Service instance as singleton?

In order to create a single instance of service in the Angular application we need to specify providedIn attribute  as root of @Injectable decorator.

When we use root then the injectable will be registered as a singleton in the application and needn't add it to the providers of any specific module we wish to use.

import { Injectable } from '@angular/core';
@Injectable({
  providedIn: 'root',-->specifies that Angular should provide the service in the root injector.
})
export class ProductService {
}

It's also possible to specify that a service should be provided in a particular @NgModule

import { Injectable } from '@angular/core';
import { ProductModule } from './product.module';
@Injectable({
  providedIn: ProductModule, -->specifies that Angular should provide the service in the User Module                                                       only.
})
})
export class ProductService {
}

OR

We can also declare a provider for the service within the module:

import { NgModule } from '@angular/core';
import { ProductService } from './product.service';
@NgModule({
  providers: [ProductService],
})
export class ProductModule {
}