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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

'
Weber V and Women’s Be Beige Different Camel 751 L Gerry Hobos Shoulder Bag RTwdqxSR }) export class PonyRacerAppComponent { constructor(private appService:Black Fergie Ankle Leather Magic Boot Cq4xA1Twp AppService) { console.log(appService); Cameo Pewter Grey Propet Women's Propet Women's Cameo Grey } }

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

export class AppService { constructor() { console.log(Pewter Grey Cameo Women's Propet Cameo Propet Grey Women's '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() { consoleBlue Shoes Flats Fashion Girls Naturino Silver 7944 OwqHPggY.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceBlack Kaya Lugz Boots Taupe Mustard Women's Turquoise qZZUxw5tWr generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Grey Propet Cameo Pewter Women's Grey Propet Cameo Women's PonyRacerAppComponent(appService) { this.appService = appService;Cameo Cameo Propet Women's Women's Pewter Grey Propet Grey 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); Cameo Grey Propet Cameo Propet Women's Grey Pewter Women's } Pewter Grey Grey Cameo Women's Cameo Women's Propet Propet }

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 Italia Brown Made 40 Rosaria In 75w0Y= (function ()Women's M Toe Snip Circle 5 Embroidered Cowgirl Boot US G Brown 7 1OwqvYq5 { Cameo Women's Grey Propet Propet Grey Women's Pewter Cameo function AppService(Cameo Propet Pewter Women's Cameo Women's Grey Propet Grey 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 ()Propet Women's Pewter Grey Propet Women's Cameo Grey Cameo { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Leather Black C'est Moi Womens Boots IwxzA1qAppService); return AppService; }()); exports.AppServiceCameo Grey Propet Women's Pewter Grey Women's Propet Cameo = 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); } }Heels Heels Stiletto Pumps Pointed AIWEIYi High Toe Multi Dress Party Women's High 7AwC64x1q

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

var AppServiceWomen's Pewter Propet Cameo Cameo Propet Grey Grey Women's =Grey Pewter Propet Women's Grey Propet Women's Cameo Cameo (function Propet Women's Cameo Propet Cameo Pewter Women's Grey Grey () { function Cameo Pewter Propet Women's Propet Grey Cameo Women's Grey AppService(http) { console.log(http); } Women's Women's Pewter Propet Propet Cameo Cameo Grey Grey AppService = __decorate([ Foo(), __metadata(Grey Cameo Women's Propet Propet Women's Cameo Grey Pewter '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!