Angular common interview questionsAngularJS

What is Angular?

Angular is a TypeScript-style open source front-end web application framework developed by Google, focusing on building single page applications (SPA).

What is AngularJS? How is it different from Angular?

AngularJS is a JavaScript framework developed by Google for building web applications. It's called "Angular1" and it was originally launched in 2010 and has since become widely used and recognized.

Unlike AngularJS, Angular is a completely rewritten framework, also known as "Angular2+". Although both frameworks are developed by Google teams, there are some fundamental differences between them. Angular has more powerful performance and better scalability, and supports component-based programming model, making it easy to write and maintain large-scale web applications. In addition, Angular adopts TypeScript as its main programming language, which provides greater type safety and code organization. In short, Angular is more modern, beautiful and advanced than AngularJS.

Please describe the role of modules in Angular.

Modules are packages of related components, directives, services, and other Angular features into a reusable collection of code. Every Angular application requires at least one root module, which is similar to the main module and imports other modules and bootstraps the application into the document browser.

What are Angular components?

Angular components are the basic unit that constitutes the front-end UI, and all the structure, style and behavior involved in the user interface (UI) are embedded in it. Components are defined through the @Component decorator, and built-in directives such as *ngFor and *ngIf are used to control their display and hiding on the page.

Please briefly describe the function of the directive.

Directives are metadata that change the behavior or display of DOM elements. Angular has some built-in common instructions, such as ngIf and ngFor, which are used to control the insertion and deletion of HTML page nodes to make the application more dynamic. It also supports custom instructions in terms of parameter passing.

What is the difference between *ngIf and [hidden]?

The result of *ngIf is to instantiate and remove the DOM tree, while [hidden] simply sets CSS display: none. ngIf should be used when the visibility of an element needs to be frequently toggled, as it can better optimize performance (reduce the number of DOM operations).

How to perform form validation (Form Validation)?

Angular provides two methods for form validation: reactive forms and template-driven forms. Responsive forms manage form status based on RxJS and use a series of operators provided by RxJS to establish data flows, which requires a large amount of code. Template-driven forms implement verification through encapsulated instructions. This method allows you to create forms in a concise and easy-to-understand manner.

Please describe the View.

A view is a component's template, which in Angular is composed of HTML, CSS, and other built-in or custom directives. Every component has a view associated with it, and when the view is destroyed, the component is also destroyed.

How to do dependency injection (DI)?

At the core of Angular, unified throughout the entire application, managed by registering services into modules or components (services are plugged into components like databases or proxies). Components only need to declare the required services, and the framework will automatically find and inject the required services.

Please describe Angular's service (Service).

Angular's service is a JavaScript class that is usually very common in applications and includes functions such as data manipulation and interaction with the server. Register it uniformly to AppModule or each component to implement dependency injection. Other components can directly call the service to obtain data and implement corresponding logic.

What is unit testing?

Unit testing is a software testing method that checks whether the code is correct during the development process to ensure that there is no current impact on code changes. In Angular, this can be achieved by writing an officially supported Angular testing library (such as @angular/core/testing).

What is linting?

Linting is a static code analysis tool used to detect issues such as programming errors, syntax errors, and convention rules to ensure code quality and consistency. For TypeScript, Angular recommends TSLint and provides official support.

What are Angular lifecycle hooks? What is their function?

Angular lifecycle hooks are a series of callback methods that are associated with the lifecycle events of a component (or directive). These methods are predefined during the life cycle of a component or directive to help developers perform necessary tasks or implement reactive behavior at specific times. Angular lifecycle hooks provide many different opportunities to customize behavior and extend Angular's behavior patterns.

Here is a summary of Angular lifecycle hooks and their uses:

1. ngOnChanges: This method is called when the value bound to the input property changes.
2. ngOnInit: This method is called after the first ngOnChanges and before the first ngDoCheck to perform the required operations when the component is initialized.
3. ngDoCheck: Used to perform custom change detection and handle any data that requires the view to be recalculated.
4. ngAfterContentInit: This method is called after projecting content display to the component content.
5. ngAfterContentChecked: This method is called immediately after the component content projection and its binding change.
6. ngAfterViewInit: This method is called immediately after the component view and its subviews are initialized.
7. ngAfterViewChecked: This method is called immediately after the component view and its subviews change.
8. ngOnDestroy: Used to release resources and any other cleanup, this method is called before the component is destroyed.

