[sp332]

I would learn the basics in a few fields that they don't generally cover in high school: graph theory, number theory, set theory, statistics, and calculus. Just getting a few of the basic ideas from each of those in your tool kit will make it easier to think about certain classes of problems. Plus, you'll have a better idea where to look for an answer when you hit a problem that requires you to go deeper.

For web dev, statistics will help you spot A/B tests that are broken or meaningless. And set theory and combinatorics will help with database queries.