jimmywartinggroupby-polyfill 
A small polyfill with zero-dependency that do just do one thing.
npm install groupby-polyfill
This is a hybrid package. It dose not import or export anything so both require
and import works.
This will conditionally polyfill global Object/Map with the missing groupBy
method using ??= but I would recommend that you conditionally import it if it's
needed. See comp. table at MDN if you need this:
Documentation and code example is in this links too.
Usage
Just need to import it.
import 'groupby-polyfill/lib/polyfill.js'