JavaScript6 继承

我需要创建一个 Orc 对象

class Orc extends new Entity(){
constructor(){
    super();

    this.lifetime = 100;
    this.move_x = 0;
    this.move_y = -1;
    this.speed = 1;
}}

继承自Entity类

class Entity{
constructor(){
   this.pos_x = 0;
   this.pos_y = 0;
   this.size_x = 0;
   this.size_y = 0;
}}

当我尝试这样做时

let obj = null;
                obj = new Orc();

我抛出一个异常：Class extends value # < Entity > is not a constructor or null

有人请帮我解决这个问题