nestedClass.js

class Bar {
  constructor() {
    this.foo = "foo from Bar";
  }
  fooFunc() {
    return this.foo;
  }
}

class Foo {
  constructor (bar) {
    this.bar = bar;
    this.foo = "foo from Foo";
  }
  fooFunc() {
    return this.foo;
  }
}

const bar = new Bar();
const foo = new Foo(bar);

console.log('foo.fooFunc(): ' + foo.fooFunc());  // expected "foo from Foo"
console.log('foo.bar.fooFunc(): ' + foo.bar.fooFunc());  // expected "foo from Bar"