Since Javascript is just a language that happens to execute client-side within the browser, why can we not use an existing language syntax (or an existing language entirely) for this purpose? For example, Go, Python, Rust, or perhaps Perl? It feels like a broken shell scripting language that really should have been replaced.
You can, but you should not run the UI script code on the same non-reentrant JS engine that executes scripts delivered from the internet at large.
By doing so (running both the UI code and scripts from the internet on the same non-reentrant JS engine) means that only one gets to run at a time (either UI code or JS from the internet). That's why FF's responsiveness sucks at times, any long running JS script from the internet also prevents the UI code from running.
The JS engine should either have been reentrant (so UI code could run even though some random internet script was still running), or there should have been two independent JS interpreters loaded, one used for the UI code only and a second for all the JS from the internet.
Since the introduction of ES2015, JavaScript has actually been a really nice language to work with. It's been really well implemented and optimized by browsers lately. So I don't really see a good reason to switch languages suddenly. That being said you'll probably be able to compile and run all sorts of languages for the web with WebAssembly in the near future.
