Angular 2 - Metadata

Metadata is used to decorate a class so that it can configure the expected behavior of the class. Following are the different parts for metadata.

Annotations − These are decorators at the class level. This is an array and an example having both the @Component and @Routes decorator.

Following is a sample code, which is present in the app.component.ts file.

@Component ({ 
   selector: 'my-app', 
   templateUrl: 'app/app.component.html' 
})

The component decorator is used to declare the class in the app.component.ts file as a component.

Design:paramtypes − These are only used for the constructors and applied only to Typescript.
propMetadata − This is the metadata which is applied to the properties of the class.

Following is an example code.

export class AppComponent {
   @Environment(‘test’)
   appTitle: string = 'Welcome';
}

Here, the @Environment is the metadata applied to the property appTitle and the value given is ‘test’.

Parameters − This is set by the decorators at the constructor level.

Following is an example code.

export class AppComponent {
   constructor(@Environment(‘test’ private appTitle:string) { }
}

In the above example, metadata is applied to the parameters of the constructor.

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