What Is Angular Singleton Service?

In Angular, two ways to make a singleton service -

1.      Include the service in the AppModule

2.      Declare that the service should be provided in the application root.

The preferred way to create a singleton service - Form beginning to Angular 6 is –

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

@Injectable({

  providedIn: 'root',

})

export class CustomerService {

}

Another way to create a singleton service - Include service in the AppModule

customer.service.ts –

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

@Injectable()

export class CustomersService {

  constructor() { }

}

And app.module.ts -

import {CustomerService} from './customers.service';

//AppModule class with @NgModule decorator

@NgModule({

  //Static, this is the compiler configuration

  //declarations is used for configure the selectors.

  declarations: [

    AppComponent

  ],

  //Composability and Grouping

  //imports used for composing NgModules together.

  imports: [

    BrowserModule

  ], 

  //Runtime or injector configuration

  //providers is used for runtime injector configuration.

  providers: [CustomerService],

  //bootstrapped entry component

  bootstrap: [AppComponent]

})

export class AppModule { }

For more detail kindly refer the link....