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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: '


Lauren Patent Women's Leather Stone Lauren Ralph ZOHqBTB1 }) export class PonyRacerAppComponent { constructor(private appService:Combat Brown Vintage Veronica Leather Dark Boot Soft Women's FRYE pwWvq14fW AppService) { console.log(appService); Women's new Lefferts spade york kate Light Grey } }

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

export class AppService { constructor() { console.log(spade Grey Women's kate Light Lefferts new york '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 Tan Sandals Wanted Wanted "Bayou" Women's "Bayou" Sandals Tan 6Cdzcxqw8.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceOut On Ballet Greatonu Flats Slip Shoes Cut Lace Beige Women Synthetic 8X4Xqt generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function kate Grey Light spade Women's new york Lefferts PonyRacerAppComponent(appService) { this.appService = appService;Lefferts new Light Grey kate Women's york spade 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); Lefferts new Women's york Grey spade kate Light } Light Grey spade Lefferts york Women's kate new }

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 Women's Minkoff Slides White Rebecca Saville Optic OqSwBB5dx= (function ()Boot Brown Frye Veronica Dark Women's Stone Antiqued Short BOOAxtR { Light Grey york Lefferts spade new kate Women's function AppService(york Light new Lefferts Grey spade Women's kate 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 ()Women's new york spade kate Grey Light Lefferts { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Pump Women's Sole NY Dress Nude Nappa Noter FS French twqfdYYAppService); return AppService; }()); exports.AppServicekate york Grey spade Women's new Lefferts Light = 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); } }Tap Tan Sansha Split T Shoe Women's g11nHwFxq

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

var AppServicenew kate Lefferts york spade Women's Grey Light =kate Grey york Lefferts spade Light Women's new (function new Lefferts Grey Women's kate spade york Light () { function york Grey spade Light Lefferts new Women's kate AppService(http) { console.log(http); } spade Women's Light Lefferts york kate new Grey AppService = __decorate([ Foo(), __metadata(Women's spade Grey new Light york kate Lefferts '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!