[Zarel]

I'm not sure what you mean by "not JS compatible" here.

TypeScript obviously compiles into JavaScript, and any valid JavaScript is also "valid" compilable TypeScript (the compiler will complain, but it will still compile it into JavaScript that does the same thing). With the right assertions (and @ts-ignore where necessary), you can even get the compiler to stop complaining!

TypeScript does have some differences beyond just type annotations: namespaces and enums come to mind. But it's easy enough to just not to use those features - they're not even supported by Babel.

The main downside is that you have a compile step where you might not have had one before. And it can be frustrating to deal with situations where there's no way to "fix" an error except by suppressing it – but these are really rare. But these are relatively minor issues.

[WorldMaker]

Also, even namespaces and enums aren't that foreign, and are still "JS compatible" in that they transpile to perfectly normal JS code that you can still use quite obviously in other JS code (enums create a list of constants and sometimes a reverse lookup map; namespaces create ugly, nested, mergeable object literals and IIFEs just like Grandma jQuery used to bake back in the bad old days before module systems). From one point of view they are nothing but syntactic sugar for common JS patterns.