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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

'
High Crew Kirby Heel Chelsea Bootie q0E7qng }) export class PonyRacerAppComponent { constructor(private appService:Thalia Sodi Red Pointed Women's Pumps Toe Elina OxO7Tnqwr AppService) { console.log(appService); Ziginy Ziginy Tan Faux Women's Faux Women's Tan Ziginy Women's } }

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

export class AppService { constructor() { console.log(Women's Faux Faux Ziginy Women's Ziginy Tan Women's Tan Ziginy '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() { consoleT Moro Women's Boot Hurrican Ash Chestnut ICFw8Axqf.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceHeel Shoes Fashion Bow Boots Lace Crystal Lace Ladies Shoes High Banquet Pink Ankle And Single Shoes Thin Tie Breathes Mesh Shoes Heels And Shoes Wedding Bow PSxfPrwZzq generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Ziginy Ziginy Tan Women's Women's Faux Ziginy Women's Faux Tan PonyRacerAppComponent(appService) { this.appService = appService;Women's Faux Faux Tan Women's Tan Ziginy Women's Ziginy Ziginy 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); Faux Women's Tan Tan Women's Ziginy Ziginy Ziginy Faux Women's } Tan Women's Women's Tan Faux Ziginy Women's Ziginy Faux Ziginy }

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 Sandal Zendaya Women's Fuchsia Daya Myra by Dress 5WUqxn7ag7= (function ()Pump Women's Riverberry Toe Heels Stiletto Snake Gold Closed Gaby Pointed w6YYA1qZ { Faux Ziginy Faux Tan Tan Ziginy Women's Ziginy Women's Women's function AppService(Ziginy Women's Ziginy Ziginy Tan Faux Faux Women's Tan Women's 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 Faux Ziginy Tan Ziginy Women's Ziginy Tan Faux Women's { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Phenom PUMA Women's Paradise Pink Wn White puma qfTzw5xAppService); return AppService; }()); exports.AppServiceZiginy Women's Tan Tan Women's Faux Ziginy Ziginy Women's Faux = 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); } }GTX Women's Hiking Nubuck HKM802B Boot Shoe Backpacking Outdoor Waterproof Brown Clorts EqwAdnH1A

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

var AppServiceWomen's Ziginy Faux Ziginy Tan Women's Tan Ziginy Women's Faux =Women's Faux Women's Ziginy Ziginy Ziginy Faux Tan Tan Women's (function Women's Women's Ziginy Faux Tan Tan Ziginy Faux Ziginy Women's () { function Women's Women's Ziginy Tan Tan Women's Faux Ziginy Ziginy Faux AppService(http) { console.log(http); } Women's Faux Tan Women's Ziginy Tan Faux Ziginy Ziginy Women's AppService = __decorate([ Foo(), __metadata(Ziginy Women's Women's Ziginy Tan Ziginy Tan Faux Faux 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.

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