David Gilbertson tells you all about polyfills.
Faced with the reality that you can’t write modern code and expect it to work for all users, you have exactly two choices:
1. Only use language features available in all the browsers you support
2. Write modern code, then do something to make it work in older browsers
If you have decided on option one then I respectfully suggest that you are bonkers, and insist that you state your case in the comments.
I believe that developers who are able to explore all the new stuff and use it in their day-to-day jobs stay happy, and being happy is important.