All in all, these lifecycle hooks help developers implement custom behaviors within the component lifecycle and better manage the state and behavior of Angular applications.

What is dependency injection? How to implement dependency injection in Angular?

Dependency Injection (DI) is a design pattern commonly used in software development to manage dependencies between objects. It achieves the design goal of loose coupling by separating the responsibilities of object creation and management from classes, and allowing external containers to take away the tight coupling between objects according to predetermined rules.

In Angular, dependency injection is a core concept. It makes it easier to create and maintain separate services that can be shared by different components and provide their own functionality.

To implement dependency injection in Angular, you can use injectors. Typically, an injector is the underlying mechanism for how an Angular application tracks and resolves component dependencies. Angular looks for dependencies based on the property or constructor parameter types and recursively creates new instances of them. This way, each component automatically gets the dependencies it needs without having to manage instance creation and lifecycle itself.

Use dependency injection in the service:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Injectable()
export class MyService {
  constructor(private http: HttpClient) {}
}

 Use dependency injection in components:

import { Component } from '@angular/core';
import { MyService } from './my.service';

@Component({
  selector: 'app-my-component',
  template: `…`,
})
export class MyComponent {
  constructor(private myService: MyService) {}
}

 In short, dependency injection is an important part of the Angular framework, which helps developers create and manage dependencies between objects more easily and safely.

What is a pipeline? How to create and use pipelines?

Pipes are a very useful feature in Angular. It allows you to declaratively filter and transform data and display the data in your application's templates.

Using pipes, you can easily operate and transform various types of data, including strings, arrays, dates, and more. Some common built-in pipes are: DatePipe, UpperCasePipe, LowerCasePipe, DecimalPipe, PercentPipe, etc.

Creating a custom pipeline requires following these steps:

        1. Create a class with an @Injectable() decorator that marks the class as an injectable dependency.

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

@Pipe({
  name: 'myCustomPipe'
})
@Injectable()
export class MyCustomPipe implements PipeTransform {
  transform(value: any, arg1?: any, arg2?: any): any {
    // your logic here
  }
}

        2. Implement the PipeTransform interface, which has only one transform method. This method accepts one or more parameters as input and returns a modified result.

        3. Define a name in the pipeline, and use this name in the template to call the transform method of the pipeline. The name in the example above is "myCustomPipe".

        4. Add custom pipes in NgModule’s declarations array so that they can be used in any part of the application.

@NgModule({
  declarations: [MyCustomPipe],
  imports: [...],
  exports:[...]
})
export class AppModule { }

        Using pipes is very simple and can be called in the template as follows: 

@Component({
  selector: 'app-my-component',
  template: `
    {
   
   { myValue | myCustomPipe }}
  `,
})
export class MyComponent {
  myValue = 'Hello World';
}

 In the above code snippet, myValue is a string value connected to the myCustomPipe pipe through the "|" symbol. This tells Angular to execute the pipe's conversion method on myValue and display the result.

In summary, pipes are a very useful and easy-to-use feature in Angular that allows you to manipulate and transform data in a declarative manner to create more flexible and reusable components.

What is routing? How to implement routing in Angular?

Routing refers to the method of determining how views in an application are mapped to URLs. In a single-page application, routing can allow users to switch between different views by clicking on a link or entering a specific URL, while also allowing the browser's history to be handled correctly and avoiding page refreshes.

There are many ways to implement routing in Angular. Among them, using the routing module provided by Angular is the most common and popular approach. The specific steps are as follows:

        1. Import the RouterModule module and Routes configuration in the app.module.ts file.

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';

const routes: Routes = [
  // define your routes here
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

        2. Define each route your application may need in the routes array. Route definitions can include path, component, redirectTo, children and other attributes.
        3. Set router-outlet where routing is required (such as app.component.ts or other components) so that the router knows how to render the view.

<router-outlet></router-outlet>

        4. As you navigate within the application, use the routerLink directive to create links to invoke related routes. 

<a routerLink="/some-path">Some Link</a>

What is template syntax? How to use template syntax?

