ES6：＆＃39;这个＆＃39;在继承的类中未定义

Question

我有两个班，分别是A和B.类定义如下所示。

class A {

    constructor(){
        ...
    }

    ...
}

//Implementation 1

class B extends A {
    constructor(){
        this.childProperty = "something"; // this is undefined.
    }
}

//Implementation 2

class B {
    constructor(){
        this.childProperty = "something"; // this is not undefined.
    }
}

为什么this中Implementation 1未定义，而Implementation 2中未定义{{1}}？我在这里做错了什么？

Answer 1

您需要先致电super()：

class B extends A {
   constructor() {
     super();
     this.childProperty = "cool"
   }
}

JsFiddle

Answer 2

尝试将super添加到您的班级：

class B extends A {
    constructor(){
        super()
        this.childProperty = "something"; 
    }
}