ES6-Symbol

let aaa = Symbol('111')
console.log(typeof aaa)//symbol

let bbb = Symbol('111')
console.log(aaa === bbb)//false

let ccc = Symbol.for('222')
let ddd = Symbol.for('222')
console.log(ccc === ddd)//true

let eee = Symbol.keyFor(ddd)
console.log(eee)//222

let obj = {
  [aaa]: 'xxx'
}
console.log(Object.keys(obj))//[]
console.log(Object.getOwnPropertySymbols(obj))//[Symbol(111)]