Pre-Angular 6 singleton service

core.module.ts

import { HttpClientModule } from '@angular/common/http';
import { NgModule } from '@angular/core';

import { PreSixModule } from './pre-six.module.ts';

@NgModule({
  imports: [
    HttpClientModule,
    PreSixModule,
  ],
})
export class CoreModule {}

pre-six-singleton.service.ts

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';

@Injectable()
export class PreSixSingletonService {
  constructor(
    private http: HttpClient,
  ) {}
}

pre-six.module.ts

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

import { PreSixSingletonService } from './pre-six-singleton.service';

@NgModule({
  providers: [
    PreSixSingletonService,
  ],
})
export class PreSixModule {}