Extending singleton class with private properties

I have a singleton class that I want to extend. Without extending the class it's fine but get
Uncaught TypeError: Cannot initialize #b twice on the same object if I use the extended version.

class A{
    static #instance = null
    #a = 1
    constructor(){
        if(!A.#instance){
            A.#instance = this
        }
        return A.#instance
    }
}
class B extends A{
    #b = 2
}

[new A(),new A()] works

[new B(),new B()] throws an error.

You have a flaw in your A::constructor():
From the constructor you return the first created instance of A saved in a static property thus reusing it as a parent instance of all instances of the class B.
Thus you try to initialize the same #b twice which throws.

To implement your singleton design pattern you could proxify the classes (luckily we can proxify a JS class!):

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const proxify = (what) => new Proxy(what, {
    instance: null,
    construct(B, args) {
        return this.instance ??= new what(...args);
    }
});
const A = proxify(class {
    #a = 1
    c = 0
});
const B = proxify(class extends A {
    #b = 2
    d = 0
});
// make sure we have a singleton for A
const a1 = new A;
const a2 = new A;
a2.c = 1;
console.log(a1.c, a2.c);
// make sure we have a singleton for B
const b1 = new B;
const b2 = new B;
b1.c = 2;
b2.d = 3;
console.log(b1.c, b2.c);
console.log(b1.d, b2.d);

<!-- end snippet -->