Why do we have to use @Injectable() in Angular?

UPDATE (2019-08-30): This blog post applies to Angular 2 to 8. Angular 9 introduced a new engine, called Ivy, which behaves differently than what is explained here.

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: '<h1>PonyRacer</h1>'
})
export class PonyRacerAppComponent {
  constructor(private appService: AppService) {
    console.log(appService);
  }
}

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

export class AppService {
  constructor() {
    console.log('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() {
      console.log('new app service');
    }
    return AppService;
}());
exports.AppService = AppService;

I skipped a bit of generated code to focus on the interesting part. The class AppService generates a pretty simple JavaScript. Let’s compare that to the PonyRacerAppComponent class:

var PonyRacerAppComponent = (function () {
    function PonyRacerAppComponent(appService) {
        this.appService = appService;
        console.log(appService);
    }
    PonyRacerAppComponent = __decorate([
        core_1.Component({
            selector: 'ponyracer-app',
            template: '<h1>PonyRacer</h1>'
        }),
        __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);
  }
}

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 = (function () {
    function AppService(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.

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

var AppService = (function () {
    function AppService(http) {
        console.log(http);
    }
    AppService = __decorate([
        core_1.Injectable(),
        __metadata('design:paramtypes', [http_service_1.HttpService])
    ], AppService);
    return AppService;
}());
exports.AppService = 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);
  }
}

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

var AppService = (function () {
    function AppService(http) {
        console.log(http);
    }
    AppService = __decorate([
        Foo(),
        __metadata('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 best practice is to add it on every service, even if it doesn’t have any dependencies on its own.

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