The other name for ES6 is also known as Harmony. This site is a stripped down version of Codecademy that focuses purely on teaching its users how to use ES6 from the very scratch. It’s a good platform to exercise your ES6 knowledge as well as strengthen it, and perhaps learn a new thing or two. This site is designed to help you learn the new features of ECMAScript (primarily ES2015 for now) through examples and exploration. Write to the console, have some fun.
Katas are a set of challenges, and this resource has quite a few of them. Instead of being terrorized by typical blog articles and tutorials with lousy challenges, read a book or two about ES6 and try to come up with solutions for this specifically tailored challenges.
One thing to remember about Udemy courses is that the moment you enroll in a specific course, you’re granted access to a special community page for that particular course — within which you are able to connect with everyone else who has enrolled in the course, giving you the opportunity to learn material faster, as well as gain help whenever you need it.