I've been using kotlin-js for the past few years. Not an obvious choice for a lot of typescript users but IMHO something that would lead to a lot of obvious improvements to typescript if more people would. Like being more strict by default and getting rid of a lot of the javascript compatibility. This library seems like a noble attempt to patch it up somewhat. But why not go all the way and just fix the language properly?
Kotlin and typescript are actually similar enough that transitioning from one to the other isn't that big of a deal. The key difference is that typescript tries to maintain compatibility with javascript whereas it's just a compilation target for kotlin-js.
Like with typescript, you use type safe wrappers for any javascript code you need to access. You can actually reuse typescript type definition files and generate Kotlin ones from them. Not perfect, sadly, but it works for simple libraries and you can manually deal with the more complicated ones. I use things like fluent-js, maplibre, and a few other libraries. I don't use react, but a lot of kotlin-js developers do. There are no real limitations on what you can use here.
The one compromise kotlin-js makes to enable interfacing with javascript code is the dynamic keyword, which you use to do the bait and switch style APIs you see a lot in the javscript world. It could be a list, a number, or a string, etc. Dynamic allows you to work with such APIs. It's a necessary evil. But otherwise, it's all good. It's strict by default. It doesn't have a mode where it is not strict. And this is what enables IDEs like intellij to be a lot smarter with Kotlin than it is with typescript.
If you ignore the few syntax and language features that are unique to either Kotlin or Typescript, the key difference is that typescript is more sloppy and it's mostly because of js compatibility. And since kotlin-js has almost none of that and can manage fine without it, it kind of proves that this level of sloppiness is simply unnecessary and redundant. A whole lot of downsides and not a lot of upsides. Typescript is much more sloppy than it needs to be. Making it less sloppy is the obvious way to improve it. It's why I use kotlin-js instead.
This is basically how I feel about rescript these days. Typescript is really amazingly complex for what it ends up giving you. It's a big improvement over js, but I don't think that's the right comparison.
Using typescript we're already paying the costs of friction & indirection in tooling and learning another language. I know typescript has a ton of momentum and is unstoppable at this point but I wish things had turned out differently.
Kotlin and typescript are actually similar enough that transitioning from one to the other isn't that big of a deal. The key difference is that typescript tries to maintain compatibility with javascript whereas it's just a compilation target for kotlin-js.
Like with typescript, you use type safe wrappers for any javascript code you need to access. You can actually reuse typescript type definition files and generate Kotlin ones from them. Not perfect, sadly, but it works for simple libraries and you can manually deal with the more complicated ones. I use things like fluent-js, maplibre, and a few other libraries. I don't use react, but a lot of kotlin-js developers do. There are no real limitations on what you can use here.
The one compromise kotlin-js makes to enable interfacing with javascript code is the dynamic keyword, which you use to do the bait and switch style APIs you see a lot in the javscript world. It could be a list, a number, or a string, etc. Dynamic allows you to work with such APIs. It's a necessary evil. But otherwise, it's all good. It's strict by default. It doesn't have a mode where it is not strict. And this is what enables IDEs like intellij to be a lot smarter with Kotlin than it is with typescript.
If you ignore the few syntax and language features that are unique to either Kotlin or Typescript, the key difference is that typescript is more sloppy and it's mostly because of js compatibility. And since kotlin-js has almost none of that and can manage fine without it, it kind of proves that this level of sloppiness is simply unnecessary and redundant. A whole lot of downsides and not a lot of upsides. Typescript is much more sloppy than it needs to be. Making it less sloppy is the obvious way to improve it. It's why I use kotlin-js instead.