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Q6: What is Angular's lifecycle, and can you explain the lifecycle hooks?
Angular lifecycle refers to the series of events that happen from the creation to the destruction of a component or directive. Angular provides lifecycle hooks that allow developers to tap into these events and perform actions at specific points.
Lifecycle Hooks:
ngOnChanges()
: Called when an input/output binding value changes.
ngOnInit()
: Called once after the first
ngOnChanges()
. Ideal for component initialization.
ngDoCheck()
: Called during every change detection run, allowing for custom change detection.
ngAfterContentInit()
: Called once after the first
ngDoCheck()
when content is projected into the component.
ngAfterContentChecked()
: Called after
ngAfterContentInit()
and after every subsequent
ngDoCheck()
.
ngAfterViewInit()
: Called once after the first
ngAfterContentChecked()
when the component's view (and child views) is initialized.
ngAfterViewChecked()
: Called after
ngAfterViewInit()
and after every subsequent
ngAfterContentChecked()
.
ngOnDestroy()
: Called just before the component or directive is destroyed. Ideal for cleanup tasks.
Q7: What is Angular Routing, and how do you set it up?
Angular Routing allows navigation from one view to another within a single-page application. It enables the application to display different components based on the URL.
Setting Up Angular Routing:
1. Import RouterModule and Routes: Import these from
@angular/router
in your main module file.
import { RouterModule, Routes } from '@angular/router';
2. Define Routes: Create an array of routes where each route maps a URL path to a component.
const routes: Routes = [
{ path: '', component: HomeComponent },
{ path: 'about', component: AboutComponent },
{ path: '**', component: NotFoundComponent }
];
3. Add RouterModule to NgModule: Add
RouterModule.forRoot(routes)
to the imports array in your module.
@NgModule({
imports: [RouterModule.forRoot(routes)],
exports: [RouterModule]
})
export class AppRoutingModule {}
4. Use Router Outlet: Add
<router-outlet></router-outlet>
in your main HTML template where you want the routed components to be displayed.
<router-outlet></router-outlet>
Q8: Explain Angular Forms and the difference between Template-driven and Reactive Forms.
Angular Forms are used to handle user input and validation. There are two types of forms in Angular:
Template-driven Forms:
ngModel
).
Example:
<form #form="ngForm">
<input name="name" ngModel>
</form>
Reactive Forms:
FormControl
and
FormGroup
for managing form state and validation.
Example:
import { FormGroup, FormControl } from '@angular/forms';
this.form = new FormGroup({
name: new FormControl('')
});
<form [formGroup]="form">
<input formControlName="name">
</form>
Q9: What are Angular Modules and why are they important?
Angular Modules (NgModules) are containers for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. An Angular app is modularized into multiple modules, and each module defines a compilation context for a set of components, directives, and pipes.
Importance of Angular Modules:
Example:
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
@NgModule({
declarations: [AppComponent],
imports: [BrowserModule],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule {}
Q10: What is Angular CLI and how does it improve development productivity?
Angular CLI (Command Line Interface) is a powerful tool to initialize, develop, scaffold, and maintain Angular applications. It provides a set of commands that simplify various development tasks.
Benefits of Angular CLI:
ng generate component
.
ng build
, optimizing the application for production.
ng serve
.
ng test
and
ng e2e
.
Example Commands:
# Create a new Angular application
ng new my-angular-app
# Serve the application locally
cd my-angular-app
ng serve
# Generate a new component
ng generate component my-component
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Q11: What is a Pipe in Angular, and how do you create a custom pipe?
A Pipe in Angular is a way to transform data in templates. Pipes can be used to format strings, dates, numbers, and more.
Creating a Custom Pipe:
1. Generate the Pipe:
ng generate pipe custom
2. Define the Pipe:
import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
name: 'custom'
})
export class CustomPipe implements PipeTransform {
transform(value: string, ...args: any[]): string {
// Transformation logic
return value.toUpperCase();
}
}
3. Use the Pipe in a Template:
<p>{{ 'hello world' | custom }}</p>
In this example, the
CustomPipe
transforms the input string to uppercase.
Q12: What is Angular Dependency Injection (DI) and how does it work?
Angular Dependency Injection (DI) is a design pattern that allows a class to receive its dependencies from external sources rather than creating them itself. DI is used to increase modularity and testability of the code.