Template syntax is a special syntax in the Angular framework for defining component views. It combines the properties, methods, etc. defined in the HTML language and Angular component classes to achieve the purpose of dynamically rendering page content.

Here are some examples using template syntax:

         1. Responsive binding

        In the template, you can associate component properties with DOM elements through two-way data binding or one-way data binding. For example:

<input type="text" [(ngModel)]="name">
<p>Welcome to {
   
   { name }}, </p>

        When the user enters a name, { { name }} will be updated. 

        2. Loop rendering

        Use the *ngFor directive to render an array into multiple list items. For example:

<ul>
  <li *ngFor="let item of items">{
   
   {item}}</li>
</ul>

        3. Conditional rendering

        The *ngIf directive can be used to determine whether to display an element based on conditional judgment. For example:

<div *ngIf="showElement">I am the visible element!</div>

        The above are just some simple examples of template syntax and are not comprehensive. In general, using template syntax can help us more conveniently control the display of elements and data on the page. At the same time, it can also make our code more readable and understandable. 

What is a form? How to use forms in Angular?

A form is a tool for collecting and submitting user data. It usually consists of a set of form elements and some action buttons. In web development, a form is an HTML element that can send user-submitted data to the server through the HTTP protocol.

In Angular, there are two types of forms: template-driven forms and reactive forms. Template-driven forms bind form elements to properties in components through template binding, while responsive forms process form values ​​by creating objects such as FormControl, FormGroup, and FormArray.

With template-driven forms, you need to bind form elements to properties in the component to get and handle form values. You can use the ngModel directive to implement two-way binding and the ngForm directive to create a form.

To use responsive forms, you need to create objects such as FormControl, FormGroup, and FormArray in the component to handle changes in form values. You can use the FormControl directive to create a single form control, the FormGroup directive to create a group of form controls, and the FormArray directive to process a form array containing multiple form controls.

Here's an example showing how to create a simple form in a template-driven form:

<form (ngSubmit)="onSubmit()">
  <div>
    <label>Name:</label>
    <input type="text" name="name" [(ngModel)]="name">
  </div>
  <div>
    <label>Email:</label>
    <input type="email" name="email" [(ngModel)]="email">
  </div>
  <button type="submit">Submit</button>
</form>

 Here's an example showing how to create a simple form in Responsive Forms:

import { Component } from '@angular/core';
import { FormControl, FormGroup } from '@angular/forms';

@Component({
  selector: 'app-root',
  template: `
    <form [formGroup]="myForm" (ngSubmit)="onSubmit()">
      <div>
        <label>Name:</label>
        <input type="text" formControlName="name">
      </div>
      <div>
        <label>Email:</label>
        <input type="email" formControlName="email">
      </div>
      <button type="submit">Submit</button>
    </form>
  `,
})
export class AppComponent {
  myForm = new FormGroup({
    name: new FormControl(''),
    email: new FormControl(''),
  });

  onSubmit() {
    console.log(this.myForm.value);
  }
}

What is the HTTP module? How to use HTTP module in Angular?

 The HTTP module is a module in the Angular framework that encapsulates common HTTP requests (such as GET, POST requests, etc.) and responses. In Angular applications, we can use HTTP module to access remote servers and get or submit data.

To use the HTTP module, you need to complete the following steps:

1. Import the HttpModule module

In your application module (usually app.module.ts), you should first import Angular's HttpModule module.

import { HttpModule } from '@angular/http';

@NgModule({
  imports: [
    HttpModule,
    // other modules
  ],
  // ...
})
export class AppModule { }

 2. Inject Http service

Then inject the HTTP service into every component or service that needs to use the HTTP module.

import { Http } from '@angular/http';

@Injectable()
export class UserService {
  constructor(private http: Http) {}
}

3. HTTP request

Various types of requests can be made using the http object, such as getting data from a remote server using the GET method:

import { Http } from '@angular/http';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

@Injectable()
export class UserService {
  private apiUrl = 'http://your-api.com/users';

  constructor(private http: Http) {}

  getUsers(): Observable<any> {
    return this.http.get(this.apiUrl)
      .pipe(map(res => res.json()));
  }
}

4. Subscription response

Finally, in the component that will use the data returned by the service, subscribe to this function and execute it:

import { Component, OnInit } from '@angular/core';
import { UserService } from './user.service';

@Component({
  // ...
})
export class UserListComponent implements OnInit {
  users: any[];

  constructor(private userService: UserService) {}

  ngOnInit() {
    this.userService.getUsers()
      .subscribe(users => {
        this.users = users;
      });
  }
}

 These are the basic steps for using the HTTP module. It should be noted that only basic code examples are provided here, which are not complete. You need to make corresponding modifications and improvements according to your own needs before you can actually apply them in your project.

