Is there a way to instantiate a new instance of a subclass from inside a super class method?

Question

I would like to be able to instantiate a new instance of a subclass from inside a superclass method.

If I have just a single class with no inheritance, it is straight forward:

class A {
    static build(opts) {
        return new A(opts)
    }   
    makeAnother() {
        return A.build(...)
    }
}

const a = new A()
const a2 = a.makeAnother()

This works. However, it doesn't work with subclassing:

class B extends A { ... }

const b = new B()
const b2 = b.makeAnother() // this returns an instance of A, not B

I suppose I could add the build & makeAnother methods to each subclass, but I would rather not repeat things.

Do you want B's `makeAnother` to return a new `B` object? If so, then I think the only way to do that is to repeat yourself — Samathingamajig, Oct 23 '20 at 22:39
Why do you need the `build()` method when you can just call the constructor directly? — Barmar, Oct 23 '20 at 22:44

score 1 · Answer 1 · answered Oct 23 '20 at 22:40

1

You can reference this.constructor inside the super class to get to the constructor of the subclass (or the super class itself, if the method is called on a super instance rather than a sub instance):

class A {
    static build(theClass) {
        return new theClass()
    }   
    makeAnother() {
        return A.build(this.constructor)
    }
}

const a = new A()
const a2 = a.makeAnother()
console.log(a2 instanceof A);

class B extends A { }

const b = new B()
const b2 = b.makeAnother()
console.log(b2 instanceof B);

answered Oct 23 '20 at 22:40

CertainPerformance

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Shouldn't it be `static build(theClass, opts)`? – Barmar Oct 23 '20 at 22:41
If an argument needs to be passed along, sure, but it wasn't being passed or used in OP's code – CertainPerformance Oct 23 '20 at 22:42

score 0 · Answer 2 · answered Oct 24 '20 at 00:35

You'll want to use

class A {
    static build(opts) {
        return new this(opts)
    }   
    makeAnother() {
        return this.constructor.build(...)
    }
}

Unless build does more than shown here, you don't need it at all of course, you'd rather directly return new this.constructor(...) in makeAnother.

Is there a way to instantiate a new instance of a subclass from inside a super class method?

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