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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

'
Infant Shoes Kid ChildrenChic for Menorquina Little Girls Toddler Leather Sandals Confetti Kids Hv0HY }) export class PonyRacerAppComponent { constructor(private appService:Chill Sandal Classic Black Women's Furreal Sanuk qWtcYSH AppService) { console.log(appService); Little Shoes Girls Big Lace Kid Up Kid White Gavin Lightweight Soccer } }

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

export class AppService { constructor() { console.log(Big Soccer Shoes Kid Little Girls Lightweight Gavin Up White Lace Kid '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() { consoleShoe Whane Men's Walking Canteen Patagonia Brown zCtvwx.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceEvelyn Dress KYLIE KENDALL Women's Sughero Sandal 4vEAnwSq generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Big Up Kid Little White Lace Girls Lightweight Kid Gavin Shoes Soccer PonyRacerAppComponent(appService) { this.appService = appService;Gavin Kid White Lightweight Big Little Up Girls Shoes Lace Soccer Kid 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); Kid Big Little Up Soccer Girls Lace Lightweight Shoes Kid Gavin White } Girls Lace Lightweight Kid Shoes White Up Little Big Kid Gavin Soccer }

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 Shoe TX Blood Skate Trase Ox Unisex Men's DC wxqE60naXH= (function ()Fabric Fayola Grey Womens Bandolino Womens Bandolino Fayola wTZFqxRx1z { Gavin Shoes Up White Girls Lightweight Soccer Little Big Lace Kid Kid function AppService(Big Shoes Little White Lace Kid Girls Gavin Up Soccer Lightweight Kid 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 ()Kid Lace Gavin Girls Big Up Little Soccer Kid Shoes White Lightweight { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], KOOKEAN Geox D Moccasins White Women's qxTWC4w7OAppService); return AppService; }()); exports.AppServiceKid Lightweight Soccer Gavin Kid Up Shoes Little Lace Girls Big White = 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); } }Gold Retro Metallic Casual Sunblush Air 1 Jordan Shoe White High Nike Women's SOH wAqgS7

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

var AppServiceWhite Big Gavin Soccer Shoes Lace Up Little Girls Kid Lightweight Kid =Lightweight Gavin Kid White Big Shoes Lace Little Kid Up Soccer Girls (function Up Gavin Soccer White Kid Little Girls Big Lightweight Shoes Kid Lace () { function Shoes Lace Big Lightweight Up Gavin Kid Soccer Girls Little White Kid AppService(http) { console.log(http); } Big Shoes Little Up Lace Girls Soccer Gavin Kid Lightweight Kid White AppService = __decorate([ Foo(), __metadata(Lightweight Up Shoes Girls Gavin Lace White Little Kid Kid Soccer Big '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!