What is RxJS? How to use RxJS with Angular?

RxJS is a reactive programming library for writing asynchronous and event-driven programs based on observable sequences and operators. It provides a simple way to handle asynchronous data streams and event sequences to handle HTTP requests, user input, route navigation, and more in Angular applications.

 There are two main steps for using RxJS with Angular:

        1. Install RxJS: To use RxJS in an Angular project, you need to install it first. It can be installed through the NPM package manager, execute the following command: npm install --save rxjs

        2. Import and use RxJS: Import the RxJS operators, observable sequences and other RxJS classes you need to use in the Angular component, and then use them in the component. For example, if you want to subscribe to an HTTP request and get the response data, you can use HttpClientclasses and Observableclasses as follows:

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-root',
  template: '<div>{
   
   { response$ | async }}</div>'
})
export class AppComponent {
  response$: Observable<any>;

  constructor(private http: HttpClient) {
    this.response$ = this.http.get('https://jsonplaceholder.typicode.com/posts');
  }
}

In the above code, HttpClientthe class is used to send HTTP requests, and Observablethe class is used to process asynchronous response data. response$It is an observable sequence that asyncsubscribes to response data through a pipeline in the component template and displays it on the page.

This is just the basics of RxJS, if you want to understand it more deeply, you can check out the official documentation and examples.

What is Zone.js? What is its role in Angular?

Zone.js is a JavaScript library that provides the ability to cross asynchronous boundaries, making it easier for developers to catch and handle errors in asynchronous operations. Zone.js plays a very important role in Angular. It provides cross-border error handling capabilities for various asynchronous operations in Angular applications (such as timers, event listeners, HTTP requests, etc.). This means that when errors occur in asynchronous operations, Zone.js can catch these errors and handle them. In addition, Zone.js can also implement change detection in Angular applications, that is, Angular can automatically detect data changes caused by asynchronous operations and automatically update the view. Therefore, Zone.js is one of the cores of Angular. Without it, Angular applications will have difficulty handling errors in asynchronous operations and completing change detection. 

What is ngfor? How to use ngfor?

ngFor is a built-in directive in Angular that is used to render a list and display its contents in a loop. This directive makes it easy to bind a data collection to an HTML template, and is very useful for scenarios where similar content is generated repeatedly.

Using ngFor requires completing the following steps:

1. Create an array containing data in the component class

export class AppComponent {
  items = ['Item 1', 'Item 2', 'Item 3'];
}

2. Use *ngFor in HTML templates to loop through rendering elements

<ul>
  <li *ngFor="let item of items">{
   
   {item}}</li>
</ul>

 In this example, the *ngFor directive iterates through the items array and creates a li element for each item.

You can also add an index variable to get the index of the currently traversed element in the array.

<ul>
  <li *ngFor="let item of items; let i=index">{
   
   {i}} - {
   
   {item}}</li>
</ul>

Here, we use the index keyword to get the index of the currently iterated item and display it on the page.

In addition, you can also use other keywords and context variables to operate ngFor, such as first, last, odd, even, etc. See the official documentation in Angular for details.

All in all, using ngFor can easily render many DOM elements with the same content, thus simplifying a lot of repetitive work in front-end development.

What is ngif? How to use ngif?

ngIf is an Angular directive used in HTML templates to determine whether to create or destroy an element or component based on the true or false value of an expression.

When using ngIf, you need to use it on an element or component and give it a value. If this expression evaluates to true, the element or component will be displayed; otherwise, it will be removed from the DOM.

 Here is an example usage of ngIf:

<div *ngIf="isDisplay">Hello World!</div>

In the above example, if the value of isDisplay is true, Hello World! will be displayed; otherwise, it will be removed from the DOM.

