React and its ecosystem (or a similar one like Vue or Preact). It’s not so much about React as a specific library but about component-based architecture, re-usability, optimization for performance and patterns. React team does a really good job overall. The library is well maintained, the community is great, the documentation is detailed and easy to follow. React is beginner friendly, so if you haven’t picked it up yet, I highly recommend it in 2019.
Webpack/Babel. Any modern front-end app has to be transpired, configured, optimized for production, split into multiple bundles, and the list goes on and on. Knowing how to fine-tune the build process is a great skill to have.