Which CS fundamentals should I focus on as I move from mathematics into full-stack development?

There isn’t an inherent limit to self-taught developers. The biggest difference is whether someone is only following tutorials or actively digging into why things work. Strong developers investigate source code, experiment, break things, notice their blind spots, and think about maintainability, security, scalability, and architectural side effects instead of fixing only the immediate bug.

A lot of developers can make code produce the right result without understanding what it costs. A nested loop may pass every test but still be O(n²), while a set or map could solve the same task in one pass. Complexity analysis, data representation, and performance reasoning are worth practicing deliberately.

For a well-rounded foundation, study how data is represented, what the operating system does, and how programs interact with memory, files, processes, and the network. Building something in a lower-level language such as C can help make arrays, pointers, allocation, and deallocation concrete, even if you ultimately stay focused on JavaScript and Python.

Database internals and query performance are easy to overlook when an ORM handles everything. Learn how indexes work, especially B-trees, understand query plans, and get familiar with the difference between writing a query that works and one that continues to work as the data grows.

Data structures and algorithms are common gaps, but the important part is understanding when and why to use them rather than just solving endless puzzle problems. You should be comfortable matching a data structure to the shape of a problem, reasoning about complexity, and recognizing tradeoffs in real applications.