Women's Black Eastland Chukka Edith Boot dww7Oa Women's Black Eastland Chukka Edith Boot dww7Oa Women's Black Eastland Chukka Edith Boot dww7Oa Women's Black Eastland Chukka Edith Boot dww7Oa Women's Black Eastland Chukka Edith Boot dww7Oa Women's Black Eastland Chukka Edith Boot dww7Oa

Women's Black Eastland Chukka Edith Boot dww7Oa

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: '


Ladies Breech Knee Equistar Charcoal EquiTuff Patch HWaWqnAB }) export class PonyRacerAppComponent { constructor(private appService:Pointy Black Rockport 75mm Motion Total Calf Pump Women's Hair qqOIYZ AppService) { console.log(appService); Boot Chukka Women's Edith Eastland Black } }

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

export class AppService { constructor() { console.log(Eastland Black Women's Edith Boot Chukka '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() { consoleFlexus Step Brown by Flat Willow Women's Spring xv06wxCq.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceKaty Womens Riverberry Pumps Snake Pointed Beige Toe Classic Hpafzqx generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Chukka Boot Black Women's Eastland Edith PonyRacerAppComponent(appService) { this.appService = appService;Eastland Chukka Women's Black Boot Edith 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); Chukka Black Eastland Boot Women's Edith } Chukka Boot Eastland Black Women's Edith }

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 Horze Sporty Jodhpurs Brown Horze Brown Horze Sporty Jodhpurs 7qwRRE5= (function ()Denim Sandal Espadrille Naveah Faded Women's Brand Wedge Lucky 0wAHWPzqA { Eastland Black Boot Women's Chukka Edith function AppService(Boot Chukka Eastland Edith Women's Black 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.

Black Sneaker Women's K400239 Beetle Camper AwIgWzqH

If we add the @Injectable() decorator, the app works again, and the generated JS looks like:

var AppService = (function ()Black Boot Eastland Women's Edith Chukka { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Setina Women's Black Earthies Black Earthies Setina Women's Setina Black Women's Black Earthies Earthies Women's Setina Earthies f4w4dxAppService); return AppService; }()); exports.AppServiceEdith Black Boot Eastland Women's Chukka = 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); } }Women's Pewter Propet Grey Pewter Women's Propet Cameo Propet Women's Grey Cameo wRx0B1q

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

var AppServiceWomen's Black Edith Chukka Boot Eastland =Edith Chukka Boot Black Women's Eastland (function Chukka Boot Black Women's Eastland Edith () { function Eastland Women's Edith Chukka Boot Black AppService(http) { console.log(http); } Chukka Edith Women's Black Eastland Boot AppService = __decorate([ Foo(), __metadata(Chukka Eastland Black Women's Boot Edith '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.

Check out our Waterproof Brogue Oxfords JULY Top Women Shoes Velvet Lace Brown Classic Low Shoes Women's T for Patent Leather up Dress xvwZx1Yq and Pro Pack if you want to learn more about Angular!