angular 6 service file

import { Injectable, EventEmitter, Output } from '@angular/core';
import { HttpClient, HttpHeaders, HttpParams } from '@angular/common/http';
import { Response } from "@angular/http";
import { Observable } from 'rxjs';
import 'rxjs/add/operator/map';

@Injectable()
export class AppService{
    isLoggedin:boolean;
    @Output() getLoggedInName = new EventEmitter(); 

    constructor(private _http: HttpClient){

    }
    
    //function get categories from database
    getCategories(){
      return this._http.get("http://localhost/api/categories")
      .map(res=>{return res;});
    }

    //function get categories from database
    getposts(params){
      return this._http.get("http://localhost/api/posts")
      .map(res=>{return res;});
    }

}



//=========== how to call in controller 
/*import { AppService } from '../shared/app.service';
allPosts;
constructor(private service:AppService) { }
this.service.getposts().subscribe(res=>{ this.allPosts= res; });*/