What is metadata?
Metadata is used to decorate a class so that it can configure the expected behavior of the class. The metadata is represented by decorators
- Class decorators, e.g. @Component and @NgModule
import { NgModule, Component } from '@angular/core';@Component({selector: 'my-component',template: '<div>Class decorator</div>',})export class MyComponent {constructor() {console.log('Hey I am a component!');}}@NgModule({imports: [],declarations: [],})export class MyModule {constructor() {console.log('Hey I am a module!');}}
- Property decorators Used for properties inside classes, e.g. @Input and @Output
import { Component, Input } from '@angular/core';@Component({selector: 'my-component',template: '<div>Property decorator</div>',})export class MyComponent {@Input()title: string;}
- Method decorators Used for methods inside classes, e.g. @HostListener
import { Component, HostListener } from '@angular/core';@Component({selector: 'my-component',template: '<div>Method decorator</div>',})export class MyComponent {@HostListener('click', ['$event'])onHostClick(event: Event) {// clicked, `event` available}}
- Parameter decorators Used for parameters inside class constructors, e.g. @Inject, Optional
import { Component, Inject } from '@angular/core';import { MyService } from './my-service';@Component({selector: 'my-component',template: '<div>Parameter decorator</div>',})export class MyComponent {constructor(@Inject(MyService) myService) {console.log(myService); // MyService}}
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