typescript is a nicely conservative set of extensions to javascript, but if you're willing to venture a bit further afield, redhat's ceylon [https://ceylon-lang

typescript is a nicely conservative set of extensions to javascript, but if you're willing to venture a bit further afield, redhat's ceylon [https://ceylon-lang.org/ https://ceylon-lang.org/] is well worth a look. it has an excellent type system, and seems to be under active development, particularly in terms of improving the runtime and code generation. i played with it for a bit before deciding that for my personal stuff i prefer more strongly ML-based languages, but if i ever have to develop and maintain a large team project i'll definitely give ceylon a very serious look.