javascript - TypeError: Object prototype may only be an Object or null: undefined

Question

Below if I import Entity I get the posts's subject error (TypeError: Object prototype may only be an Object or null: undefined), but if I replace the import with the actual Entity declaration the code runs fine.

Stackblitz demo here.

This is Customer.ts in the form that produces the error when I run the code with ts-node:

index.ts

export { Customer } from "./Customer";
export { Entity } from "./Entity";

Customer.ts

import { Entity } from "./index";

export class Customer extends Entity {
  sku: string;
  constructor(po: any) {
    super();
    this.sku = po.sku;
  }
}

Entity.ts

export abstract class Entity {
  id?: string;
}    

Run.ts (The test code)

import {Customer} from "./";

let c = new Customer({
  name: "Bob"
});
console.log(c);

If I replace the Entity import with the declaration like this:

export abstract class Entity {
  id?: string;
}    

export class Customer extends Entity {
  sku: string;
  constructor(po: any) {
    super();
    this.sku = po.sku;
  }
}

Then Run.ts logs this:

Customer { sku: undefined }

In other words it runs fine and produces no errors. Thoughts?

Answer 1

正如我所怀疑的，您的原始程序具有循环导入。 Run.ts进口index.ts，进口Customer.ts，index.ts再次进口。由于index.tsis 已经在加载过程中并且它本身依赖于Customer.ts，因此只是将of （尚未设置）import { Entity } from "./index";绑定到of ，即使没有完成加载，也会继续执行。然后在您尝试扩展它时未定义。您可能会争辩说循环导入应该是一个错误，或者 JavaScript 引擎应该使用其他一些可以正确处理您的场景的算法；我没有资格评论为什么选择当前的设计。（其他人可以随意添加有关此的信息。）Entityindex.tsEntityCustomer.tsindex.tsEntity

如您所见，更改Customer.ts为./Entity直接导入而不是./index中断循环，并且一切都按预期工作。另一种解决方案是颠倒index.ts.

Answer 2

您可以尝试此命令并检查应用程序：

1. ng update @angular/cli @angular/core --force
2. npm install
3. ng serve -o