js调用基类构造函数

Question

Is it possible to call a baseclass constructor from a class?

class BaseCls {
}

class Cls extend BaseCls {
    constructor(options){
        super(options)
    }
}

var instance = new Cls();

Now I want an instance of the baseclas. Something like this:

var baseInstance = new Cls.parent()

I know that I could just call new BaseCls(), but doing it the other way allows me to have only one import.

Answer 1

The superclass is the prototype of the subclass* (which is why superclass static methods are accessible on the subclass), so Object.getPrototypeOf will give you the superclass:

 class BaseCls { constructor() { console.log("BaseCls"); } } class Cls extends BaseCls { constructor(options){ super(options) console.log("Cls"); } } var base = Object.getPrototypeOf(Cls); var instance = new base(); 

You don't have to separate the statements, but if you want to combine them, you have to put () around the call to Object.getPrototypeOf (otherwise new tries to consume it):

var instance = new (Object.getPrototypeOf(Cls))();

And of course, if you wanted to do this generically from a reference to an instance of Cls , it would be:

var superInstance = new (Object.getPrototypeOf(instance.constructor))();

...provided instance doesn't have an own constructor property. Or the rather more convoluted:

var superInstance = new (Object.getPrototypeOf(Object.getPrototypeOf(instance).constructor))();

...if it may have its own constructor property.

---

* Yes, really. When you use B extends A , there are two parallel lines of inheritance set up: A.prototype is made the prototype of B.prototype , and A is made the prototype of B (whereas in ES5 and earlier, the prototype of a function was always Function.prototype ).

Given:

class A {
}

class B extends A {
}

the inheritance looks like this:

B −−−−−−−−−−−−−−> A −−−−−−−−−−−−−−> Function.prototype

B.prototype −−−−> A.prototype −−−−> Object.prototype

 class A { } class B extends A { } console.log(Object.getPrototypeOf(B) === A); console.log(Object.getPrototypeOf(A) === Function.prototype); console.log(Object.getPrototypeOf(B.prototype) === A.prototype); console.log(Object.getPrototypeOf(A.prototype) === Object.prototype);