Another supporter here. I was very reluctant at the beginning but after seeing how it helped catch bugs earlier, I can't go back. I actually think that TypeScript makes JS more flexible because of the patterns it enables. I enjoy it even more than C#.

When implementing overloaded functions, explicitly defining an interface would allow for much easier debugging than testing it afterwards. Running tests (that can take up to seconds sometimes) is far less efficient than seeing the problem nearly immediately

I would guess that compatibility with existing JS libs is far easier with Typescript. You can trivially consume an existing JS lib from TS or export/compile your TS lib to pure JS, which other JS then can seamlessly consume.

Although I don't have bigger experience with Elm, Purescript, Scala.js, Ceylon, Websharper & Co I guess its more complicated with them. An advantage here is also that Typescript does not bring along it's own standard library (e.g. new collection types) which could cause problems on interoperability.

Another advantage is getting other people on board. Getting developers from pure JS to typescript is not difficult, especially when they are used to Java/C++/C# (which most developers are). The learning curve for the other languages will be higher for most developers.

Well those 2 are entirely new languages, not just javascript with types added so they are not the same thing.

Not everyone knows or want to learn Haskell