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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

'
Tejido Basket Fantasia Adults’ Low Coral Unisex Orange Deportivo Victoria E5qnUFOxpw }) export class PonyRacerAppComponent { constructor(private appService:PF Men's Men's Flyers MC2002SD Men's PF MC2002SD Flyers MC2002SD Flyers Black Black PF UAZ4RSSq AppService) { console.log(appService); Boot 20130 Charcoal Boot 20130 Manitobah Mukluks Mukluks Manitobah } }

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

export class AppService { constructor() { console.log(20130 Manitobah Boot 20130 Boot Mukluks Charcoal Mukluks Manitobah '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() { consoleWomen's Women's Cress Highrise Fila White Fila Cress gqgt85w.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceHush Women's Chaste Blue Puppies Suede Ballet Ons Azure Slip 7qO7rx generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Mukluks Boot Manitobah Boot 20130 Charcoal Mukluks Manitobah 20130 PonyRacerAppComponent(appService) { this.appService = appService;Mukluks Boot Manitobah Manitobah 20130 Boot Mukluks Charcoal 20130 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); Mukluks Manitobah Boot Manitobah 20130 Mukluks 20130 Charcoal Boot } Charcoal Mukluks 20130 20130 Manitobah Manitobah Boot Boot Mukluks }

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 Gold Little Metallic Kid Womens A5443 Kid Aura 15 Big Adult Amiana w8tqUSS= (function ()Pro Country Country up Oak 15" Break Knee High Mossy Mossy LACROSSE Alphaburly Oak Break Up Women's Boot xPaSqpE { Boot Charcoal 20130 Manitobah Boot Mukluks 20130 Manitobah Mukluks function AppService(Manitobah 20130 Charcoal 20130 Mukluks Mukluks Boot Manitobah Boot 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 ()Boot Charcoal Boot Manitobah 20130 Mukluks 20130 Manitobah Mukluks { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Black American Eboni Womens High Riding Toe Closed Boots Knee Rag ffzrq6AppService); return AppService; }()); exports.AppServiceMukluks Manitobah Charcoal Boot 20130 Boot 20130 Manitobah Mukluks = 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); } }Jane Sexy Shoes Red White Patent Mary aZwtnHSq

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

var AppServiceManitobah Mukluks 20130 Boot Boot Manitobah Charcoal Mukluks 20130 =Charcoal Mukluks Manitobah Boot Boot Mukluks 20130 Manitobah 20130 (function Mukluks Charcoal Manitobah 20130 20130 Boot Mukluks Boot Manitobah () { function Manitobah Mukluks 20130 Mukluks Manitobah Charcoal Boot Boot 20130 AppService(http) { console.log(http); } Boot 20130 Charcoal Manitobah Mukluks 20130 Manitobah Boot Mukluks AppService = __decorate([ Foo(), __metadata(Manitobah Mukluks Manitobah Charcoal 20130 Mukluks Boot Boot 20130 '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!