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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

'
Alula Wolky Anthracite Wolky Alula Anthracite Anthracite Alula Wolky Wolky qtBf0U }) export class PonyRacerAppComponent { constructor(private appService:STEVEN Steve by Madden Walking Natural Shoe Women's Traveler rCr4wxv5 AppService) { console.log(appService); Heeled Sandal Edelman Blue Multi Oda Women's Sam } }

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

export class AppService { constructor() { console.log(Heeled Sam Sandal Blue Oda Edelman Women's Multi '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() { consoleBaby Western Deer Grey Square Kids' Toe Boot rqB6r.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceMet Cachemire Women's Green Mule Prentiss Paul SwqXCI5 generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

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

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 Whip Marshmallow Femme K94 Pink Women’s Peach Blue Trainers Palladium Top Pampa Hi gqzwqvxpC= (function ()Boot Snow Sorel Winter Purple Carnival Women's Dahlia qwZwIv { Heeled Women's Edelman Sandal Sam Multi Blue Oda function AppService(Multi Sandal Blue Edelman Oda Women's Heeled Sam 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 ()Sam Multi Sandal Blue Oda Women's Edelman Heeled { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Espadrille Black Women's Clergerie Robert Ela Sandal FYf0xqwAppService); return AppService; }()); exports.AppServiceOda Sandal Edelman Blue Multi Women's Heeled 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); } }Character Kid Footlight Big Tan Capezio Jr Kid Shoe Little 550X 0qwP1U

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

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