<span style="color:red">{{usererror.username}}  </span>
...other items here
<span style="color:red">{{usererror.password}}  </span>

export class LoginComponent implements OnInit{
 public usererror={
    username:"",
    password:""
  }

  //this function is called after a post request
onLoginError(error:any){

let message= JSON.parse( error._body);
  console.log(error._body); //first log
  console.log(message)  //second log


}

}

[{"field":"username","message":"Username cannot be blank."},
  {"field":"password","message":"Password cannot be blank."}]

onLoginError(error:any){

let message= JSON.parse( error._body);
   let message= JSON.parse( error._body);

    for (var i=0; i<message.length; i++){
     this.usererror.message[i].field = message[i].message;  //this fails since the
         //usererror doesnt have (message[i].field) property

     }
 }