If I could pick one thing that I wish I would have known, it would be the understanding of functional programming techniques. Adhering to FP principles like fun

If I could pick one thing that I wish I would have known, it would be the understanding of functional programming techniques. Adhering to FP principles like functional purity and composition has made my applications infinitely cleaner, more maintainable, and easier to test. My work mostly involves React, which is very well suited to functional-style programming, so if you're doing something with heavy graphics or some other application, YMMV. However for react apps or anything that returns virtual DOM nodes as a response to state change, FP is the way to go IMO.