Angular Providers , providedIn , Injectable Key points

 "Using the @Injectable() providedIn property is preferable to the @NgModule() providers array because with @Injectable() providedIn, optimization tools can perform tree-shaking, which removes services that your application isn't using and results in smaller bundle sizes.Tree-shaking is especially useful for a library because the application which uses the library may not"

"If a module defines both providers and declarations (components, directives, pipes), then loading the module in multiple feature modules would duplicate the registration of the service. This could result in multiple service instances and the service would no longer behave as a singleton."

"If you have a module which has both providers and declarations, you can use this technique to separate them out and you may see this pattern in legacy applications. However, since Angular 6.0, the best practice for providing services is with the @Injectable() providedIn property."