创建类的对象时，将引发错误

Question

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

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

var b = new B();

output:

ReferenceError: this is not defined at B (repl:4:1) at repl:1:9 at REPLServer.defaultEval (repl.js:262:27) at bound (domain.js:287:14) at REPLServer.runBound [as eval] (domain.js:300:12) at REPLServer. (repl.js:431:12) at emitOne (events.js:82:20) at REPLServer.emit (events.js:169:7) at REPLServer.Interface._onLine (readline.js:211:10) at REPLServer.Interface._line (readline.js:550:8)

Answer 1

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";
    }
}

UPDATED:

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, eg 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
    }
}

Thanks to Axel Rauschmayer .
For more see here https://esdiscuss.org/topic/super-on-class-that-extends