symbol.js

const hiddenSymbol = Symbol();
const obj = { a: 1 };
obj[hiddenSymbol] = 'hidden';

// print all keys
const keys = Object.keys(obj);
console.log(keys); // prints ['a']

// print value of prop hiddenSymbol
console.log(obj[hiddenSymbol]); // prints 'hidden'

// retrieve all symbol properties
console.log(Object.getOwnPropertySymbols(obj)); // prints [Symbol()], our symbol type