Learn ES5, and the affordances that HTML5 give to it. Then, learn a bit about underscore/lodash for functional work, and node.js streams. Learn about promises.

Learn ES5, and the affordances that HTML5 give to it. Then, learn a bit about underscore/lodash for functional work, and node.js streams. Learn about promises. Later, you can learn more modern stuff (ES2015), but getting that solid foundation back under your feet will be useful. You will also skip a lot of mucking around with custom build systems.