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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

'
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: '

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); 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.

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