How DI Works:
1. Injectable Decorator: Services and other classes can be marked as injectable using the
@Injectable
decorator.
import { Injectable } from '@angular/core';
@Injectable({
providedIn: 'root'
})
export class MyService {
constructor() { }
}
2. Provider Configuration: Services can be provided in modules, components, or other services.
@NgModule({
providers: [MyService]
})
export class AppModule { }
3. Injection via Constructor: Dependencies are injected via the constructor.
import { Component } from '@angular/core';
import { MyService } from './my-service.service';
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent {
constructor(private myService: MyService) { }
}
Q13: What is Angular Change Detection and how does it work?
Angular Change Detection is a mechanism that automatically synchronizes the model and the view. Angular's change detection process checks the component's data-bound properties for any changes and updates the DOM accordingly.
How Change Detection Works:
Change Detection Strategy: Angular provides two change detection strategies:
import { ChangeDetectionStrategy, Component } from '@angular/core';
@Component({
selector: 'app-my-component',
changeDetection: ChangeDetectionStrategy.OnPush,
templateUrl: './my-component.component.html'
})
export class MyComponent {
// Component logic
}
Change Detection Cycle: Angular performs change detection in a top-down manner, starting from the root component down to the child components.
Zone.js: Angular uses Zone.js to detect asynchronous operations and trigger change detection automatically.
Q14: What is Angular Universal and why is it used?
Angular Universal is a technology for server-side rendering (SSR) of Angular applications. It allows the application to be rendered on the server and then sent to the client, improving the initial load time and SEO.
Benefits of Angular Universal:
Setting Up Angular Universal:
1. Install Angular Universal:
ng add @nguniversal/express-engine
2. Build the Application:
npm run build:ssr
3. Serve the Application:
npm run serve:ssr
Q15: What is NgZone and how is it used in Angular?
NgZone is a service in Angular that provides a way to execute code inside or outside of the Angular zone. This can be useful for optimizing performance by running heavy operations outside the Angular zone to prevent triggering change detection unnecessarily.
Using NgZone:
1. Inject NgZone:
import { Component, NgZone } from '@angular/core';
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent {
constructor(private ngZone: NgZone) {}
runOutsideAngular() {
this.ngZone.runOutsideAngular(() => {
// Code to run outside Angular's zone
});
}
runInsideAngular() {
this.ngZone.run(() => {
// Code to run inside Angular's zone
});
}
}
2. Use
NgZone.runOutsideAngular
: Perform operations that do not need to trigger change detection outside the Angular zone.
3. Use
NgZone.run
: Bring the code back into the Angular zone if it needs to update the UI.
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Q16: What are Angular Guards and how are they used?
Angular Guards are used to control access to routes in an Angular application. They are functions that can control whether a user can navigate to a particular route or not. There are four types of guards:
Using CanActivate Guard:
1. Create a Guard:
ng generate guard auth
2. Implement the Guard:
import { Injectable } from '@angular/core';
import { CanActivate, ActivatedRouteSnapshot, RouterStateSnapshot, UrlTree } from '@angular/router';
import { Observable } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class AuthGuard implements CanActivate {
canActivate(
next: ActivatedRouteSnapshot,
state: RouterStateSnapshot): Observable | Promise | boolean | UrlTree {
// Logic to determine if the route can be activated
return true; // or false
}
}
3. Apply the Guard to a Route:
const routes: Routes = [
{
path: 'protected-route',
component: ProtectedComponent,
canActivate: [AuthGuard]
}
];
Q17: What is the difference between Promises and Observables in Angular?
Both Promises and Observables are used to handle asynchronous operations, but they have some key differences:
1. Promises:
then()
,
catch()
,
finally()
.
const promise = new Promise((resolve, reject) => {
// asynchronous operation
});
promise.then(value => {
// handle resolved value
}).catch(error => {
// handle error
});
2. Observables:
unsubscribe()
.
subscribe()
,
unsubscribe()
, operators like
map
,
filter
,
merge
.
import { Observable } from 'rxjs';
const observable = new Observable(observer => {
// asynchronous operation
observer.next(value);
observer.complete();
});
const subscription = observable.subscribe({
next(value) {
// handle emitted value
},
error(err) {
// handle error
},
complete() {
// handle completion
}
});
// To cancel the subscription
subscription.unsubscribe();
Q18: What is Angular's HttpClient, and how do you use it for making HTTP requests?
