在创建类的对象时，会抛出错误

问题描述

class A{
    constructor(){
        this.name="A";
    }
    M1(){
        return "M1";
    }
}

class B extends A{
    constructor(){
        this.id="B";
    }
    M2(){
        return "M2";
    }
}

var b = new B();

输出

推荐答案

你必须调用 super的构造函数。当你调用基类构造函数时，它会创建这个和然后你可以使用这个。

You must call super's constructor.When you call the base class constructor it creates the this and then you can use this.

class A{
    constructor(){
        this.name="A";
    }
    M1(){
        return "M1";
    }
}

class B extends A{
    constructor(){
        super();
        this.id="B";
    }
    M2(){
        return "M2";
    }
}

更新：

你需要从派生类构造函数调用超构造函数的原因是由于ES6分配实例的地方 - 它们是由/在基类中分配的（这是必要的）这样构造函数可以被子类化，具有异常实例，例如Array）：

The reason why you need to call the super-constructor from a derived class constructor is due to where ES6 allocates instances – they are allocated by/in the base class (this is necessary so that constructors can be subclassed that have exotic instances, e.g. Array):

// Base class
class A {
    // Allocate instance here (done by JS engine)
    constructor() {}
}
// Derived class
class B extends A {
    constructor() {
        // no `this` available, yet
        super(); // receive instance from A
        // can use `this` now
    }
}
// Derived class
class C extends B {
    constructor() {
        // no `this` available, yet
        super(); // receive instance from B
        // can use `this` now
    }
}

感谢 Axel Rauschmayer 。

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