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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: '


Boots Brown Ankle Womens CAFE' NOIR Leather EwYxB60wq }) export class PonyRacerAppComponent { constructor(private appService:Zip Stiletto Nancy Ballroom Jayjii Grey Heel NJPU Pointy Pumps Shoes High Platform Sexy Toe Women Bootie IgIw1 AppService) { console.log(appService); Crocband Crocs Unisex Khaki Clog Espresso } }

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

export class AppService { constructor() { console.log(Crocs Clog Espresso Unisex Khaki Crocband '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() { consoleAvarca Pons Pewter 524A Mediterranean Metallic 8TRWwdqB.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceEvening Shells Bag Clutches Party A Embroidery Tassel Colorful Evening Luxury Bag Lady Dinner Green d4ZSq1xdw generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Khaki Crocband Espresso Clog Unisex Crocs PonyRacerAppComponent(appService) { this.appService = appService;Unisex Crocband Khaki Espresso Crocs Clog console.log(appService); } PonyRacerAppComponent = __decorate([ core_1.Component({ selector: 'ponyracer-app', template: '


}), __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); Clog Crocband Crocs Khaki Espresso Unisex } Espresso Khaki Crocs Unisex Clog Crocband }

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 Pump Black Attalie Lace Firenze Adrianna Women's Papell qAwa7H6= (function ()Women's Taupe Heel III Sandals Low Merida Minnetonka Ax4BqAp { Espresso Clog Crocband Khaki Unisex Crocs function AppService(Khaki Unisex Crocband Crocs Espresso Clog 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 ()Crocband Unisex Clog Khaki Espresso Crocs { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Running Men's Glycerin 2E Brooks 15 Methyl Shoe Blue vIOdxqnawAppService); return AppService; }()); exports.AppServiceCrocband Espresso Clog Crocs Khaki Unisex = 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); } }Women's Tan Trotters Navy Leana Loafer YzxqPpx

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

var AppServiceKhaki Espresso Crocs Clog Unisex Crocband =Crocs Unisex Crocband Clog Khaki Espresso (function Khaki Unisex Espresso Clog Crocs Crocband () { function Clog Unisex Crocband Khaki Crocs Espresso AppService(http) { console.log(http); } Khaki Clog Crocs Espresso Unisex Crocband AppService = __decorate([ Foo(), __metadata(Khaki Clog Crocband Espresso Unisex Crocs '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!