Rust because it’s the other language I see most often see coders learn to build their own apps. It has a good reputation for developer experience, security and speed.
I’ve heard enthusiasm for lisp-based languages for a couple decades and don’t doubt the merits of Racket.
Disclaimer: my lisp experience is largely with Clojure, and not at all with writing Racket, just earnestly reading about it.
If you’re picking just one language to learn, and limiting your learning to only that one language for a significant period of time, you’ll probably want to learn Rust. You won’t get all of the benefits of also learning a lisp, but you’ll get a lot of the ones which generalize well.
If you’re open to learning 1+N and otherwise lean towards TS, I would say you’re better off learning Racket (or any lisp really) in tandem than learning TS alone.
Familiarity with the core concepts and idioms of FP are broadly beneficial in any language, and lisps tend to be a good balance of those with low incidental mental overhead, and reasonable escape hatches to do imperative stuff where it makes sense. Racket is probably a particularly good candidate because its optional typing is another overlapping story between the two.
I haven’t written in any lisp for close to a decade now, but I still find that prior experience beneficial every day since. I sometimes miss the simplicity and flexibility of the parentheses. But I can take what I learned—how to reason about state, data flow, data-driven abstractions—anywhere. And those abstractions are particularly useful in a structural type system like TS.
Rust also is functional by default with clearly distinguished imperative mechanisms. In my (admittedly limited) experience, Rust hews more towards the type theory of MLs and generally encourages programming with expressions rather than statements. The syntax is definitely less lispy but not so much that it’s hard to metaprogram.
