如何修复“属性没有初始值设定项并且未在构造函数中明确分配”错误？

我对这些课程有疑问。我想使用该方法doSomething()班级独有B无需每次都进行类型转换，但是当我指定属性时a属于类型B，它告诉我它没有在构造函数中分配，这有点错误，因为父构造函数进行了分配。

class A {

}

class B extends A {
  doSomething() { }
}

class One {
  constructor(protected a: A){  }
}

class Two extends One {
  protected a: B // Property 'a' has no initializer and is not definitely assigned in the constructor.

  constructor(){
    super(new B());
    // If I enter "this.a = new B();" here then the error disappears, but the code is redundant.
  }

  doStuff() {
    this.a.doSomething()
  }
}

我究竟做错了什么？

操场 https://www.typescriptlang.org/play?#code/MYGwhgzhAECC0G8BQSC%20LSRgIWgUwA8AXPAOwBMZ5lppyB7AZXoFs8iALAS1IHMAKAJSJo6dEkxRoAeVJ5ESWsHqkIRAE4BXYEXrr%20AB3X0SOvOWhgAXHEEJaYjOCkAVAO718xMpRlyFtEYmeGYW1tC4APSR0AAKxgZ46kQAntAA5GDp0ByQ0KSePFxEXGAgXABeSZYU0FwwBUR0eABmRSQgaVhcvHIWPNCc8sqqGtq66gB0KEoqalo6ekI0tNAQmon6cm4RQoIA3Iqr0dAAki1n%20KQk6tAARJz1k2DQALz5eDvYQvt3OUnyIakQYceRJYy3cj1MAGRJgdQQAA00AARpomkNoMpyPJ6tB1OZNBQwNdpg4ZnQmERNC0WkIAqtHhBnpMGMw2I8%20EIjmIgA

问题是提议添加类字段声明 https://github.com/tc39/proposal-class-fields#class-field-declarations-for-javascriptJavaScript 的语义与您可能期望的语义以及 TypeScript 设计者将它们添加到 TypeScript 时的期望不同。事实证明，在 JavaScript 中，类字段声明将通过以下方式初始化Object.defineProperty() https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty而不是通过赋值，所有此类没有初始化器的声明字段都将被初始化为undefined。所以最终你可以期望像你这样的代码生成设置的 JavaScripta to undefined在子类中，即使您的目的只是缩小基类的类型。布莱奇。

所以，在 TypeScript 3.7 中，a --useDefineForClassFields添加了标志，以及declare属性修饰符 https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#the-usedefineforclassfields-flag-and-the-declare-property-modifier。如果你使用--useDefineForClassFields，编译器将输出符合预期的代码Object.defineProperty()类字段的语义：

如果你使用该标志按原样运行您的代码 https://www.typescriptlang.org/play?useDefineForClassFields=true#code/MYGwhgzhAECC0G8BQSC+LSRgIWgUwA8AXPAOwBMZ5lppyB7AZXoFs8iALAS1IHMAKAJSJo6dEkxRoAeVJ5ESWsHqkIRAE4BXYEXrr+AB3X0SOvOWhgAXHGEJRaDOCkAVAO718xMpRlyFtEYmeGYW1tC4APSR0AAKxgZ46kQAntAA5GDp0ByQ0KSePFxEXGAgXABeSZYU0FwwBUR0eABmRSQgaVhcvHIWPNCc8sqqGtq66gB0KEoqalo6ekIBtNAQmon6cm4RQoIA3Iqr0dAAki1n+KQk6tAARJz1k2DQALz5eDvYQvt3OUnyIakQYceRJYy3cj1MAGRJgdQQAA00AARpomkNoMpyPJ6tB1OZNBQwNdprRxLQGIwiJoWi1ljRVo8IM9JlTWOxuHwhEcxChttB3PQhGymDS6UIgA，你会在运行时看到这个问题：

new Two().doStuff()
// [ERR]: "Executed JavaScript Failed:" 
// [ERR]: this.a is undefined 

解决方案是使用declare属性修饰符缩小子类属性而不发出任何相应的Object.defineProperty() code:

class Two extends One {
  declare protected a: B // okay

  constructor() {
    super(new B());
  }

  doStuff() {
    this.a.doSomething()
  }
}

new Two().doStuff(); // okay now

Playground 代码链接 https://www.typescriptlang.org/play?useDefineForClassFields=true#code/MYGwhgzhAECC0G8BQSC+LSRgIWgUwA8AXPAOwBMZ5lppyB7AZXoFs8iALAS1IHMAKAJSJo6dEkxRoAeVJ5ESWsHqkIRAE4BXYEXrr+AB3X0SOvOWhgAXHGEJRaDOCkAVAO718xMpRlyFtOR4mOryRibBJBbW0LgA9HHQ9ADWYACeKEoqalo6ekIBtNAQmgZ4+nJusUKCANyKDg0MjESaAGZtBTRFnFwQAHRg-c2s7Nx8Qg1iKJXQ7vRCw0ytHQUJ0KSe5cbqQA