typescript循环引用问题

Question

有两个类如下

// Parent.ts
import {Sub} from "./Sub";

export class Parent {
   public getSub() {
        return new Sub();
    }
}

// Sub.ts
import {Parent} from "./Parent";

export class Sub extends Parent {}

经过ts编译后运行js会抛出错误

Uncaught TypeError: Object prototype may only be an Object or null: undefined

似乎是因为ts编译器看到Parent类里有引用Sub类便先定义Sub类,而此时Parent类还未定义,就会导致Parent为undefined,抛出TypeError.

请问一下要怎么改才能让ts编译器先定义Parent呢