[gingerb]

I use types.js (https://www.npmjs.com/package/types.js) dynamic type checker already for years in production without any issues. I simply don't want yet another JS syntax or transpiler only for type checking like Typescript, Flow etc.. For me that's way over the top.

[seanmcdirmid]

Typescript was designed in such a way that the transpired mostly merely removes type annotations from the TS file to the JS one. They also went with a loose optional structural type system to be as non invasive as possible (ironically enough, it can be less strict than a nominal dynamic type system).