Well, unless you are not using a transpiler AND you are targeting older browsers, you can begin to take advantage of ES6 features. If you need to get up to speed with ES6, here are a few resources.
First, I find it helpful to check support for ES6 features for the browsers or environments I am targeting. You can do that by going to: https://kangax.github.io/compat-table/es6/. The compatibility table is easy to use and it shows all the different environments that you may be targeting.
Second, the best reference source I have found for explaining all of the ES6 features is the book: ES6 & Beyond by Kyle Simpson. It is part of the You Don’t Know JS series. It is the most complete resource I have found.