public signin(){
  return this.http.post<any>(`${env.API_URL}/user/signin`, { username: user.username, password: user.password }).toPromise();
}

public signin(){
  return this.http.post<any>(`${env.API_URL}/user/signin`, { username: user.username, password: user.password }).pipe(
    tap(
      data => this.customFunction(data),
      error => error
    )
  ).toPromise();
}