Common http provider for Angular 4

CommonServiceProvider.ts

import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
import 'rxjs/add/operator/map';
import { CommonUtilsProvider } from '../../providers/common-utils/common-utils';
import { CommonVariablesProvider } from '../../providers/common-variables/common-variables'
/*
 *  Common Services Provider
 *  @author Alfred Francis
 *  @date 10/06/2017
*/
@Injectable()
export class CommonServiceProvider {

  constructor(public http: Http,
    private commonUtilsProvider: CommonUtilsProvider,
    private commonVariablesProvider: CommonVariablesProvider) {
  }

  invoke(payload) {
  /**
   * For calling mypay db router
   * @param {json}  payload - json payload to be sent
   * @return {json}
   */
    return new Promise(
      (resolve, reject) => {
        //retrive loginKey
        let userInfo = this.commonUtilsProvider.getUserInfo()
        if (userInfo) {
          //loginKey is set
          payload["aLoginKey"] = userInfo.loginkey;
          this.http.post("http://172.30.13.21:8078/mypayroute/route",
            payload)
            .map(result => result.json())
            .subscribe(data => {
              resolve(data)
            },
            errData => {
              reject(errData)
            }
            )
        }
        else {
          //loginKey not set
          reject("Invalid loginKey")
        }

      }
    )
  }
  invokeHTTP(method, context, payload = null) {
   /**
   * For using common HTTP functions
   * @param {string} method - GET,POST,PUT,DELETE
   * @param {string} context - context path
   * @param {json}  payload - json payload to be sent
   * @return {json}
   */
    let url = this.commonVariablesProvider.apiRouteUrl + context
    return new Promise(
      (resolve, reject) => {
        switch (method) {
          case "GET": {
            this.http.get(url)
              .map(result => result.json())
              .subscribe(result => {
                resolve(result)
              },
              err => {
                reject(err)
              }
              )
            break;
          }
          case "POST": {
            this.http.post(url, payload)
              .map(result => result.json())
              .subscribe(result => {
                resolve(result)
              },
              err => {
                reject(err)
              }
              )
            break;
          }
          case "PUT": {
            this.http.put(url, payload)
              .map(result => result.json())
              .subscribe(result => {
                resolve(result)
              },
              err => {
                reject(err)
              }
              )
            break;
          }
          default: {
            reject(new Error("Invalid HTTP method."))
            break;
          }
        }
      }
    )
  }
}