In addition, ngIf has other uses, such as:

<ng-container *ngIf="isDisplay; else notDisplay">
  <div>Hello World!</div>
</ng-container>
<ng-template #notDisplay>Not Display</ng-template>
 

In the above example, if the value of isDisplay is true, the ng-container will be displayed, which contains a Hello World div element; otherwise, the ng-container will be removed, and its else part will be displayed, which contains A notDisplay ng-template element.

What is two-way data binding? How to implement two-way data binding in Angular?

Two-way data binding is a data binding method used to establish a two-way connection between the UI (user interface) and the data model. When the user enters data in the UI, the data model is updated and vice versa. This makes it easier for developers to handle forms and user input.

In Angular, two-way data binding is implemented through the [(ngModel)] directive. The [(ngModel)] directive can bind the value of a form element (such as input, select, and textarea) to the property of an Angular component in the template. When the user enters data into the UI, this directive automatically assigns the new data to the component's properties and vice versa.

What is a one-time binding? How to implement one-time binding in Angular?

One-time binding means that the view is only updated when the bound value changes for the first time, and subsequent value changes will no longer update the view. One-time binding can improve the performance of your application because it only updates the view when necessary.

In Angular, one-time binding can be achieved using { {value | oneTime}} syntax. Among them, value is the bound value, and oneTime is a one-time bound filter.

Note that one-time binding only works with simple data types, such as strings or numbers. If you want to bind an object or array, you need to use other more complex techniques to achieve one-time binding. 

What is event binding? How to implement event binding in Angular?

Event binding is the process of associating a specified event with a specific handler function. In Angular, common event binding is to associate events in the component template with methods in the component so that when the user triggers the event, the corresponding action is performed.

There are two ways to implement event binding in Angular:

1. Use template syntax

To use event binding in component templates, you can use the following syntax:

<button (click)="handleClick()">Click me</button>

In the above code, (click)it is the event binding symbol and handleClick()the handler function defined in the component.

1. Use event binder

In component classes, event binders can be used to dynamically bind events. For example:

<button [ngClass]="{ 'active': isActive }" (click)="handleClick()">Click me</button>

In the above code, [ngClass]it is the attribute binding symbol that binds the style class to the button element. (click)Is an event binding symbol that handleClick()associates the click event of the button element with the method. When isActivethe attribute is true, the button element will have activethe style class added.

In short, event binding is a very important feature in Angular, which can easily add behaviors in response to user operations to elements in the template.

What is an asynchronous pipe? How to use async pipes in Angular?

Asynchronous pipeline is a pipeline type in Angular, which is used to handle asynchronous data flow and asynchronous operations. It can be used to handle various operations such as API calls, lazy loading, and other asynchronous operations.

Using asynchronous pipes in Angular is simple. First, use pipe syntax in your template to specify the value to be converted. Then, implement the pipeline interface in your pipeline class and write the asynchronous transformation code.

Below is an example using an asynchronous pipe. We will create a asyncPipepipe named which will convert the string to uppercase letters and simulate an asynchronous operation.

import { Pipe, PipeTransform } from '@angular/core';
import { Observable, of } from 'rxjs';
import { delay } from 'rxjs/operators';

@Pipe({
  name: 'asyncPipe'
})
export class AsyncPipe implements PipeTransform {
  transform(value: string): Observable<string> {
    return of(value.toUpperCase()).pipe(delay(1000));
  }
}

In this example, we use RxJS delayoperators to simulate a 1 second asynchronous operation and return a Observable. When using a pipeline in a template asyncPipe, it will automatically subscribe to this Observableand display the converted results.

asyncPipeNow you can use pipes in your template :

<p>{
   
   { 'hello world' | asyncPipe }}</p>

This template will be displayed when the application starts HELLO WORLD. After one second, the pipe will return a new value and the template will be rendered again HELLO WORLD.

What is NgModule? What does it do?

NgModule is a modular system in Angular. It is used to organize a set of related codes in an application and provide relevant dependency injection and compilation context for these codes.

The role of NgModule is:

1. Divide the application into a set of functionally related modules.
2. Provides a mechanism for adding declarations of components, directives, pipes, etc. to your application.
3. Allows developers to reuse code by importing other NgModules.
4. Control the compilation order and load order of applications.
5. Provides a dependency injection mechanism for applications.

