Migrate to ES6 syntax

[kadenzipfel]

Is there any reason that we're not on ES6+ syntax? I noticed in the tutorials that we're using var instead of let or const. I also noticed that we're not using arrow functions where, as far as I can tell, we should.

Perhaps there is something that I'm misunderstanding or am not aware of, so please correct me or enlighten me.

Otherwise, if there is interest in migrating to ES6, let me know and I'd be happy to make a pull request.