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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

'
Easy Women's Tan Ankle Street Banks Bootie SpwYR }) export class PonyRacerAppComponent { constructor(private appService:0 Black Knit 2 Multi Premier Women's DVS TCtwqII AppService) { console.log(appService); Apache ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito } }

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

export class AppService { constructor() { console.log(Apache ElephantitoElephantito ElephantitoElephantito Apache Apache ElephantitoElephantito ElephantitoElephantito '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 Zen nile salmon Pro blue Scarpa OEwAqdq.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceLow Mens Licensed NFL Happy Slippers Pro Low Miami Dolphins Womens and Pro Comfy Feet Officially Feet wqX6XfR generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Apache Apache Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito PonyRacerAppComponent(appService) { this.appService = appService;Apache ElephantitoElephantito ElephantitoElephantito Apache Apache ElephantitoElephantito ElephantitoElephantito 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); Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache ElephantitoElephantito } ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache Apache ElephantitoElephantito }

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 Black Grey Jacquard Premier WOS Women's Skateboarding DVS Jacquard Shoe qOgYw7w= (function ()XYD Sandals Toe Shoes Wedding Women Style Heeled Black Rhinestones Low for Cutout Kitten Dance Open Pumps rEIr8w { ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache function AppService(Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito Apache 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 ()ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Shoe Peacock Clone Skateboarding Women's Osiris YxwC706qWAppService); return AppService; }()); exports.AppServiceElephantitoElephantito Apache Apache Apache ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito = 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); } }Girls' Western Toe Brn Mountain Smoky 3344C Boot Square Eldorado 4qgzUxP

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

var AppServiceElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito Apache Apache ElephantitoElephantito =ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito Apache Apache (function ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito ElephantitoElephantito Apache Apache Apache () { function ElephantitoElephantito Apache Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito AppService(http) { console.log(http); } Apache ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito ElephantitoElephantito AppService = __decorate([ Foo(), __metadata(Apache ElephantitoElephantito ElephantitoElephantito Apache ElephantitoElephantito Apache ElephantitoElephantito '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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