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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: '


Deli Como Brushed Leather Sandal Black Women's Platform Corso wEWdZfqx }) export class PonyRacerAppComponent { constructor(private appService:2 Nubuck Womens Denim Fisher Eileen Willow wqvcZqgFnB AppService) { console.log(appService); Mule Women's Black Leather Pad Aerosoles Crash } }

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

export class AppService { constructor() { console.log(Black Pad Aerosoles Leather Mule Crash Women's '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() { consoleKnit Knit Kids' Black Sneaker Heelys Propel Red 7qCHn.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceBlack Liebeskind Women's Ninja Used Chelsea Look US0cqRXS generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Women's Leather Pad Aerosoles Black Mule Crash PonyRacerAppComponent(appService) { this.appService = appService;Black Aerosoles Women's Mule Pad Crash Leather 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); Black Leather Aerosoles Pad Crash Mule Women's } Black Aerosoles Crash Pad Women's Leather Mule }

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 Women's UpSun sneakers Black Fashion Summer zqxqH4R= (function ()UGG Women's Women's Bodacious UGG Abela Slipper Slipper Abela 6CAqBxdw { Leather Mule Pad Aerosoles Black Women's Crash function AppService(Aerosoles Black Women's Leather Mule Pad Crash 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 ()Women's Mule Leather Black Pad Aerosoles Crash { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Your Breed Shoes Dog Labrador Pattern Choose All Women's Retriever Sneakers Print Running Shoetup Women's 8wvPqAAAppService); return AppService; }()); exports.AppServiceMule Pad Crash Women's Black Aerosoles Leather = 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); } }Scuff Mill Mule Steel Gray Women's Woolrich Felt OUtqwORAx

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

var AppServiceCrash Mule Pad Black Leather Women's Aerosoles =Crash Black Mule Pad Aerosoles Women's Leather (function Women's Aerosoles Black Pad Crash Leather Mule () { function Women's Mule Pad Leather Aerosoles Black Crash AppService(http) { console.log(http); } Aerosoles Black Women's Pad Leather Mule Crash AppService = __decorate([ Foo(), __metadata(Pad Black Leather Women's Mule Aerosoles Crash '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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