Symbols
How to deal with ES6 symbols?
Rollup: https://buble.surge.sh/#typeof%20Symbol()%20%3D%3D%3D%20%22symbol%22%3B
Only it's basic features are supported by a few browsers. Internet Explorer doesn't support this at all.
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Symbol
Babel have a solution like this: https://babeljs.io/docs/plugins/transform-es2015-typeof-symbol/