How does typescript decorators allow angular 2 to discover type metadata -

i don't understand how typescript decorator @injectable captures type information , how later knows constructor parameter corresponds type when no explicit @inject(...) provided within constructor parameter list? how replicate such behavior , in simple terms create own injector own library.

@injectable() export class appservice { }  export class appcomponent {     public constructor(private appservice: appservice) {      } } 

you may see insight within compiled code on how decorators works, example

compiled code looks below,

    componentclass = __decorate([         core_1.component({             moduleid: module.id,             selector: 'component-selector',             templateurl: 'component.html',             styleurls: ['component.css'],             providers: [component_service_1.componentservice]         }),          __metadata('design:paramtypes', [component_service_1.componentservice])     ], componentclass); 

when angular core looks uses reflect.js metadata info component.

to create own decorator, can try below,

mycustomdecorator

import "reflect-metadata";  interface icustomdecoratormeta{     var1: string }  export var mycustomdecorator =     (metadata: <icustomdecoratormeta>) => {         return (target) => {            reflect.definemetadata("mycustomdecorator", metadata, target);         }     } 

how use it,

@mycustomdecorator({    var1 : "hello" }) export class myclass(){}   // retrieve metadata can use below,  var metadata = reflect.getmetadata('mycustomdecorator', myclass); 

hope helps!!