babel/doc/caveats.md

Caveats

In order for certain features to work they require certain polyfills. You can satisfy all 6to5 feature requirements by using the included polyfill.

You may alternatively selectively include what you need:

Feature	Requirements
Abstract References	experimental, Symbol
Array destructuring	Array.from
Async functions, Generators	experimental, regenerator runtime
Comprehensions	experimental, Array.from
For Of	Symbol, prototype[Symbol.iterator]
Modules	Object.assign*
Object spread/rest	experimental, Object.assign
Spread	Array.from

*Only required for exporting a non-function default with additional exports.

ES5

Since 6to5 assumes that your code will be ran in an ES5 environment it uses ES5 functions. So if you're using an environment that has limited or no support for ES5 such as lower versions of IE then using the es5-shim along with the 6to5 polyfill will add support for these methods.

Internet Explorer

Classes (10 and below)

If you're inheriting from a class then static properties are inherited from it via __proto__, this is widely supported but you may run into problems with much older browsers.

NOTE: __proto__ is not supported in IE <= 10 so static properties will not be inherited. A possible workaround is to use super();:

class Foo {
  static foo() {

  }
}

class Bar extends Foo {
  static foo() {
    super();
  }
}

Getters/setters (8 and below)

In IE8 Object.defineProperty can only be used on DOM objects. This is unfortunate as it's required to set getters and setters. Due to this if you plan on supporting IE8 or below then the user of getters and setters isn't recommended.

Reference: MDN.