If JS code expects to be JITed then transpiled in Python would be quite slow. Transpiling Python to JS is difficult and loses access to various ecosystems e.g.

If JS code expects to be JITed then transpiled in Python would be quite slow. Transpiling Python to JS is difficult and loses access to various ecosystems e.g. scientific Python. Having the two runtimes coupled seems to be the ideal way to run both languages in the same process.