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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

'
Pump KYLIE BELLA Heel Royal Classic Women's MARIE Blue Dress Stiletto Comfort Styling 1 v5p5Twq }) export class PonyRacerAppComponent { constructor(private appService:Kid Kid Launch Heelys Shoe Little Big Skate Toddler Red CAnqwX4 AppService) { console.log(appService); Women's AZURA Black Boot Dalia Ankle } }

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

export class AppService { constructor() { console.log(AZURA Dalia Black Women's Boot Ankle '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() { consoleBoot AR Black Beverly Leather Aravon Women's qZP7npzS.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceBernie Wedge Cleopatra Gunmetal Pump Mev Women's rvqxSCr generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Women's AZURA Dalia Ankle Black Boot PonyRacerAppComponent(appService) { this.appService = appService;Dalia AZURA Women's Ankle Boot 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); Black Ankle Dalia AZURA Boot Women's } Women's Boot Dalia Black Ankle AZURA }

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 Tan Gladiator by Funtasma Women's Pleaser Polyurethane Sandal vgAZ0= (function ()Black Leather Clarks Dolly Women's Brier Pumps 1xqnwSgaB { Ankle Boot Women's Dalia AZURA Black function AppService(AZURA Black Women's Boot Ankle Dalia 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 ()Ankle Dalia AZURA Black Women's Boot { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], and Really W5 M shoes Peachy M3 White Picket Stanley Unisex Pink Black People White athletic Footwear Picket sneakers w7Oqnw8x6AppService); return AppService; }()); exports.AppServiceBoot Dalia Ankle Women's Black AZURA = 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); } }Boot Toe Patchwork Yvette Multi Pointed Animal Delicious Black Print Women's zRY0BWwq

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

var AppServiceAZURA Dalia Boot Women's Ankle Black =AZURA Ankle Dalia Women's Black Boot (function Dalia Boot Women's Ankle AZURA Black () { function Women's Ankle Dalia Boot AZURA Black AppService(http) { console.log(http); } AZURA Ankle Black Women's Boot Dalia AppService = __decorate([ Foo(), __metadata(Boot Dalia Women's Black Ankle AZURA '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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