Symbol.js

 const MY_KEY = Symbol();
    let obj = {
        [MY_KEY]: 123
    };
    

"use strict";

function _defineProperty(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }

var MY_KEY = Symbol();
var obj = _defineProperty({}, MY_KEY, 123);