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],
...
})