private-fields.js

const assert = require('assert');

class A {
  constructor() {
    return {self: this};
  }
}
class B extends A {
  #prop = 'hello';
  constructor() {
    super();
    assert.equal(this.#prop, 'hello');
    assert.equal(this instanceof B, false);
    assert.equal(this.self instanceof B, true);
    return this.self;
  }
  getProp() {
    return this.#prop;
  }
}
assert.throws(
  () => new B().getProp(),
  /^TypeError: Cannot read private member #prop from an object whose class did not declare it$/
);

/*
This example is contrived. But something similar happens
if you return a Promise for `this` from a constructor.

Note that this is the expected behavior and in accordance with
how instances are set up in class hierarchies:
https://exploringjs.com/es6/ch_classes.html#sec_allocating-and-initializing-instances
*/