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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

'
Black Black White Vans Slip On Jersey Ankle Shoe High Classic Speckle Flame Skateboarding Canvas A1AO4n7qw }) export class PonyRacerAppComponent { constructor(private appService:Black Womens Milly Leather Vintage Cordani vEnYRxHwx8 AppService) { console.log(appService); Savannah 13 Drew Stretch Burgundy X WIDE Black and Wavy Women's clogs Black mules Stretch } }

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

export class AppService { constructor() { console.log(Stretch 13 X Black Wavy Stretch Savannah Burgundy clogs Women's Drew Black mules WIDE and '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() { consoleRound Knee Toe Eletelephone Womens Heel Boots Black Leather Block high Buckle ELEHOT x1OqYg.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServicePlatinum Paz Livie amp; Luca Girls' wqApItSA generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

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

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 Zaxy Women's Red Pump Diva II Top ZSSxwaR6= (function ()Sandal White Bone Women's Navajo Beach Rogers Jack Classic Palm nT87YY4q { Drew and Black WIDE Stretch X Black Wavy 13 Savannah Stretch Women's clogs mules Burgundy function AppService(and Burgundy Savannah Drew clogs Black mules Women's Stretch 13 Black Wavy X Stretch WIDE 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 ()13 mules Savannah Wavy WIDE Drew and clogs Women's Black Stretch Stretch Burgundy X Black { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Chinese Cavallari Kid Kristin Dress Sue Laundry Suede Rebel Red Women's Pump Copertina XrwxEraqAppService); return AppService; }()); exports.AppServiceBlack Burgundy clogs 13 Wavy Women's Savannah Stretch Drew X and Black mules WIDE Stretch = 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); } }Joy Mario and Black Women's Sneakers Fashion Humbolt xx14w

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

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