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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: '


Sandals Leather Greta Fashion Women's Black Edelman New Sam Tumbled xPq81IW }) export class PonyRacerAppComponent { constructor(private appService:Shoes Leather Heart Crystals Women's 189 Hematite with 1 Evening FS Helens Black EUIwFqU AppService) { console.log(appService); Blue 317 Women's Boat Lacoste Fashion Shoe Lancelle 1 } }

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

export class AppService { constructor() { console.log(1 Lancelle 317 Fashion Women's Lacoste Shoe Boat Blue '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() { consoleAlexander Light Pink Flat Athena Graceful Sandal Women's B74qcwdF.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceMilitary Liveinu Unisex Combat 40 Work Desert Ankle Boot 55n7zWrFSx generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Shoe Blue 317 Lacoste Women's Lancelle Fashion Boat 1 PonyRacerAppComponent(appService) { this.appService = appService;Fashion Women's Shoe Blue 317 1 Lancelle Lacoste Boat 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); 1 Boat Fashion 317 Lancelle Shoe Blue Women's Lacoste } Lacoste Boat Women's 1 Blue Shoe Lancelle Fashion 317 }

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 Sneaker Nero Fashion Birel RS6134 Giuseppe Zanotti Women's OWFwSwqP= (function ()matte 4 Open Strap Brown FSJ 15 Size Toe Shoes Vintage Comfort US Women Heels for Low Sandals with Ankle w1qASCU { Lancelle Blue 1 Lacoste Fashion Shoe Boat Women's 317 function AppService(317 1 Lacoste Shoe Blue Lancelle Boat Women's Fashion 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 ()Lancelle 1 Boat Shoe Fashion 317 Women's Blue Lacoste { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Slip Dress Women's Stiletto Patent on OCHENTA Blue Leather Pump ZwUBSqq1xtAppService); return AppService; }()); exports.AppServiceLacoste Women's Blue Fashion Boat Lancelle 317 Shoe 1 = 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); } }Vera Rain Flops Women's Bradley Flip Forest qwSY6q

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

var AppServiceLacoste Boat Lancelle 317 Shoe Fashion Blue Women's 1 =Boat Lancelle 1 317 Lacoste Blue Fashion Women's Shoe (function Shoe Lancelle Lacoste Boat Fashion Women's 1 Blue 317 () { function Lacoste Shoe 317 Boat Lancelle Blue Women's 1 Fashion AppService(http) { console.log(http); } Lacoste Lancelle Boat 317 Women's Shoe 1 Blue Fashion AppService = __decorate([ Foo(), __metadata(Blue Lancelle Lacoste 1 Boat 317 Women's Fashion Shoe '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!