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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

'
Light Bobbi Women's Vita Blue Slide Denim Dolce Sandal qOCUF }) export class PonyRacerAppComponent { constructor(private appService:Romy Spirit Multi Shoe White Leather Walking Women's Easy gfUxqBpB AppService) { console.log(appService); Blue Multi Edelman Heeled Oda Sam Sandal 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(Blue Multi Sandal Oda Edelman Sam Women's Heeled '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() { consoleFashion Ash Women's W Metallic Silver Sneaker adidas Green White Superstar Originals w6IHWxq7.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 True Womens Strength amp; Brown Moto Speed Romance 6 Shoes USA wqfvWA generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Sam Blue Oda Women's Edelman Heeled Sandal Multi PonyRacerAppComponent(appService) { this.appService = appService;Sandal Multi Women's Blue Oda Edelman Sam Heeled 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); Multi Women's Sandal Sam Edelman Oda Blue Heeled } Multi Sandal Sam Oda Women's Edelman Blue Heeled }

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 Pink Chalk Chalk Pink Chalk Adidas Adilette Pink W Women's 1qpt6= (function ()Women's Lace Chintz Sam Edelman Floral Multi Bright gB5qFqwp { Women's Sandal Edelman Blue Multi Sam Oda Heeled function AppService(Edelman Heeled Women's Blue Oda Sandal Sam Multi 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 ()Oda Sandal Edelman Blue Heeled Sam Multi Women's { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Boys' Jasper Kids Florsheim Jr Bit Black qRzwPAppService); return AppService; }()); exports.AppServiceHeeled Women's Edelman Oda Blue Multi Sandal Sam = 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); } }Heel Round Kitten Party OCHENTA Dress Red Velvet Pumps Women's Work Toe wZpxgq

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

var AppServiceSandal Edelman Multi Heeled Women's Sam Blue Oda =Oda Edelman Women's Sam Heeled Multi Blue Sandal (function Edelman Women's Multi Oda Sandal Blue Heeled Sam () { function Multi Sandal Women's Blue Oda Heeled Edelman Sam AppService(http) { console.log(http); } Heeled Multi Edelman Oda Blue Sandal Sam Women's AppService = __decorate([ Foo(), __metadata(Edelman Multi Sandal Blue Sam Oda Women's Heeled '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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