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What Are Components in Angular 5,4 and 2?

The Concepts of Angular Components  -
Components are the most basic building block of a UI in Angular applications and it controls views (HTML/CSS). They also communicate with other components and services to bring functionality to your applications.

Technically components are basically TypeScript classes that interact with the HTML files of the components, which get displayed on the browsers.

The component is the core functionality of Angular applications but you need to know to pass the data into the components to configure them.

Angular applications must have a root component that contains all other components.
Components are created using @Component decorator that is part of @angular/core module.
You can create your own project using Angular CLI, this command allows you to quickly create an Angular application like - generate components, services, pipes, directive, classes, and modules, and so on as per your requirements.

Create your own component (login) using below command line –
ng g component login


After executing the above Angular CLI command in your project directory, the result looks like –
D:\Angular\DemoApp>ng g component login
  create src/app/login/login.component.html (24 bytes)
  create src/app/login/login.component.spec.ts (621 bytes)
  create src/app/login/login.component.ts (265 bytes)
  create src/app/login/login.component.css (0 bytes)
  update src/app/app.module.ts (394 bytes)


And the application login files are created by default and it looks like –
1.      login.component.html
2.      login.component.spec.ts
3.      login.component.ts
4.      login.component.css
5.      app.module.ts

And Angular CLI commands also import the Login component in the Angular module.
See the example in details -

login.component.ts-
import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-login',
  templateUrl: './login.component.html',
  styleUrls: ['./login.component.css']
})
export class LoginComponent implements OnInit {

  constructor() { }

  ngOnInit() {
  }
}

The above component class shows some of the most useful @Componentconfiguration options –
1.      Selector
2.      TemplateUrl
3.      StyleURLs

The selector – It is a CSS selector that tells Angular to create an instance of this component wherever it finds the corresponding tag in template HTML. For example, it is -

The templateUrl – It is the module-relative address of this component's HTML template and you can also provide the inline HTML template.

The styleUrls - It can be used for CSS rules and it will affect the style of the template elements and you can also provide the inline style CSS.

The components provide you some additional metadata configurations–
@Component({
  changeDetection?: ChangeDetectionStrategy
  viewProviders?: Provider[]
  moduleId?: string
  templateUrl?: string
  template?: string
  styleUrls?: string[]
  styles?: string[]
  animations?: any[]
  encapsulation?: ViewEncapsulation
  interpolation?: [string, string]
  entryComponents?: ArrayTypeany> | any[]>
  preserveWhitespaces?: boolean
  // inherited from core/Directive
  selector?: string
  inputs?: string[]
  outputs?: string[]
  host?: {...}
  providers?: Provider[]
  exportAs?: string
  queries?: {...}
})

app.module.ts
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { LoginComponent } from './login/login.component';

@NgModule({
  declarations: [
    AppComponent,
    LoginComponent
  ],
  imports: [
    BrowserModule
  ],

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What Are Components in Angular 5,4 and 2?

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