Angular's HttpClient is a service used to make HTTP requests to remote servers. It is part of the
@angular/common/http
package and provides a simplified API for making HTTP calls.
Using HttpClient:
1. Import
HttpClientModule
:
import { HttpClientModule } from '@angular/common/http';
@NgModule({
imports: [HttpClientModule],
})
export class AppModule {}
2. Inject HttpClient:
import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class DataService {
private apiUrl = 'https://api.example.com/data';
constructor(private http: HttpClient) {}
getData(): Observable<any> {
return this.http.get<any>(this.apiUrl);
}
}
3. Make HTTP Requests:
import { Component, OnInit } from '@angular/core';
import { DataService } from './data.service';
@Component({
selector: 'app-data',
template: `<div *ngFor="let item of data">{{ item }}</div>`
})
export class DataComponent implements OnInit {
data: any[];
constructor(private dataService: DataService) {}
ngOnInit() {
this.dataService.getData().subscribe(data => {
this.data = data;
});
}
}
Q19: What is Ahead-of-Time (AOT) Compilation in Angular and what are its benefits?
Ahead-of-Time (AOT) Compilation is a process in Angular where the Angular compiler compiles the application during the build time, before the browser downloads and runs the code. This is opposed to Just-in-Time (JIT) compilation, which happens in the browser during runtime.
Benefits of AOT Compilation:
Using AOT Compilation:
AOT compilation can be enabled by running the build command with the
--aot
flag:
ng build --aot
Q20: What is the difference between Angular's Renderer2 and ElementRef?
Both Renderer2 and ElementRef are used to interact with the DOM in Angular, but they serve different purposes and have different use cases.
1. ElementRef:
import { Component, ElementRef, OnInit } from '@angular/core';
@Component({
selector: 'app-sample',
template: `<div #myDiv>Sample</div>`
})
export class SampleComponent implements OnInit {
constructor(private el: ElementRef) {}
ngOnInit() {
this.el.nativeElement.querySelector('#myDiv').style.color = 'blue';
}
}
2. Renderer2:
import { Component, OnInit, Renderer2 } from '@angular/core';
@Component({
selector: 'app-sample',
template: `<div #myDiv>Sample</div>`
})
export class SampleComponent implements OnInit {
constructor(private renderer: Renderer2, private el: ElementRef) {}
ngOnInit() {
const div = this.el.nativeElement.querySelector('#myDiv');
this.renderer.setStyle(div, 'color', 'blue');
}
}
Renderer2 is generally preferred for DOM manipulation to ensure compatibility and security.
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(Basic)
import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
@Component({
selector: 'app-data',
template: `<ul>
<li *ngFor="let item of data">{{ item.name }}</li>
</ul>`
})
export class DataComponent implements OnInit {
data: any[];
constructor(private http: HttpClient) {}
ngOnInit() {
this.http.get('https://api.example.com/data')
.subscribe(response => this.data = response);
}
}
Error: The type of response in subscribe should be any[].
Corrected Code:
import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
@Component({
selector: 'app-data',
template: `<ul>
<li *ngFor="let item of data">{{ item.name }}</li>
</ul>`
})
export class DataComponent implements OnInit {
data: any[] = [];
constructor(private http: HttpClient) {}
ngOnInit() {
this.http.get<any[]>('https://api.example.com/data')
.subscribe(response => this.data = response);
}
}
(Basic)
import { Component } from '@angular/core';
@Component({
selector: 'app-user-list',
template: `
{{ user.name }}
`
})
export class UserListComponent {
users = [{ name: 'John' }, { name: 'Jane' }];
addUser() {
this.users.push({ name: 'New User' });
}
}
(Basic)
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-timer',
template: `<div>{{ message }}</div>`
})
export class TimerComponent implements OnInit {
message: string = 'Waiting...';
ngOnInit() {
setTimeout(() => {
this.message = 'Time's up!';
}, 3000);
}
}
Output: Initially, the component will display "Waiting...". After 3 seconds, it will update to "Time's up!"
Explanation: The setTimeout function updates the message property after 3 seconds, triggering Angular's change detection to update the view.
