angular 6 dependency injection

As always when multiple solutions are available it depends on what you want to achieve. But the documentation gives you some directive to choose.

Sometimes it's not desirable to have a service always be provided in the application root injector. Perhaps users should explicitly opt-in to using the service, or the service should be provided in a lazily-loaded context. In this case, the provider should be associated with a specific @NgModule class, and will be used by whichever injector includes that module.

So basically you will use providedIn: 'root' for any services that are application wide. For other services keep using the old version.

Don't forget that on you already had the choice to provide service differently. For instance it's also possible to declare Injectable at component level (this doesn't change in V6).

  @Component({
    selector: 'app-my-component',
    templateUrl: './my.component.html',
    providers: [ MyService ]
  })

This way the service becomes available only in MyComponent and its sub-component tree.

Basically you can use either, But as per new CLI provideIn will be automatically added while creating service

#providedIn

There is now a new, recommended, way to register a provider, directly inside the @Injectable() decorator, using the new providedIn attribute. It accepts 'root' as a value or any module of your application. When you use 'root', your injectable will be registered as a singleton in the application, and you don’t need to add it to the providers of the root module. Similarly, if you use providedIn: UsersModule, the injectable is registered as a provider of the UsersModule without adding it to the providers of the module.

This new way has been introduced to have a better tree-shaking in the application. Currently a service added to the providers of a module will end up in the final bundle, even if it is not used in the application, which is a bit sad.

For more information please refer here

• https://blog.ninja-squad.com/2018/05/04/what-is-new-angular-6/
• https://angular.io/guide/dependency-injection#injectable-ngmodule-or-component
• https://angular.io/guide/hierarchical-dependency-injection#moduleinjector [As suggested by Tuan-Tu in comment below]