via DataResolver.
ngModel is a directive used to create two-way data binding in Angular forms. It binds an input field (or other form elements) to a component’s property. It keeps the view (HTML) and the model (component data) in sync. This means that when the value of the input field changes, the component property is updated, and vice versa.
ngModel is typically used in template-driven forms for two-way binding.
Example:
<input [(ngModel)]="username" placeholder="Enter username">
In this case, when the user types into the input field, the username property in the component will automatically be updated with the value of the input field. Likewise, if the username property changes, the input field will update with the new value.
A custom pipe in Angular allows you to transform data in your templates. You can create your own pipe when you need to perform custom transformations that are not provided by the built-in pipes.
Steps to create a custom pipe:
Example:
import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
name: 'reverse'
})
export class ReversePipe implements PipeTransform {
transform(value: string): string {
return value.split('').reverse().join('');
}
}
@NgModule({
declarations: [ReversePipe],
})
export class AppModule { }
<p>{{ 'Angular' | reverse }}</p>
This will display the text "ralugnA".
Angular modules (NgModules) are the building blocks of an Angular application. They provide a way to organize the application’s functionality into cohesive blocks of code. Each module can define components, directives, pipes, and services, and can import other modules to provide the necessary functionality.
The Key Benefits of Angular Modules:
Example:
@NgModule({
declarations: [AppComponent],
imports: [BrowserModule],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule { }
Here, AppModule is the root module of the application, which imports BrowserModule and declares the AppComponent.
Both ngOnChanges() and ngDoCheck() are lifecycle hooks in Angular, but they serve different purposes:
Example:
ngOnChanges(changes: SimpleChanges) {
console.log(changes);
}
Example:
ngDoCheck() {
console.log('Change detection is running');
}
Key Difference: ngOnChanges() is used for input property changes, while ngDoCheck() is a more general hook that is invoked every time Angular runs change detection.
Angular’s module system allows you to organize code into different functional blocks. The imports, exports, and providers are key aspects of how modules work:
Imports: Modules can import other modules to access their functionality. This allows you to use the components, directives, and services from other modules.Example:
imports: [CommonModule]
Exports: Modules can export components, directives, and pipes so that other modules can use them.Example:
exports: [MyComponent]
Providers: Modules can define services or other providers that can be injected into components, directives, or other services in the application. This helps in managing the dependency injection system.Example:
providers: [MyService]
Global error handling in Angular is typically done using an ErrorHandler service. You can extend Angular's built-in ErrorHandler class to create a custom error handler for logging errors or displaying error messages globally.
Example:
import { Injectable, ErrorHandler } from '@angular/core';
@Injectable()
export class GlobalErrorHandler implements ErrorHandler {
handleError(error: any): void {
console.error('An error occurred:', error);
// You can log the error to an external server or display a user-friendly message
}
}
@NgModule({
providers: [
{ provide: ErrorHandler, useClass: GlobalErrorHandler }
]
})
export class AppModule { }
With this approach, all uncaught errors in the application will be caught by the GlobalErrorHandler and handled appropriately.
Optimizing large Angular applications involves various techniques to improve performance, reduce memory consumption, and enhance user experience. Here are some key strategies:
Use Angular’s built-in Router to load modules lazily:
const routes: Routes = [
{
path: 'feature',
loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
}
];
Example:
<div *ngFor="let item of items; trackBy: trackById">
{{ item.name }}
</div>
trackById(index: number, item: any): number {
return item.id;
}
In Angular, ngFor and ngForOf are essentially the same, but there’s a subtle difference in how they are used:
ngFor is a shorthand syntax for ngForOf and is the commonly used version. It is typically used as a structural directive to iterate over a list and render elements in the DOM.Example:
<div *ngFor="let item of items">
{{ item }}
</div>
ngForOf is used when you want to explicitly reference the directive, which is less common. You might see it in more complex use cases or when you're manually adding directives to templates.Example:
<div *ngFor="let item of items as itemsArray">
{{ item }}
</div>
Although ngForOf exists, Angular's templating system encourages the use of ngFor in most cases because it is simpler and more concise.
Angular uses change detection to keep the DOM in sync with the component data. By default, Angular uses ChangeDetectionStrategy.Default, which checks every component in the component tree during every change detection cycle.
