Prototype-changing objects in Javascript

mutating.js

class Mutating {
    constructor(x) {
        this._thing = x;
    }

    get thing() { return this._thing; }
    set thing(x) {
        this._thing = x;
        this.change();
    }
}

class MutatingA extends Mutating {
    change() { Object.setPrototypeOf(this, MutatingB.prototype); }

    announce() { return 'A...'; }

    get stuff() { return 'A!'; }
}

class MutatingB extends Mutating {
    change() { Object.setPrototypeOf(this, MutatingA.prototype); }

    announce() { return 'B?'; }

    get stuff() { return 'B!'; }
}