(Basic)
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-greeting',
template: `<h1>{{ greeting }}</h1>`
})
export class GreetingComponent implements OnInit {
greeting: string;
constructor() {}
ngOnInit() {
this.greeting = 'Hello, World!';
}
}
Error: The greeting property is not initialized. Fix: Ensure that the greeting property is initialized properly.
Corrected Code:
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-greeting',
template: `<h1>{{ greeting }}</h1>`
})
export class GreetingComponent implements OnInit {
greeting: string = '';
constructor() {}
ngOnInit() {
this.greeting = 'Hello, World!';
}
}
(Intermediate)
import { Component, Input, OnInit } from '@angular/core';
@Component({
selector: 'app-counter',
template: `<div>{{ counter }}</div>`
})
export class CounterComponent implements OnInit {
@Input() counter: number;
constructor() {}
ngOnInit() {
setInterval(() => {
this.counter++;
}, 1000);
}
}
Optimization: Use
ChangeDetectionStrategy.OnPush
to minimize change detection cycles.
import { Component, Input, OnInit, ChangeDetectionStrategy, ChangeDetectorRef } from '@angular/core';
@Component({
selector: 'app-counter',
template: `<div>{{ counter }}</div>`,
changeDetection: ChangeDetectionStrategy.OnPush
})
export class CounterComponent implements OnInit {
@Input() counter: number;
constructor(private cd: ChangeDetectorRef) {}
ngOnInit() {
setInterval(() => {
this.counter++;
this.cd.markForCheck();
}, 1000);
}
}
(Intermediate)
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';
@Injectable({
providedIn: 'root'
})
export class DataService {
constructor(private http: HttpClient) {}
getData(): Observable<any> {
return this.http.get('https://api.example.com/data');
}
}
Improvement: Use caching to avoid multiple API calls if the data is already available.
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, of } from 'rxjs';
import { tap } from 'rxjs/operators';
@Injectable({
providedIn: 'root'
})
export class DataService {
private data: any;
constructor(private http: HttpClient) {}
getData(): Observable<any> {
if (this.data) {
return of(this.data);
} else {
return this.http.get('https://api.example.com/data').pipe(
tap(response => this.data = response)
);
}
}
}
(Intermediate)
import { Component, Input } from '@angular/core';
@Component({
selector: 'app-item-list',
template: `
<ul>
<li *ngFor="let item of items">{{ item }}</li>
</ul>
`
})
export class ItemListComponent {
@Input() items: string[];
}
Optimization: Use
ChangeDetectionStrategy.OnPush
to minimize unnecessary re-renders.
import { Component, Input, ChangeDetectionStrategy } from '@angular/core';
@Component({
selector: 'app-item-list',
template: `
<ul>
<li *ngFor="let item of items">{{ item }}</li>
</ul>
`,
changeDetection: ChangeDetectionStrategy.OnPush
})
export class ItemListComponent {
@Input() items: string[];
}
(Intermediate)
import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
name: 'formatNames'
})
export class FormatNamesPipe implements PipeTransform {
transform(names: string[]): string {
return names.map(name => name.toUpperCase()).join(', ');
}
}
Improvement: Use
pure: true
in the pipe's metadata to ensure the pipe is only recalculated when the input changes.
import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
name: 'formatNames',
pure: true
})
export class FormatNamesPipe implements PipeTransform {
transform(names: string[]): string {
return names.map(name => name.toUpperCase()).join(', ');
}
}
(Advanced)
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, of } from 'rxjs';
import { tap } from 'rxjs/operators';
@Injectable({
providedIn: 'root'
})
export class PostService {
private posts: any[];
constructor(private http: HttpClient) {}
getPosts(): Observable<any[]> {
if (this.posts) {
return of(this.posts);
} else {
return this.http.get<any[]>('https://jsonplaceholder.typicode.com/posts').pipe(
tap(response => this.posts = response)
);
}
}
}
(Advanced)
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-counter',
template: `<div>{{ count }}</div>`
})
export class CounterComponent implements OnInit {
count: number = 0;
ngOnInit() {
setInterval(() => {
this.count++;
}, 1000);
}
}
Output: The component will display the value of count, which increments by 1 every second.
Explanation: The setInterval function increments the count property every 1000 milliseconds (1 second), and Angular's change detection will update the view to reflect the new count value.
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Overview of Angular