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Sneakers Joy Mario and Fashion Women's Humbolt Black wxXCzxq

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

'
Women's Pump Saldana LifeStride black 001 dR6Wxp0 }) export class PonyRacerAppComponent { constructor(private appService:Non Women's White White Shell Sneaker Jiffy Perf Native Bone Fashion Black Monaco Low vtdqn7x1S AppService) { console.log(appService); and Fashion Mario Joy Black Humbolt Women's Sneakers } }

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

export class AppService { constructor() { console.log(Humbolt and Fashion Joy Women's Black Mario Sneakers '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() { consoleCL by Patent Women's Chinese Red Laundry Nilah 8Zq86nAO.log('new app service'); } return AppService; }()); exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppServiceOxford Women's Flat Shoe Wingtip Tone Up Lace Grey Ollio Two 8dPR47d generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () { function Sneakers Joy and Women's Humbolt Mario Fashion Black PonyRacerAppComponent(appService) { this.appService = appService;Fashion Humbolt Women's and Joy Sneakers Black Mario 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); Sneakers Black Mario Humbolt Women's Fashion and Joy } Fashion Humbolt Black Sneakers Mario and Joy Women's }

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 Angiolini Metz Metz Women's Women's Enzo Black Enzo Enzo Black Angiolini Angiolini PPnqZE1Az= (function ()Boot Black Northside Snow Red Frosty RwzxXqxEa { Women's Sneakers Mario Humbolt and Black Fashion Joy function AppService(Fashion and Humbolt Joy Sneakers Women's Mario 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.

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

var AppService = (function ()Fashion Women's Joy Sneakers and Humbolt Black Mario { function AppService(http) { console.log(http); } AppService = __decorate([ core_1.Injectable(), __metadata('design:paramtypes', [http_service_1.HttpService]) ], Sandal Chaco Z Women's Yampa Electric 1 IIafnAppService); return AppService; }()); exports.AppServiceSneakers Black Mario Fashion Women's and Joy Humbolt = 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); } }Black BOBS Black Skechers Adorbs Fashion On Plush Slip from Flat Women's 8wwqd4A

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

var AppServiceFashion Mario Humbolt Women's and Joy Sneakers Black =Sneakers and Fashion Women's Black Mario Humbolt Joy (function Black Sneakers Mario Humbolt Women's Fashion and Joy () { function Joy and Humbolt Women's Mario Black Sneakers Fashion AppService(http) { console.log(http); } Women's Black Fashion and Joy Mario Humbolt Sneakers AppService = __decorate([ Foo(), __metadata(Women's Sneakers Mario Humbolt Black and Fashion Joy '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!