[jasim]

* Learn to think about types first, and the algorithms as a natural consequence of the structure of your data. Richard Feldman: https://www.youtube.com/watch?v=IcgmSRJHu_8,

* Use a statically typed language that allows fast refactoring. Ideally a typed functional language like Elm, Reason, OCaml, or Haskell.

I recently shipped a complex Javascript project (without types) and it is one of the few instances of my decade-old professional career where I had that level of confidence in the robustness of a piece of code I wrote. It remained sturdy in production even in the presence of unforeseen conditions. This was thanks to the habits I picked up from typed FP: parsing and validating all data at the boundaries of the system, better design by modelling data to make inconsistent states impossible, and automatically handling all edge-cases at all times.

So even if you don't actively use such a language in your workplace, there really is a payoff to learning it in making you a better programmer overall.

[julvo]

Came here to say something similar: Learn a functional language.

Not because you'll be using functional languages afterwards for work, but it changes your coding style - even in imperative languages. More towards pure functions and a cleaner separation between data and logic

[smush]

I agree with you 100% and will amplify your comment with one of my own.

I suspect this won't be immediately possible as I'm not certain what languages OP is allowed to use, but if OP has some wiggle room in what PL to use, then I would recommend that he look at the Domain Modelling Made Functional book by Scott W.

Like you say, the principles in the book apply to other languages as well, but it can be a faster feedback cycle to learn these lessons in a language that has first-class support for them. He will be able to use F# or its close syntax cousins OCaml, ML to model new features as domains and employ option types, pattern matching, railway-oriented-programming, and functions as interfaces to make his code more resilient, like you mention.

The bonus is that if OP uses the likes of F# or Scala, it can interop with C# or Java respectively within the same codebase.