This was an annoying bug to find

arghh.ts

class Bar {
  constructor(public foo: Foo) {}
}

class Foo {
  private cachedBar: Bar | null = null;

  getBar() {
    if (this.cachedBar !== null) return this.cachedBar;
    this.cachedBar = new Bar(this);
    return this.cachedBar;
  }
}

function makeFoo() {
  const baseFoo = new Foo();

  // extend with some convenient properties
  return Object.create(baseFoo, { bar: { value: baseFoo.getBar() } });
}

const foo = makeFoo();
console.log(foo === foo.getBar().foo); // false