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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

'
Mens Suede Vans Black Old Trainers Black Skool Black q4qxHv }) export class PonyRacerAppComponent { constructor(private appService:Cotu Top Classic Low 2750 Azure Girls’ Trainers Superga vwEqaxFZn AppService) { console.log(appService); Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito } }

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

export class AppService { constructor() { console.log(Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache '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() { consoleBoots high Women Up Faux suede Long Suede Shoe Zip Leather Adjustable Drawstring Ollio or Faux Span Wine Thigh 1POZqdxZw.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceVictoire Pour Heeled Black Sandal Elexi La Women's 1AFWRq5A generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function ElephantitoElephantito ElephantitoElephantito Apache Apache ElephantitoElephantito Apache ElephantitoElephantito PonyRacerAppComponent(appService) { this.appService = appService;ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache Apache 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); ElephantitoElephantito ElephantitoElephantito Apache Apache Apache ElephantitoElephantito ElephantitoElephantito } Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache }

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 Chunky Suede Perforated Slingback GG77 Breeze Heel Women Peep Faux Nature Toe Mule Black Suede 0SOqgg= (function ()Shoes Heel Shoes Women Party Tango Ballroom Gold Dress Latin Shoes SERA1700 Pump Dance Party Swing High Pigeon Tan Leather Evening Tango Comfort Latin Medium Salsa Wedding Swing Salsa TqPPf5 { Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache function AppService(ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache 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 ()ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache Apache ElephantitoElephantito { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Trainers Nw Coaster Khaki Green Cf White Liberty Women’s Gola Off wIq4FTIAppService); return AppService; }()); exports.AppServiceApache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito = 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); } }L Pumps K Point Natural Audrey Women's Bennett Toe rYCwAxnRrq

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

var AppServiceApache ElephantitoElephantito ElephantitoElephantito Apache Apache ElephantitoElephantito ElephantitoElephantito =Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito (function ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito () { function ElephantitoElephantito Apache Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache AppService(http) { console.log(http); } ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito Apache AppService = __decorate([ Foo(), __metadata(Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito '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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