With NgModules, applications can be broken down into smaller functional blocks that can be managed and maintained more easily. At the same time, NgModule can also improve the performance and maintainability of applications because it can control the loading and compilation order of applications, thereby reducing application startup time and loading time.

What are dynamic components? How to implement dynamic components in Angular?

Dynamic components refer to components that are dynamically loaded and rendered at runtime. Different components can be loaded based on conditions in the application or user input.

In Angular, dynamic components can be implemented using Angular's dynamic component constructor. Here's a simple example:

1. First, you need to add a placeholder to the component's template for dynamically loading the component:

<ng-template #container></ng-template>

1. Then in the component class, import ComponentFactoryResolver and ViewChild to dynamically create and obtain components:

import { Component, ComponentFactoryResolver, ViewChild, ViewContainerRef } from '@angular/core';

@Component({
  selector: 'app-dynamic-component',
  template: `
    <ng-template #container></ng-template>
  `
})
export class DynamicComponent {
  @ViewChild('container', { read: ViewContainerRef }) container: ViewContainerRef;

  constructor(private componentFactoryResolver: ComponentFactoryResolver) {}

  loadComponent() {
    // 动态加载组件
    const componentFactory = this.componentFactoryResolver.resolveComponentFactory(MyComponent);
    const componentRef = this.container.createComponent(componentFactory);
  }
}

In the above code, ViewChild is used to obtain placeholders, ComponentFactoryResolver is used to load components to be dynamically created, and the createComponent method is used to create instances.

1. Finally, this dynamic component can be used anywhere in the application, such as loading on a button click event:

<button (click)="loadComponent()">加载组件</button>

In the loadComponent method, call the createComponent method to create a component instance, and then set the component's properties and event listeners, as shown below:

loadComponent() {
  // 动态加载组件
  const componentFactory = this.componentFactoryResolver.resolveComponentFactory(MyComponent);
  const componentRef = this.container.createComponent(componentFactory);

  // 设置组件属性
  componentRef.instance.myData = 'Hello World';

  // 监听组件事件
  componentRef.instance.myEvent.subscribe(data => console.log(data));
}

The above is an example of implementing dynamic components in Angular.

What is ng-content? How to use ng-content?

 ng-content is a built-in directive in Angular that is used to pass content from parent components to child components.

The steps to use ng-content are as follows:

1. In the child component, use the ng-content directive to define a slot.

2. In the parent component, place the content you want to pass to the child component in the middle of the child component tag.

Here's a simple example:

Subcomponent code:

<ng-content></ng-content>

Parent component code:

<app-child>
  <h1>Hello World</h1>
</app-child>

In this example, the ng-content directive defines a slot in the child component. In the parent component, include a title tag in the middle of the child component tag, and the title tag will be passed to the slot in the child component and displayed in the child component.

It should be noted that the ng-content directive can also specify a selector to pass only specific types of content to child components. For example, if you only want to pass elements with a specific class name to the child component, you can use the ng-content directive like this:

<ng-content select=".my-class"></ng-content>

What is Angular Material? How to use Angular Material with Angular?

 Angular Material is a UI component library developed by Google and used in Angular applications. It provides a ready-made set of Material Design components that can be used to build beautiful web applications and can be easily used in any Angular application.

To use Angular Material with Angular, follow these steps:

1. Install Angular Material: You can use npm to install Angular Material. Run the following command:

npm install --save @angular/material @angular/cdk @angular/animations

1. Import Angular Material modules: In your Angular application, you need to import the modules provided by Angular Material. You can import required modules in app.module.ts file.

import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
import { MatInputModule } from '@angular/material/input';
import { MatIconModule } from '@angular/material/icon';
import { MatButtonModule } from '@angular/material/button';

@NgModule({
  imports: [BrowserAnimationsModule, MatInputModule, MatIconModule, MatButtonModule],
  exports: [BrowserAnimationsModule, MatInputModule, MatIconModule, MatButtonModule],
})
export class MaterialModule { }

1. Using Angular Material components in HTML templates: In your HTML templates, you can use imported Angular Material components. For example:

<mat-form-field>
  <input matInput placeholder="Username">
  <mat-icon matSuffix>account_circle</mat-icon>
</mat-form-field>

<button mat-raised-button color="primary">Submit</button>

The above example demonstrates how to use Angular Material's input box and button components in Angular.

