Well, I don't know if some codebases would have problem with Kotlin, I'm talking about Java 5 or older code. In those cases, some refractory will be needed.
I've read something about it, it is necessary a gradual migration from JS code to TS code. It is not obligatory but it's needed if developers want to use types.
Of course, such old code will need to be upgraded to recent version of Java anyways.
I did recently a migration from JS to TS, and it is still quite painful. You have to hunt a lot of TS config options to make them just right so that JS + TS can work together and not get into any 'import' errors.
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