To optimize change detection, you can use ChangeDetectionStrategy.OnPush, which tells Angular to check a component for changes only when:
This significantly reduces the number of components Angular needs to check, improving performance.
Example:
@Component({
selector: 'app-optimized-component',
changeDetection: ChangeDetectionStrategy.OnPush,
templateUrl: './optimized.component.html',
})
export class OptimizedComponent {
@Input() data: any;
}
When to use OnPush:
Here are some performance best practices for Angular:
The async pipe in Angular automatically subscribes to an observable or a promise and unwraps the value for you. It manages subscriptions and automatically unsubscribes when the component is destroyed, helping you avoid memory leaks.
Example:
Suppose you have an observable that fetches data:
export class AppComponent {
data$: Observable<Data>;
constructor(private dataService: DataService) {
this.data$ = this.dataService.getData();
}
}
In your template, use the async pipe to bind the data:
<div *ngIf="data$ | async as data">
<p>{{ data.name }}</p>
</div>
Here, the async pipe subscribes to data$, and when the data is available, it renders the content.
HttpClient is the recommended choice for new Angular applications. To use it, you must import HttpClientModule.
Example:
import { HttpClientModule } from '@angular/common/http';
@NgModule({
imports: [HttpClientModule]
})
export class AppModule { }
The ngFor directive is used for iterating over a list or array of items and displaying them dynamically in the DOM. It creates an instance of the template for each item in the list, which is useful for displaying lists or collections of data.
Example:
<ul>
<li *ngFor="let item of items">
{{ item }}
</li>
</ul>
ngFor allows you to specify the item to iterate over and automatically creates the necessary DOM elements for each item in the list.
In Angular, you can dynamically load a component using ViewContainerRef and ComponentFactoryResolver or, in newer versions of Angular, the ngComponentOutlet directive.
Example:
Using ViewContainerRef and ComponentFactoryResolver (Angular 9 and earlier):
import { Component, ViewChild, ViewContainerRef, ComponentFactoryResolver } from '@angular/core';
import { DynamicComponent } from './dynamic/dynamic.component';
@Component({
selector: 'app-dynamic-loader',
template: `<ng-template #container></ng-template>`
})
export class DynamicLoaderComponent {
@ViewChild('container', { read: ViewContainerRef }) container: ViewContainerRef;
constructor(private resolver: ComponentFactoryResolver) {}
loadDynamicComponent() {
const factory = this.resolver.resolveComponentFactory(DynamicComponent);
this.container.clear();
this.container.createComponent(factory);
}
}
ngOnDestroy() is a lifecycle hook that is called when a component or directive is destroyed. It is commonly used for cleanup operations, such as:
Using ngOnDestroy() helps prevent memory leaks by ensuring that resources are released when the component is no longer needed.
Example:
ngOnDestroy() {
if (this.subscription) {
this.subscription.unsubscribe();
}
}
Some common routing issues in Angular include:
Ahead-of-Time (AOT) compilation is a key feature of Angular that allows the application to be compiled during the build process rather than at runtime. This provides several benefits:
To enable AOT in Angular, you can build the application using the following command:
ng build --prod
This command triggers the AOT compilation along with other optimizations like minification and tree shaking.
Angular Universal is Angular’s platform for server-side rendering (SSR) of Angular applications. It allows you to pre-render the HTML of an Angular application on the server, then send it to the client, making the application more SEO-friendly and improving initial load performance.
Benefits of Angular Universal:
To implement Angular Universal, you can generate an SSR-enabled application using the Angular CLI:
ng add @nguniversal/express-engine
Improving the performance of an Angular application can be done using several strategies:
TrackBy with ngFor: Use trackBy with ngFor to avoid unnecessary re-renders when the list changes.
