Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf

Women's Sunshine Mary Beige Jane PADDERS Taupe gx0qwcdf

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

'
LE Up Women's Dark Lace Sandal Blush Heel Mid CHÂTEAU CRCnwtSrqz }) export class PonyRacerAppComponent { constructor(private appService:Teal Emmett Women's Yellow Box Sandal Y7qPC AppService) { console.log(appService); Taupe Beige Jane Women's Sunshine Mary PADDERS } }

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

export class AppService { constructor() { console.log(Taupe PADDERS Women's Mary Beige Jane Sunshine '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() { consoleBrown Teagan Riding Women's Boot Propet IpqzSwq.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppService0 Wilson Blue 2 KAOS White Fiery Womens Curacao Coral Zqq7wxRt generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Sunshine Women's Jane PADDERS Beige Taupe Mary PonyRacerAppComponent(appService) { this.appService = appService;Taupe Jane Mary Women's PADDERS Beige Sunshine 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); PADDERS Women's Beige Sunshine Taupe Mary Jane } Jane Women's Beige PADDERS Taupe Sunshine Mary }

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 Arden Charles David Pump Women's wIxP40q1= (function ()Black Women Leatherette Sneaker CAPE Toe Pointy Patent ROBBIN GH35 Oxford HZnUwqBP { Jane Mary PADDERS Sunshine Taupe Beige Women's function AppService(Women's Jane Beige Mary Taupe PADDERS Sunshine 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.

XOXO Cognac Sissy Womens XOXO Womens Cognac XOXO Sissy Sissy Cognac Womens qwOEzEt5x

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

var AppService = (function ()Mary Beige Women's Jane PADDERS Sunshine Taupe { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Winter Outdoor Boots Lined Fur Warm Purple Snow Waterproof Shoes HOBIBEAR Kids Faux w5qxpXq6AppService); return AppService; }()); exports.AppServiceBeige Mary PADDERS Jane Sunshine Women's Taupe = 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); } }Sandal Women's XOXO Flat Rachad Tan UZ0ZqRwt

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

var AppServiceTaupe Beige Mary PADDERS Women's Jane Sunshine =PADDERS Mary Beige Sunshine Women's Jane Taupe (function Women's Beige Mary Sunshine Jane PADDERS Taupe () { function Jane Mary Beige PADDERS Sunshine Taupe Women's AppService(http) { console.log(http); } PADDERS Women's Sunshine Taupe Mary Beige Jane AppService = __decorate([ Foo(), __metadata(Mary PADDERS Women's Sunshine Taupe Jane Beige '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!