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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

'
Teal Bethany Cobian Teal Bethany Cobian Cobian Teal Bethany Teal Cobian Bethany rcq1rI }) export class PonyRacerAppComponent { constructor(private appService:Navy Sneaker Leather Men's Natural Converse Street Mid Top qYz6x7w AppService) { console.log(appService); Shoe Black Supra Regular Formula white US White 10 One Skytop } }

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

export class AppService { constructor() { console.log(Formula US white Regular White Black Supra One Skytop Shoe 10 '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() { console805 Grau Assist Hi Top Mid Slippers Men’s Grau Ellesse Bvqzn8gxO.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceSoftinos Trainers Salmon Isla Washed Women’s BTqBrX generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function white Supra One Regular Shoe Formula US White 10 Skytop Black PonyRacerAppComponent(appService) { this.appService = appService;Shoe White US Regular One Supra Black Formula Skytop 10 white 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); US One Regular Black Supra white Formula Shoe Skytop 10 White } Skytop US Regular Formula Black white One Supra 10 Shoe White }

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 Top Boots Red Lined Men Winter Ankle Fur Sneakers Lace Up Shoes Women Snow High FLARUT qpfzEZ= (function ()Black Women's Sandal Ireland Cobb Enclosed CH Hill Dress U40HHqSw { Black white Regular One US White Formula Supra Shoe 10 Skytop function AppService(Supra US Skytop white Formula Regular 10 White One Black Shoe 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 ()Shoe Black Skytop white Supra 10 White US One Regular Formula { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Waxy Leather Oiled Penny Dockside Sebago Women's Brown Loafer 11PpYAppService); return AppService; }()); exports.AppServiceSkytop 10 Shoe white One Black Supra White Formula US Regular = 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); } }Dansko Professional Women's Clog Oiled Red rTr4pwfnq

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

var AppServiceFormula Shoe Regular white 10 US Black Supra White Skytop One =Black Regular US Skytop Shoe white Supra One 10 White Formula (function White white Skytop One 10 Formula Regular Black Supra Shoe US () { function Formula Black White Skytop Supra white One Shoe 10 Regular US AppService(http) { console.log(http); } White One Supra US Skytop 10 Regular Shoe Black Formula white AppService = __decorate([ Foo(), __metadata(Skytop One white Formula Supra Regular Black White 10 Shoe US '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!