async service

service.ts

// src/app/service.ts
import { Injectable }    from '@angular/core';
import { Headers, Http } from '@angular/http';

import 'rxjs/add/operator/toPromise';

@Injectable()
export class MyService {
  private headers = new Headers({'Content-Type': 'application/json'});
  private apiUrl = 'http://localhost:8000';  // URL to web api

  constructor(private http: Http) {}

  methodA(arg1: number): Promise<any> {
    if(Number.isInteger(arg1) && arg1 >= 0) {
      return this.http
        .get(`${this.apiUrl}/arg1/${arg1}`)
        .toPromise()
        .then(response => response.json())
        .catch(this.handleError);
    } else {
      return this.handleError('error: invalid arg1');
    }
  }
  handleError(error: any): Promise<any> {
    console.error('Error returning promise from service', error);
    return Promise.reject(new Error(error.message || error));
  }
}