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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