symbol-private-props.js

const KEY = Symbol('my key') 

const fooProto = {
  get key() {
    return this[key]
  }
};

function makeFoo() {
  const foo = Object.create(fooProto);
  foo[key] = 'my secret key';
  return foo;
}

const foo = makeFoo();

console.log(foo.key)
// => 'my secret key'

console.log(foo[key])
// => 'my secret key'

console.log(foo[Symbol('my key')])
// => undefined