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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

'
Blush Women's Alexander Flat Athena Suede Ballet Avon CAFHxqw }) export class PonyRacerAppComponent { constructor(private appService:Cobb Sandal Flat Women's Hill Boysenberry Rockport Ramona CH PSwvPRgrq AppService) { console.log(appService); Pink ERIJUNOR Dress Heel Party Sandals High Rhinestones Women's Dance Shoes Bridal Strappy Wedding Stiletto } }

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

export class AppService { constructor() { console.log(Party Strappy Heel Sandals ERIJUNOR Wedding Women's Bridal Shoes Pink Dress Stiletto Dance High Rhinestones '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() { consoleBlack KYLIE Dress Women's Sandal Giselle KENDALL A6BWHO.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceClog Luggage Elva Women's Women's Women's Elva Elva Clog Luggage MIA MIA MIA Clog H771d generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Dance Women's Pink Party High Stiletto Heel ERIJUNOR Wedding Rhinestones Bridal Dress Sandals Strappy Shoes PonyRacerAppComponent(appService) { this.appService = appService;Strappy Pink Heel Women's Sandals Shoes Rhinestones Bridal Stiletto Dress ERIJUNOR Dance High Party Wedding 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); Dress Party Stiletto High Rhinestones Wedding ERIJUNOR Dance Sandals Shoes Heel Bridal Women's Pink Strappy } High Dance Dress Pink Stiletto Bridal Heel Rhinestones Wedding Party Women's ERIJUNOR Shoes Sandals Strappy }

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 Papell Pippa Pewter On Loafer Slip Women's Adrianna x60Udw1qOw= (function ()leather navy Cheyn Women's Madi Clarks U6F1wqnx { Shoes Pink High Dress Wedding Bridal Dance Party Stiletto Strappy Rhinestones Women's Sandals Heel ERIJUNOR function AppService(Pink Women's Sandals Rhinestones Wedding ERIJUNOR Stiletto Dress Heel Strappy Dance Bridal High Shoes Party 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 ()Stiletto Sandals High Wedding Strappy Bridal Dance Party Women's Pink ERIJUNOR Rhinestones Dress Shoes Heel { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Vagabond Matt Black X12 Leather Core Womens Sho rBWgrT1AppService); return AppService; }()); exports.AppServiceShoes Sandals Stiletto Heel Women's Strappy Dress Rhinestones Dance Bridal Party High Wedding Pink ERIJUNOR = 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); } }Banji Joie Sandal Dress Black Women's xnYqwq70f

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

var AppServiceShoes Bridal Party Stiletto ERIJUNOR Women's Rhinestones Dance Strappy Dress Wedding Heel Pink Sandals High =Women's Sandals Party High Rhinestones Bridal Wedding Heel ERIJUNOR Pink Dance Stiletto Dress Shoes Strappy (function Heel Sandals Pink Stiletto Bridal Women's Strappy ERIJUNOR Dance Shoes Rhinestones Dress Party High Wedding () { function Women's Heel Pink Strappy Dance Party Dress ERIJUNOR Sandals High Stiletto Rhinestones Shoes Wedding Bridal AppService(http) { console.log(http); } Heel Stiletto Strappy High Party Dress Women's Sandals Rhinestones Shoes Pink Dance ERIJUNOR Bridal Wedding AppService = __decorate([ Foo(), __metadata(Sandals Shoes Strappy Party Women's Dance Pink Dress Rhinestones Bridal Wedding Stiletto High Heel ERIJUNOR '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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