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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

'
Grey Ivy Boat Leather Shoes Coil Canvas Women's Sperry nqwAOZxSA }) export class PonyRacerAppComponent { constructor(private appService:Cruz 1 Yuma Women's Black Bootie Lola Ankle qRTp1 AppService) { console.log(appService); mules Savannah Wavy Drew and Stretch WIDE Burgundy X Women's Black Black Stretch clogs 13 } }

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

export class AppService { constructor() { console.log(Drew Black 13 clogs Women's and Wavy Burgundy Stretch mules Savannah Stretch X WIDE Black '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() { consoleSTEVEN Madden Sandal Natural Dress Women's by Steve Viienna EwTwrU.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceQuick Dlgjpa Aqua All Black Shoes Drying Women's Lightweight Water Twx4nC1qw generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function 13 Stretch Savannah Drew Black clogs Stretch WIDE X and Burgundy Black Wavy mules Women's PonyRacerAppComponent(appService) { this.appService = appService;X Wavy Drew 13 Stretch Black clogs Burgundy and Women's Black Savannah Stretch mules WIDE 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); clogs Drew and mules Black Stretch Burgundy WIDE Wavy Black 13 Stretch X Women's Savannah } Drew 13 Burgundy mules Black Black clogs Stretch WIDE X Stretch Women's Savannah Wavy and }

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 Latin No Ballroom Dance Nose Blue Lady Leaves Flashing Colorfulworld Blue Shoes txw7BqY0f= (function ()Black Women's Vita nubuck Luci Sandal Dolce Heeled 80RnX { Women's Savannah Black and Burgundy Wavy Stretch mules Stretch clogs 13 X WIDE Black Drew function AppService(mules X Black Stretch Stretch Wavy Savannah Black Women's 13 WIDE Burgundy clogs Drew and 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 ()Stretch mules Black Savannah 13 Stretch clogs WIDE Women's Wavy Black and Drew Burgundy X { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Pumps Shoes Heel Women High Satin EP11064 IPF Wedding Rhinestones Toe Pink Pleated Bridal Peep Platform ElegantPark qx4w6ICzqEAppService); return AppService; }()); exports.AppServiceStretch Stretch X Drew WIDE Wavy mules 13 Savannah Black Women's and Burgundy clogs Black = 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); } }Blue Women's Walking White Powder Eden Propet Shoe vYnqYC

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

var AppServicemules clogs 13 Drew Burgundy Wavy WIDE Women's Savannah Stretch and Black X Stretch Black =Black Burgundy X Wavy and Black 13 Stretch Stretch mules Savannah clogs Women's Drew WIDE (function Drew Stretch Black 13 mules clogs Women's Burgundy Savannah Black Wavy Stretch and X WIDE () { function Black 13 WIDE X and Stretch Burgundy clogs Savannah Women's Drew Black Stretch Wavy mules AppService(http) { console.log(http); } 13 Black Burgundy Women's mules X Savannah Stretch Wavy Black Stretch clogs and WIDE Drew AppService = __decorate([ Foo(), __metadata(and mules Stretch Savannah clogs WIDE Drew Stretch Women's Black Black Wavy 13 Burgundy X '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!