postData(credentials,type){
  return new Promise((resolve, reject) =>{
    let headers = new HttpHeaders();
    this.http.post(apiUrl+type, JSON.stringify(credentials), {headers: headers}).
    subscribe(res =>{
      resolve(res.json());
    }, (err) =>{
      reject(err);
    });
  });
 }

public postData(credentials: Object, type: string): Promise<any> {
    return new Promise((resolve, reject) => {
      this.http.post(apiUrl + type, credentials).subscribe(
        response => {
          resolve(response);
        },
        exception => {
          reject(exception);
        }
      );
    });
  }