Starting from Angular 6, if you want to provide a singleton service it’s recommended to use providedIn:

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class UserService {
}

…instead of the providers array we’ve always used in the past:

@NgModule({
  ...
  providers: [UserService],
  ...
})

(examples are copied from the documentation)

OK, so what’s the big deal? I just realized this is not just a fancy syntax, but it makes your services tree-shakeable, as noted in the docs. Since they’re no longer referenced directly by AppModule, the services can be moved to the lazily-loaded bundles which need them. As a result, switching to providedIn would most probably shrink your main bundle considerably, making the initial load slightly faster!