构造函数代码不可访问 [英] Constructor code not reachable

问题描述

我正在将类方法作为参数传递给这样的新类实例:

I am passing a class method as a parameter to a new class instantiation like this:

class Abc {
    constructor() {
        this.a = () => { };
    }
    b = new Def(this.a);
}

我在浏览器控制台中得到无法读取未定义的属性a".为什么在 b = new Def(this.a)中的 a undefined 是未定义的?在调试时，我发现浏览器抛出错误，并且构造函数代码从未到达.为什么会这样?

I get 'cannot read property a of undefined' in browser console. Why is a undefined inside b = new Def(this.a)? On debugging, I found that browser throws the error and the constructor code is never reached. Why is this happening?

注意:我正在使用babel，所以我可以使用类字段，因此 b = new Def()是有效的语法.

Note: I am using babel, so I can use class fields and hence b = new Def() is a valid syntax here.

推荐答案

这是类字段的工作方式，它们在构造函数主体之前(但在 super()之后)进行求值.第1行在第2行之前求值，而构造函数和 b 字段的顺序无关紧要:

That's how class fields work, they are evaluated before constructor body (but after super()). Line 1 is evaluated before line 2, and the order in which constructor and b field are ordered doesn't matter:

constructor() {
    this.a = () => { }; // 2
}
b = new Def(this.a); // 1

由于类字段已经在使用中，为了维持正确的执行顺序，它应该是:

Since class fields are already in use, in order to maintain proper execution order it should be:

a = () => { }; // 1
b = new Def(this.a); // 2

constructor() {}

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