Self Hosted Array Supporting Private State

my-array-with-privates.js

class X {
    constructor(len) {
        let target = Object.create(new.target.prototype);

        let handler = {
            // handler stuff - special logic for "length" and integer properties
            // Otherwise, use target's properties
        };

        return Reflect.construct(Proxy, [target, handler], new.target);
    }
}

class Y extends X {
    #foo;
    constructor() {
        super(10);
    }
    get foo() { return this.#foo }
}