Typescript isn't exactly a pseudo-language. It looks just like JavaScript with Flow annotations. In fact, Typescript is intended to be JavaScript + Type Annotations, so it's just as "native" as JavaScript + Flow annotations, with the addition of some Babel-style ES6 transpiling as well. The only difference is the explicit build process versus running the transforms in browser, and I've seen projects use the Typescript compiler (itself written in JS) as their in-browser transpiler (SystemJS supports it as a first-class transpiler option alongside Babel and Traceur, even).