Symbols
ES6 Symbols, in your ES5.
If Symbol is already defined, it will be used. Otherwise, this implements as much of the Symbol spec as is possible with plain JavaScript.
Usage
var Symbol = require('symbol');
var key = Symbol();
var obj = {};
obj[key] = 'foo';
console.log(obj[key]); // 'foo'
console.log(Object.keys(obj)); // []