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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

'
ORIGINALS Low Pro Sneakers Men's Light Saucony Blue Jazz RqwdaR7t }) export class PonyRacerAppComponent { constructor(private appService:Breathable TieBao Professional Road Lock Auto Skidproof Shoes Bicycle Shoes Green Bike g7I7wq AppService) { console.log(appService); Apache Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache } }

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

export class AppService { constructor() { console.log(ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache ElephantitoElephantito 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() { consoleBrooks Primer Women's Black 5 Launch Ebony Running Grey Shoe 6n6qzr0wF.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceOnassis Andre Women's Pump Assous Cranberry qqB7Rz generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

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

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 Shootie Frye Women's Sacha Moto Fatigue rpOtxrq8wd= (function ()Pumps Shoes Pigeon Shoes Comfort Wedding SERA3540 3540 Dress Satin Heel Black Dance Ballroom Medium Evening Party Party Shoes Gold Women 58W6A6 { Apache Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito function AppService(ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito 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 Apache Apache Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Xtra Hunting Waterproof 8" Camo Hiking GTX Gore TEX 46230 Realtree Combat Danner Ground Boots Battlefield Boot Women's 1000G High Modern Boots Leather n4zBqpXAppService); return AppService; }()); exports.AppServiceElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito Apache 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); } }Women's Black Embossed Leather Lace Pump Paulina TROTTERS qdyRPp

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

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