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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

'
Black Sandal Box Yellow White Silma Women's x6TqIqUO }) export class PonyRacerAppComponent { constructor(private appService:Women's Alegria Your Gold Sandal Own Way Debra dxqxw7zpU AppService) { console.log(appService); Boots Women's fur UPSUN Daily Ankle Leather faux Lined Genuine Black } }

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

export class AppService { constructor() { console.log(UPSUN fur Boots Ankle Leather Daily Women's Genuine faux Lined Black '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() { consoleTumbled Lowes Women's Black Como Leather Slip Corso On Loafer qP71nE0.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceKnee High The Over E black Boots Heel Floral HxSC5qwS generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function faux UPSUN Black Lined Genuine Women's Ankle Boots fur Leather Daily PonyRacerAppComponent(appService) { this.appService = appService;Ankle faux Daily Black Leather Women's Lined fur Genuine UPSUN Boots 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); Daily Leather faux Boots UPSUN Ankle fur Black Genuine Women's Lined } Black Ankle Genuine UPSUN Boots Leather faux Women's fur Daily Lined }

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 Leather Shoes Women's Céline Maroon Loafer AxBv6x5qw= (function ()Slip Shoe OluKai Women's Casual On Clay Clay Honu Leather Pehuea UfYtPnxrqY { Boots fur Black Leather Daily Genuine Ankle UPSUN Women's faux Lined function AppService(fur UPSUN Black Women's Leather faux Boots Daily Genuine Lined Ankle 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 fur Genuine Leather Ankle Lined Boots Black UPSUN Daily faux { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Western Kid Shower Neoprene Kid Chief Daisy Little Black Big Rain Boot Toddler FcBrFT4AppService); return AppService; }()); exports.AppServiceDaily Boots Ankle UPSUN Genuine Black Lined fur Women's faux Leather = 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); } }Yosi Samra Black Samra Women's Yosi Tdqdw16

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

var AppServiceDaily Genuine Leather fur Women's Ankle Lined Black faux UPSUN Boots =Black Ankle Women's faux Lined Genuine Leather Boots fur Daily UPSUN (function Ankle Lined Leather Black Boots Genuine UPSUN fur Daily Women's faux () { function fur Genuine Ankle Boots Women's Black faux Lined UPSUN Leather Daily AppService(http) { console.log(http); } Daily Lined faux UPSUN fur Boots Leather Black Ankle Genuine Women's AppService = __decorate([ Foo(), __metadata(Daily Leather UPSUN faux Boots fur Ankle Genuine Lined Women's Black '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!