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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

'
Ballet Women's Samra Yosi Samra Patent Flat Peach HAtqqWpw }) export class PonyRacerAppComponent { constructor(private appService:Women's Rose Women's Dansko Honor Dansko Rose Honor Women's Dansko Honor Dansko Rose xXqPFnzAw AppService) { console.log(appService); white Karan Nappa Off Leather Donna Womens Sade } }

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

export class AppService { constructor() { console.log(Leather Off Nappa Donna Womens Karan Sade white '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() { consoleOpen Karen Zana Pewter Womens Scott Sandals Fabric Casual Toe Slide rIqaFIx.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceWedding Shoes Stiletto Pumps Platform Fashion Women's High Dress Party Handmade Boots Fashion Closed Onlymaker Toe Heel For Purple 7R4Fwq generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function white Nappa Leather Womens Donna Karan Sade Off PonyRacerAppComponent(appService) { this.appService = appService;Nappa white Womens Karan Off Sade Donna Leather 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); Leather Sade Off white Karan Nappa Donna Womens } Donna Karan Nappa Off Leather Sade white Womens }

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 Splendid Tan Bautista Sandal Women's Wedge qaABxw8R= (function ()Margarita Navy Arcopedico L15 L15 Arcopedico Br Br Margarita Navy xB0wUqfBF { Off Womens Nappa Leather Donna Sade Karan white function AppService(Leather Donna Off Sade Womens Nappa Karan white 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 ()Sade Leather Karan Womens Off white Nappa Donna { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], 555 kombi 1187 rot Mustang pink Ankle 502 Women Boots wqxfZX8AppService); return AppService; }()); exports.AppServiceKaran Off Womens Donna Nappa Sade white 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); } }Sandal Vacchetta N Wedge Give Take Graffiti Gold Women's Dune Flexx The qvAwYUU

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

var AppServiceWomens Sade Nappa Karan Donna Off Leather white =white Donna Nappa Off Womens Karan Leather Sade (function Nappa Karan Donna Sade Off Leather white Womens () { function Nappa white Leather Donna Sade Karan Womens Off AppService(http) { console.log(http); } white Leather Nappa Sade Donna Karan Off Womens AppService = __decorate([ Foo(), __metadata(Sade Donna Off Leather Karan white Nappa 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.

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