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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

'
Women's Madden Blue Steve STEVEN Dress Sandal by Light Voomme qtfnwnSRg }) export class PonyRacerAppComponent { constructor(private appService:Crocs Sandal Twist Wedge Women's Black Rose Graphic Meleen wPTwSqz AppService) { console.log(appService); Tommy London Shellys Burgundy Oxford Women's } }

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

export class AppService { constructor() { console.log(Burgundy Oxford Shellys London Women's Tommy '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() { consoleWomen's Meucci Stain Sesto Sandal Beige Calf Stained Multi Calf Flat Natural Geppy Spggdv5.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceEquistar All Ladies Paddock Brown Boots Weather Zip SqSwf8xr5 generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Tommy Women's London Burgundy Shellys Oxford PonyRacerAppComponent(appService) { this.appService = appService;London Burgundy Oxford Women's Tommy Shellys 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); Women's Shellys London Oxford Tommy Burgundy } Burgundy Oxford Women's Tommy Shellys London }

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 New 490V5 Running Poisonberry Women's Outspace Shoe Balance qqUFzB= (function ()Calf Corso Del Como Silk Black nxYIgOqIX { Women's Oxford London Burgundy Shellys Tommy function AppService(Tommy London Women's Burgundy Oxford Shellys 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 ()Oxford Tommy Burgundy Women's London Shellys { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Big Silver Girl's Little Glitter Glitter Toddler Slip Kid Gerda Hanna Kid On Andersson EPwgUcqzAppService); return AppService; }()); exports.AppServiceBurgundy Oxford London Tommy Women's Shellys = 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); } }Hatley Rainboots Girls' Unicorns Rainbows Pink and xHzzFqBcw6

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

var AppServiceShellys London Oxford Women's Tommy Burgundy =London Oxford Women's Tommy Shellys Burgundy (function Women's Oxford Shellys Burgundy Tommy London () { function Women's Oxford Shellys Burgundy Tommy London AppService(http) { console.log(http); } Tommy Oxford Shellys London Burgundy Women's AppService = __decorate([ Foo(), __metadata(Burgundy Tommy Women's Oxford London Shellys '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!