How do you provide a singleton service?

There are two possible ways to provide a singleton service.

1. Set the providedIn property of the @Injectable() to "root". This is the preferred way(starting from Angular 6.0) of creating a singleton service since it makes your services tree-shakable.

   import { Injectable } from '@angular/core';
   
   @Injectable({
     providedIn: 'root',
   })
   export class MyService {}

2. Include the service in root module or in a module that is only imported by root module. It has been used to register services before Angular 6.0.

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