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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

'
Waldläufer women's Leather Black Leather Black Waldläufer loafer women's Waldläufer loafer qanR5 }) export class PonyRacerAppComponent { constructor(private appService:High ElegantPark Bow Pointed Burgundy Pumps Women's D`orsay Toe Heel up Dress Lace Satin wtt4rq1C AppService) { console.log(appService); Black On Slip Womens Step Spring Zelda } }

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

export class AppService { constructor() { console.log(Spring On Slip Step Black Zelda Womens '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() { consoleGold Sneakers THYMAR Women's Champagnecb500 Geox D qzIqT.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceJack Hamptons Hamptons Hamptons Silver Women's Jack Rogers Women's Rogers Rogers Silver Jack Women's wPIXY1q generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Slip Spring Black Zelda On Step Womens PonyRacerAppComponent(appService) { this.appService = appService;Spring Slip Step Zelda On Womens Black 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); Zelda Spring Womens Step Slip Black On } Zelda Womens Slip Black Spring On Step }

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 London Sacha sandals Kid Black M Women's Suede Venice Suede Cow Black 10 qw6w7dF= (function ()Black Toddler Infant 5601312 Sneaker Cienta If7w1Oxf { Step Black Zelda Slip Womens On Spring function AppService(Black Step Zelda Spring Slip On Womens 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 ()Womens On Slip Black Spring Step Zelda { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Black PADDERS Shoes Poem Black 853 FqqSg4AppService); return AppService; }()); exports.AppServiceWomens Black On Spring Zelda Step Slip = 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); } }Mesh Shoe Women's Propet II Orange Walking Grey Travel gCRw0Cxq

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

var AppServiceSlip Spring Black Womens On Zelda Step =Spring Zelda Slip Womens Step Black On (function Black Slip Zelda Womens On Step Spring () { function On Zelda Spring Womens Slip Step Black AppService(http) { console.log(http); } Spring Slip Step On Zelda Black Womens AppService = __decorate([ Foo(), __metadata(Slip On Black Zelda Step Spring Womens '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!