Hope this helps you get started with Angular Material!

What is Angular Universal? What does it do?

 Angular Universal is an open source framework based on Angular that allows you to use Angular to build web applications that can be rendered on the server side and client side. Its main role is to render Angular applications into HTML files so that search engines can index them page by page, which is an important part of SEO. Additionally, Angular Universal provides faster first-page load times and a better user experience because page content can be preloaded before rendering. It also helps improve the accessibility of web applications and provides better performance and reliability.

What are JIT and AOT compilers? What's the difference?

 JIT (Just-In-Time) compiler and AOT (Ahead-Of-Time) compiler are both types of compilers. Their differences are as follows:

The JIT compiler dynamically compiles source code or intermediate code into local machine code while the program is running, and saves it in memory. When a certain part of the program needs to be executed, the JIT compiler loads the corresponding machine code into the CPU for execution. The advantage of the JIT compiler is that it can optimize the code at runtime, thus improving the execution efficiency of the program. The disadvantage is that it takes longer to start because it needs to be compiled first.

The AOT compiler compiles the source code or intermediate code into local machine code before the program is run, and saves it to disk. When a certain part of the program needs to be executed, the AOT compiler will directly load the corresponding machine code for execution. The advantage of the AOT compiler is that the startup time is short because the code has already been compiled into machine code. The disadvantage is that it cannot be optimized for program runtime, so it may not be able to achieve the execution efficiency of the JIT compiler.

In general, JIT compilers are suitable for scenarios that require program optimization, such as game development, etc.; while AOT compilers are suitable for scenarios that require quick startup, such as mobile applications.

What are HttpClient Interceptors? How to use HttpClient Interceptors in Angular?

HttpClient Interceptors are a mechanism provided by Angular to intercept HTTP requests and responses. By using interceptors, you can do some processing on the request before it is sent out or the response comes back.

Using HttpClient Interceptors in Angular is very simple. You can implement an Interceptor by creating a class that implements HttpInterceptorthe interface. In this class, you can perform some processing on the request or response, such as adding headers, handling errors, etc. Then add the Interceptor to the interceptor list of HttpClient.

Here is a simple interceptor example:

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpHandler, HttpRequest, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class MyInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    // 在请求头中添加Authorization信息
    const authToken = 'my-auth-token';
    const authReq = req.clone({ setHeaders: { Authorization: authToken } });
    // 将修改后的请求传递给下一个拦截器或者HttpClient
    return next.handle(authReq);
  }
}

In the above example, we added an Authorization header to the request header and passed the modified request to the next interceptor or HttpClient. To use this interceptor, you just need to provide it to token in your NgModule HTTP_INTERCEPTORS.

import { NgModule } from '@angular/core';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';
import { MyInterceptor } from './my-interceptor';

@NgModule({
  imports: [
    HttpClientModule,
  ],
  providers: [
    { provide: HTTP_INTERCEPTORS, useClass: MyInterceptor, multi: true },
  ],
})
export class MyModule { }

In the above example, we MyInterceptoradded the HTTP_INTERCEPTORS token to the provider list. This tells Angular to use this interceptor before sending each HTTP request. The last use multi: truetells Angular that this is a multi-provider and multiple interceptors can be added.

What is Angular Ivy? What does it do?

 Angular Ivy is a new rendering engine introduced in Angular 9 version. It is a compilation and rendering engine based on a more advanced technical architecture. It features many improvements and optimizations compared to Angular's previous rendering engine (View Engine), including:

1. Smaller application size: Ivy can generate smaller applications, reducing download time and loading time.

2. Faster compilation time: Ivy will be faster at compile time, reducing developer waiting time.

3. Faster runtime performance: Ivy processes templates more efficiently at runtime, improving application performance.

4. Better readability and maintainability: The code generated by Ivy is easier to read and maintain, making the developer's code clearer.

The role of Ivy is mainly to improve the performance and maintainability of Angular applications, and to provide a better development experience and user experience.