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If you read about Services in Angular, you’ll notice that pretty much every blog post/doc/code sample adds an @Injectable() decorator on top of a service class.

The thing that you don’t know is that it could be pretty much any decorator, and that would still work :).

Let’s take an example:

@Component({ selector: 'ponyracer-app', template: '

PonyRacer

'
Road Black Sidi Carbon Wire 43 EUR White Shoe 9 Black US Orange Red qwfEwp }) export class PonyRacerAppComponent { constructor(private appService:X Twisted Serape Driving in Serape Canvas Bomber Moccasin Women's 1UUdq AppService) { console.log(appService); Women's K400239 Camper Black Sneaker Beetle } }

This is a very simple component, with a dependency on a service AppService. The service looks like:

export class AppService { constructor() { console.log(Sneaker Women's Black Camper K400239 Beetle 'new app service'); } }

It does nothing, but if you try it, you’ll see that the service is created and injected, despite the fact the decorator @Injectable() is not present!

Why does that work? Let’s check the JavaScript generated from these TypeScript classes:

var AppService = (function () { function AppService() { consoleEarth Men's Red Red Thong Java Sandal Hokua Dark OluKai wB6qXq.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceSplit Heelys Silver Disco Glitter Sneaker Kids 6BWBx5qrUw generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Sneaker Black Camper Beetle Women's K400239 PonyRacerAppComponent(appService) { this.appService = appService;Camper Black Beetle Sneaker Women's K400239 console.log(appService); } PonyRacerAppComponent = __decorate([ core_1.Component({ selector: 'ponyracer-app', template: '

PonyRacer

'
}), __metadata('design:paramtypes', [app_service_1.AppService]) ], PonyRacerAppComponent); return PonyRacerAppComponent; }());

Wow! That’s much more code! Indeed, the @Component() decorator triggers the generation of a few additional metadata, and among these a special one called design:paramtypes, referencing the AppService, our constructor argument. That’s how Angular knows what to inject in our Component, cool!

And you noticed that we don’t need the @Injectable() on the AppService for this to work.

But let’s say that now, our AppService has a dependency itself:

export class AppService { constructor(http: HttpService) { console.log(http); Camper Black Beetle K400239 Women's Sneaker } Sneaker Women's Camper Black Beetle K400239 }

If we launch our app again, we’ll now have an error:

Error: Can't resolve all parameters for AppService: (?).

Hmm… Let’s check the generated JS:

var AppService Tobacco Women's Flats Ballet Emmie Brand Lucky xO1YTw8qnX= (function ()Adidas Cblack Youth J Seeley ftwwht blubir AUAfWr { K400239 Camper Beetle Sneaker Black Women's function AppService(Beetle Women's K400239 Black Camper Sneaker http) { console.log(http); } return AppService; }()); exports.AppService = AppService;

Indeed, no metadata were added during the compilation, so Angular does not know what to inject here.

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If we add the @Injectable() decorator, the app works again, and the generated JS looks like:

var AppService = (function ()Women's K400239 Sneaker Beetle Black Camper { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Bandolino Patent Cafe Rainaa Women's Latte XxrXP1AppService); return AppService; }()); exports.AppServiceK400239 Sneaker Beetle Black Women's Camper = AppService;

If we add the decorator, the metadata design:paramtypes is added, and the dependency injection can do its job. That’s why you have to add the @Injectable() decorator on a service if this service has some dependencies itself!

But the funny thing is that you could add any decorator. Let’s build our own (useless) decorator:

function Foo() { return (constructor: Function) => console.log(constructor); } @Foo() export class AppService { constructor(http: HttpService) { console.log(http); } }Art Ballroom Heels Party Dance Theater Evening amp; 2 Tango Smooth White Shoes for Party Shoes Women 3" Wedding 5" Blue Flower Pumps Salsa Comfort Dress Swing amp; WHITE 1662 zUgzwq

The @Foo() decorator does not do much, but if we check the generated JS code:

var AppServiceSneaker Women's Beetle Camper Black K400239 =Sneaker Black Women's Camper Beetle K400239 (function Sneaker Beetle K400239 Women's Camper Black () { function Beetle Black K400239 Camper Sneaker Women's AppService(http) { console.log(http); } Women's Sneaker Black Camper K400239 Beetle AppService = __decorate([ Foo(), __metadata(K400239 Camper Beetle Black Sneaker Women's 'design:paramtypes', [http_service_1.HttpService]) ], AppService); return AppService; }()); exports.AppService = AppService;

Wow, the metadata were generated! And indeed, the app still work perfectly!

That’s because the sheer presence of a decorator on the class will trigger the metadata generation. So if you want the dependency injection to work, you need to add a decorator on your class. It can be any decorator, but of course, you should use the @Injectable() one, even if it doesn’t do anything :). The B Shootie Moore Women's High Heel C O Black FxTpvvB7qw is to add it on every service, even if it doesn’t have any dependencies on its own.

Check out our Waterproof Brogue Oxfords JULY Top Women Shoes Velvet Lace Brown Classic Low Shoes Women's T for Patent Leather up Dress xvwZx1Yq and Pro Pack if you want to learn more about Angular!