<div *ngFor="let item of items; trackBy: trackById">
{{ item.name }}
</div>
Reactive Forms and Template-Driven Forms are two approaches to managing forms in Angular. Here are the key differences:
Feature
Reactive Forms
Template-Driven Forms
Approach
Code-centric (more control in TypeScript)
Template-centric (logic in the template)
Form Model
Synchronous model in the component
Asynchronous model in the template
Validation
Built-in and custom validators directly in the form model
Uses directives in templates like required
Form Control Initialization
Programmatically using FormControl, FormGroup
Automatically through template directives
Use Case
Complex forms with dynamic validation and interactions
Simple, static forms with less complex validation
Scaling
More scalable for larger forms
Better for simple forms
Example of Reactive Form:
this.form = this.fb.group({
name: ['', [Validators.required]],
email: ['', [Validators.required, Validators.email]]
});
Example of Template-Driven Form:
<form #form="ngForm">
<input name="name" ngModel required />
<input name="email" ngModel required email />
</form>
Dependency Injection (DI) in Angular is a design pattern used to implement IoC (Inversion of Control). It allows Angular to manage the creation and injection of dependencies into components, services, and other constructs.
Key Concepts:
When Angular creates a component, it injects all required dependencies (services, other components) through the constructor using the @Injectable() decorator.
Example of Dependency Injection in Angular:
@Injectable()
export class MyService {
constructor() {}
}
@Component({
selector: 'app-root',
providers: [MyService]
})
export class AppComponent {
constructor(private myService: MyService) {}
}
In this example, Angular’s DI system will inject an instance of MyService into AppComponent automatically.
Dependency Injection Scopes:
Lazy loading in Angular allows you to load feature modules on demand instead of loading them upfront when the application starts. This improves the initial load time of your application.
Steps to configure lazy loading:
Create a feature module (if not already created):
ng generate module feature
Create a routing module for the feature:
ng generate module feature/feature-routing --flat --module=feature
2.
Define the routes in the feature module:
const routes: Routes = [
{
path: '',
component: FeatureComponent
}
];
@NgModule({
imports: [RouterModule.forChild(routes)],
exports: [RouterModule]
})
export class FeatureRoutingModule {}
Configure lazy loading in the main routing module (app-routing.module.ts):
const routes: Routes = [
{
path: 'feature',
loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
}
];
@NgModule({
imports: [RouterModule.forRoot(routes)],
exports: [RouterModule]
})
export class AppRoutingModule {}
This configures the feature module to be lazily loaded when the /feature route is accessed.
The Angular CLI provides several advantages when working with large applications:
Decorators in Angular are special types of functions used to add metadata to classes, methods, and properties. They enable Angular to associate certain functionality with specific classes or methods.
Some key Angular decorators include:
@Component: Defines a component and its associated template and styles.
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent {}
@Injectable: Marks a class as available for dependency injection.
@Injectable()
export class MyService {}
@NgModule: Defines a module and its dependencies.
@NgModule({
declarations: [AppComponent],
imports: [BrowserModule],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule {}
Decorators help in organizing code, making it declarative and easy to maintain.
Angular provides mechanisms to manage memory effectively, but developers must take care to avoid memory leaks, especially in larger applications.
Common Causes of Memory Leaks:
Both ngOnInit and ngAfterViewInit are lifecycle hooks that are executed during different stages of the component lifecycle.
Example:
ngOnInit() {
console.log('Component Initialized');
}
ngAfterViewInit() {
console.log('Component View Initialized');
}
The primary difference is that ngOnInit is for initializing the component’s input properties, whereas ngAfterViewInit is for working with the component’s view after the view’s DOM elements are available.
Cross-Site Scripting (XSS) attacks occur when malicious scripts are injected into a web page, which can then be executed in the browser of other users. In Angular, several mechanisms and best practices are used to prevent XSS:
Automatic HTML Escaping: Angular automatically escapes content interpolated inside {{ }} syntax. This ensures that user input or dynamic content is rendered as plain text rather than HTML or JavaScript, preventing any embedded scripts from being executed.
<div>{{ userInput }}</div>
Sanitization: Angular's DomSanitizer service helps sanitize dynamic content, such as HTML, URLs, styles, etc. For example, when rendering dynamic HTML or a URL, you can use DomSanitizer to ensure it's safe to use.
import { DomSanitizer } from '@angular/platform-browser';
constructor(private sanitizer: DomSanitizer) {}
getSafeHtml(dangerousHtml: string) {
return this.sanitizer.bypassSecurityTrustHtml(dangerousHtml);
}
