symbol-private-props.js

const KEY = Symbol('my key') 

function makeFoo(){
  return {
    [KEY]: 'my secret key',
    get key(){
      return this[KEY];
    }
  };
}

const foo = makeFoo();

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

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

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